www.fairchildsemi.com

REV. 1.1.7 8/29/02

Features

· CPU Core power: 0.925V to 2.0V output range
· ±1% reference precision over temperature
· Dynamic voltage setting with 5-bit DAC
· 5V to 24V input voltage range
· 2 phase interleaved switching
· Active droop to reduce output capacitor size
· Differential remote voltage sense
· High efficiency:

>90% efficiency over wide load range
>80% efficiency at light load

· Excellent dynamic response with Voltage Feed-Forward

and Average Current Mode control

· Dynamic duty cycle clamp minimizes inductor current

build up

· Lossless current sensing on low-side MOSFET or

Precision current sensing using sense resistor

· Fault protections: Over-voltage, Over-current, and

Thermal Shut-down

· Controls: Enable, Forced PWM, Power Good, Power

Good Delay

· QSOP28, TSSOP28

Applications

· AMD Mobile Athlon

CPU V

CORE

Regulator

· AMD Mobile Duron

CPU V

CORE

Regulator

General Description

The FAN5240 is a single output 2-Phase synchronous buck
controller to power AMD's mobile CPU core. The FAN5240
includes a 5-bit digital-to-analog converter (DAC) that
adjusts the core PWM output voltage from 0.925VDC to
2.0VDC, which may be changed during operation. Special
measures are taken to allow the output to transition with
controlled slew rate to comply with AMD's Power Now

technology. The FAN5240 includes a precision reference,
and a proprietary architecture with integrated compensation
providing excellent static and dynamic core voltage regula-
tion. The regulator includes special circuitry which balances
the 2 phase currents for maximum efficiency.

At light loads, when the filter inductor current becomes
discontinuous, the controller operates in a hysteretic mode,
dramatically improving system efficiency. The hysteretic
mode of operation can be inhibited by the FPWM control
pin.

The FAN5240 monitors the output voltage and issues a
PGOOD (Power-Good) when soft start is completed and the
output is in regulation. A pin is provided to add delay to
PGOOD with an external capacitor.

A built-in over-voltage protection (OVP) forces the lower
MOSFET on to prevent the output from exceeding a set
voltage. The PWM controller's overcurrent circuitry moni-
tors the converter load by sensing the voltage drop across the
lower MOSFET. The overcurrent threshold is set by an exter-
nal resistor. If precision overcurrent protection is required,
an optional external current-sense resistor may be used.

FAN5240

Multi-Phase PWM Controller for AMD Mobile
Athlon

and Duron

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Typical Application

Figure 1. AMD Mobile Athlon/Duron CPU Core Supply

Table 1. BOM for Figure 1

Description

Qty

Ref.

Vendor

Part Number

Capacitor 22µF, Ceramic X7R 25V

C1, C2

TDK

Capacitor 1µF, Ceramic

C3,C7,C9

Any

Capacitor 0.1µF, Ceramic

C4C6, C8, C11, C12

Any

Capacitor 0.22µF, Ceramic

C10

Any

Capacitor 270µF, 2V, ESR 15m

C13C16

Panasonic

EEFUE0D271R

10K

, 5% Resistor

Any

, 1% Resistor

R2, R3, R6

Any

56.2K

, 1% Resistor

Any

Schottky Diode 40V

D1, D2

Fairchild

MBR0540

Inductor 1.6µH, 20A, 2.4m

L1, L2

Panasonic

ETQP6F0R8LFA

N-Channel SO-8 MOSFET, 11m

Q1, Q4

Fairchild

FDS6694

N-Channel SO-8 SyncFETTM MOSFET, 6m

Q2, Q3, Q5, Q6

Fairchild

FDS6676S

VCORE +

C12

CORE

Phase 1

Phase 2

VCC

ISNS2

AGND

PGND2

SW2

HDRV2

ISNS1

PGND1

LDRV1

BOOT2

HDRV1

SW1

BOOT1

VIN

LDRV2

PGOOD

VIN (BATTERY)

= 5 to 24V

VIN

VCORE D

VID0

VID1

VID2

VID3

VID4

FPWM

ILIM

DELAY

C13

C14

C16

C11

C10

C15

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

Pin Configuration

Pin Definitions

Pin

Number

Pin

Name

Pin Function Description

LDRV2
LDRV1

Low-Side Drive.

The low-side (lower) MOSFET driver output.

PGND2
PGND1

Power Ground.

The return for the low-side MOSFET driver.

BOOT2
BOOT1

BOOT.

The positive supply for the upper MOSFET driver. Connect as shown in Figure 1.

HDRV2
HDRV1

High-Side Drive.

The high-side (upper) MOSFET driver output.

SW2
SW1

Switching node.

The return for the high-side MOSFET driver.

ISNS2
ISNS1

Current Sense input.

Monitors the voltage drop across the lower MOSFET or external

sense resistor for current feedback.

7 - 11

VID4 -

VID0

Voltage Identification Code.

Input to VID DAC. Sets the output voltage according to the

codes set as defined in Table 2. These inputs have 1

A internal pull-up.

FPWM

Forced PWM mode.

When logic high, inhibits the chip from entering hysteretic operating

mode. If tied low, hysteretic mode will be allowed.

ILIM

Current Limit.

A resistor from this pin to GND sets the current limit.

ENABLE.

This pin enables IC operation when either left open, or pulled up to VCC.

Toggling EN will also reset the chip after a latched fault condition.

AGND

Analog Ground.

This is the signal ground reference for the IC. All voltage levels are

measured with respect to this pin.

DELAY

Power Good / Over-Current Delay.

A capacitor to GND on this pin delays the PGOOD

from going high as well delaying the over-current shutdown.

18
17

VCORE+
VCORE

VCORE Output Sense.

Differential sensing of the output voltage. Used for regulation as

well as PGOOD, under-voltage and over-voltage protection and monitoring. A resistor in
series with this VCORE+ sets the output voltage droop.

PGOOD

Power Good Flag.

An open-drain output that will pull LOW when the core output below

825mV. PGOOD delays its low to high transition for a time determined by CDELAY when
VCORE rises above 875mV.

LDRV2

PGND2

BOOT2

HDRV2

SW2

ISNS2

VID4

VID3

VID2

VID1

VID0

FPWM

ILIM

FAN5240

VCC

LDRV1

PGND1

BOOT1

HDRV1

SW1

ISNS1

VIN

PGOOD

VCORE+

VCORED

DELAY

AGND

QSOP-28 or TSSOP-28

= 90

°C/W

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Absolute Maximum Ratings

Absolute maximum ratings are the values beyond which the device may be damaged or have its useful life
impaired. Functional operation under these conditions is not implied.

Recommended Operating Conditions

Soft Start.

A capacitor from this pin to GND programs the slew rate of the converter during

initialization as well as in operation. This pin is used as the reference against which the
output is compared. During initialization, this pin is charged with a 25

A current source.

Once this pin reaches 0.5V, its function changes, and it assumes the value of the voltage
as set by the VID programming. The current driving this pin is then limited to ±500

A, that

together with C

sets a controlled slew rate for VID code changes.

VIN

Input voltage from battery.

This voltage is used by the oscillator for feed-forward

compensation of input voltage variation.

VCC

VCC.

This pin powers the chip. The IC starts to operate when voltage on this pin exceeds

4.6V (UVLO rising) and shuts down when it drops below 4.3V (UVLO falling).

Parameter

Min.

Typ.

Max.

Units

VCC Supply Voltage:

6.5

VIN

BOOT, SW, HDRV Pins

BOOT to SW

6.5

All Other Pins

0.3

VCC+0.3

Junction Temperature (T

)

150

°C

Storage Temperature

150

°C

Lead Soldering Temperature, 10 seconds

300

°C

Parameter

Conditions

Min.

Typ.

Max.

Units

Supply Voltage VCC

4.75

5.25

Supply Voltage VIN

Ambient Temperature (T

)

°C

Pin Definitions

(continued)

Pin

Number

Pin

Name

Pin Function Description

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

Electrical Specifications

(VCC = 5V, VIN = 6V24V, and T

= recommended operating ambient temperature range using circuit of Figure 1,

unless otherwise noted.)

Note 1:

Guaranteed by slew rate testing.

Parameter

Conditions

Min.

Typ.

Max.

Units

Power Supplies

VCC Current

Operating, C

= 10pF

Shut-down (EN=0)

VIN Current

Operating

Shut-down (EN=0)

UVLO Threshold

Rising VCC

4.3

4.45

4.6

Falling VCC

3.8

3.95

4.10

Regulator / Control Functions

Output voltage

per Table 2

0.925

2.00

Error Amplifier Gain

Error Amplifier GBW

2.7

MHz

Error Amplifier Slew Rate

VCORE+ Input Current

ILIM Voltage

ILIM

= 30K

0.89

0.91

ILIM T

HOLDOFF

DELAY

= 22nF

1.16

Over-voltage Threshold

2.2

2.35

2.5

Over-voltage Protection delay

EN, input threshold

Logic LOW

0.8

Logic HIGH

Phase to Phase current mismatch

IC contribution only
Guaranteed by design

±5

Over-Temperature Shut-down

150

°C

Over-Temperature Hysteresis

°C

Output Drivers

(note 1)

HDRV Output Resistance

Sourcing

3.8

Sinking

1.6

LDRV Output Resistance

Sourcing

3.8

Sinking

0.8

1.5

Oscillator

Frequency

255

300

345

KHz

Ramp Amplitude, pkpk

VIN = 16V

Ramp Offset

0.5

Ramp Gain

125

mV/V

Reference, DAC and Soft-Start

VID input threshold

Logic LOW

0.8

Logic HIGH

2.0

VID pull-up current

to VCC

DAC output accuracy

Soft Start Charging current (I

)

< 90% of Programmed output

> 90% of Programmed output

350

500

650

RampAmplitude

----------------------------------------------

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Electrical Specifications

(continued)

Figure 2. IC Block Diagram

Parameter

Conditions

Min.

Typ.

Max.

Units

PGOOD

VCORE Lower Threshold

Falling Edge

800

825

850

Rising Edge

850

875

900

PGOOD Output Delay

Low to High, C

DELAY

= 22nF

PGOOD Output Low

PGOOD

= 4mA

0.5

Leakage Current

PULLUP

= 5V

HYST

PGOOD

VDD

PWM

VIN

POR/UVLO

BOOT

VDD

CURRENT

PROCESSING

HDRV

LDRV

PGND

BOOT

ISNS1

VIN

OUT

VCORE

ILIM

I
LIM

SENSE1

MODE

ISNS1

OUT

ISNS2

SENSE2

VCORE-

30mA

ISNS1+ISNS2

EA1

DUTY

CYCLE

CLAMP

OSC

RAMP

CLK

ILIM
det.

VCORE+

TO PH 2
MODULATOR

VOUT'

DAC

Soft Start &

OVP

ISNS2-ISNS1

ISNS1-ISNS2

ISNS2

HYST

PWM

FPWM

PWM/HYST

S/H

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

Circuit Description

Overview

The FAN5240 is a 2-phase, single output power management
IC, which supplies the low-voltage, high-current power to
modern processors for notebook PCs. Using very few exter-
nal components, the IC controls a precision programmable
synchronous buck converter driving external N-Channel
power MOSFETs. The output voltage is adjustable from
0.925V to 2.0V by changing the DAC (VID) code settings
(see Table 2). The output voltage of the core converter can be
changed on-the-fly with programmable slew rate, which
meets a key requirement of AMD's Mobile Athlon/Duron

processors.

The converter can operate in two modes: fixed frequency
PWM, and variable frequency hysteretic depending on the
load. At loads lower than the point where filter inductor cur-
rent becomes discontinuous, hysteretic mode of operation is
activated. Switchover from PWM to hysteretic operation at
light loads improves the converter's efficiency and prolongs
battery run time. As the filter inductor resumes continuous
current, the PWM mode of operation is restored.

Output Voltage Programming

The output voltage of the converter is programmed by an
internal DAC in discrete steps of 25mV from 0.925V to
1.300V and then in 50mV steps from 1.300V to 2.00V:

Table 2.

Output voltage VID

1 - Logic High or open, 0 = Logic Low

VID04 pins will assume a logic 1 level if left open as each
input is pulled up with a 1

A internal current source.

VID4

VID3

VID2

VID1

VID0

VOUT to CPU

0.000

0.925

0.950

0.975

1.000

1.025

1.050

1.075

1.100

1.125

1.150

1.175

1.200

1.225

1.250

1.275

0.000

1.300

1.350

1.400

1.450

1.500

1.550

1.600

1.650

1.700

1.750

1.800

1.850

1.900

1.950

2.000

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Initialization, Soft Start and PGOOD

Assuming EN is high, FAN5240 is initialized when power is
applied on VCC. Should VCC drop below the UVLO thresh-
old, an internal Power-On Reset function disables the chip.

The IC attempts to regulate the VCORE output according to
the voltage that appears on the SS pin (V

). During start-up

of the converter, this voltage is initially 0, and rises linearly
to 90% of the VID programmed voltage via the current sup-
plied to C

by the 25

A internal current source. The time it

takes to reach this threshold is:

where T

90%

is in seconds if C

is in

µF.

At that point, the current source changes to 500

µA, which

establishes the slew rate of voltage changes at the output in
response to changes in VID.

This dual slope approach helps to provide safe rise of volt-
ages and currents in the converters during initial start-up and
at the same time sets a controlled speed of the core voltage
change when the processor commands to do so.

Figure 3. Soft-Start function

typically is chosen based on the slew rate desired in

response to a VID change. For example, if the spec requires a
500mV step to occur in 100

µS:

Assuming VID is set to 1.5V, with this value of C

, the

time for the output voltage to rise to 0.9 of V

VID

is found

using equation 1:

The transition from 90% VID to 100% VID occupies 0.5%
of the total soft-start time, so TSS is essentially T

90%

The PGOOD delay (TDLY, Figure 3) can be programmed
with a capacitor to GND on pin 16 (C

DELAY

For 12mS of TDLY, C

DELAY

= 22nF.

DELAY

is typically chosen to provide 1mS of "blanking"

for the over-current shut-down (see Over-Current Sensing,
on page 12).

The following conditions set the PGOOD pin low:

Under-voltage - VCORE is below a fixed voltage.

Chip shut-down due to over-temperature or over-current
as defined below.

Converter Operation

(see Figure 2)

At nominal current the converter operates in fixed frequency
PWM mode. The output voltage is compared with a refer-
ence voltage set by the DAC, which appears on the SS pin.
The derived error signal is amplified by an internally com-
pensated error amplifier and applied to the inverting input
of the PWM comparator. To provide output voltage droop for
enhanced dynamic load regulation, a signal proportional to
the output current is added to the voltage feedback signal
at the + input of A1. Since the processor specifies a +100mV/
-50mV tolerance on VCORE, a fixed positive offset of 30
mV is created with a 30

µA current source and external 1K

resistor. Phase load balancing is accomplished by adding
a signal proportional to the difference of the two phase
currents before the error amplifier (at nodes A and B). This
feedback scheme in conjunction with a PWM ramp propor-
tional to the input voltage allows for fast and stable loop
response over a wide range of input voltage and output
current variations. For the sake of efficiency and maximum
simplicity, the current sense signal is derived from the volt-
age drop across the lower MOSFET during its conduction
time. This current sense signal is used to set droop levels as
well as for phase balancing and current limiting.

The PWM controller has a built-in duty cycle clamp in the
path from the error amplifier to the PWM comparator.
During a severe load step, the output signal from the error
amp can go to its rail, pushing the duty cycle to almost 100%
for a significant amount of time. This could cause a severe
rise in the inductor current, especially at high battery volt-
age, and lead to a long recovery time or even failure of the
converter. To prevent this, the output of the error amplifier is
clamped to a fixed value after two clock cycles if a large
output voltage excursion is detected. Sensitivity of this
circuit is set in such a way as not to affect the PWM control
during transients normally expected from the load.

90%

0.9

VID

--------------------------------------------

(1)

PGOOD

1.5V

TDLY

1.35V

DAC

------------------

500

µA

500mV

-------------------

100µS 0.1µF

(2)

90%

1.35V

0.1

------------------------------

5.4mS

DELAY

in nF

(

)

1.8

TDLY in mS

(

)

(3)

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

Operation Mode Control

The mode-control circuit changes the converter's mode of
operation from PWM to Hysteretic and visa versa, based
on the voltage polarity of the SW node when the lower
MOSFET is conducting and just before the upper MOSFET
turns on. For continuous inductor current, the SW node is
negative when the lower MOSFET is conducting and the
converters operate in fixed-frequency PWM mode as shown
in Figure 4. This mode of operation achieves high efficiency
at nominal load. When the load current decreases to the point
where the inductor current flows through the lower MOSFET
in the `reverse' direction, the SW node becomes positive,
and the mode is changed to hysteretic, which achieves higher
efficiency at low currents by decreasing the effective switch-
ing frequency.

To prevent accidental mode change or "mode chatter" the
transition from PWM to Hysteretic mode occurs when the
SW node is positive for eight consecutive clock cycles
(see Figure 4). The polarity of the SW node is sampled at the
end of the lower MOSFET's conduction time. At the transi-
tion between PWM and hysteretic mode both the upper and
lower MOSFETs are turned off. The phase node will `ring'
based on the output inductor and the parasitic capacitance on
the phase node and settle out at the value of the output volt-
age.

The boundary value of inductor current, where current
becomes discontinuous, can be estimated by the following
expression.

Hysteretic Mode

Conversely, the transition from Hysteretic mode to PWM
mode occurs when the SW node is negative for 8 consecutive
cycles.

A sudden increase in the output current will also cause a
change from hysteretic to PWM mode. This load increase
causes an instantaneous decrease in the output voltage due to
the voltage drop on the output capacitor ESR. If the load

causes the output voltage (as presented at VSNS) to drop
below the hysteretic regulation level (20mV below VREF),
the mode is changed to PWM on the next clock cycle. This
insures the full power required by the increase in output
current.

In hysteretic mode, the PWM comparator and the error
amplifier that provide control in PWM mode are inhibited
and the hysteretic comparator is activated. In hysteretic
mode the low side MOSFET is operated as a synchronous
rectifier, where the voltage across V

DS(ON)

is monitored,

and its gate switched off when V

DS(ON

) goes positive

(current flowing back from the load) blocking reverse
conduction

The hysteretic comparator initiates a PFM signal to turn on
HDRV when the output voltage (at VSNS) falls below the
lower threshold (10mV below VREF) and terminates the
PFM signal when VSNS rises over the higher threshold
(5mV above VREF).

The switching frequency is primarily a function of:

Spread between the two hysteretic thresholds

LOAD

Output Inductor and Capacitor ESR

A transition back to PWM (Continuous Conduction Mode or
CCM) mode occurs when the inductor current rises suffi-
ciently to stay positive for 8 consecutive cycles. This occurs
when:

where

HYSTERESIS

= 15mV and ESR is the equivalent

series resistance of C

OUT

Because of the different control mechanisms, the value of the
load current where transition into CCM operation takes place
is typically higher compared to the load level at which transi-
tion into hysteretic mode occurs.

LOAD DIS

(

)

OUT

(

OUT

--------------------------------------------------

(4)

LOAD CCM

(

)

HYSTERESIS

2 ESR

-----------------------------------------

(5)

Figure 4. Transitioning between PWM and Hysteretic Mode

PWM Mode

Hysteretic Mode

PWM Mode

CORE

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Current Processing Section

The following discussion refers to Figure 6.

Setting RSENSE

Each phase current is sampled about 200nS after the SW
node crosses 0V. For proper converter operation, choose an
RSENSE value of:

which is about 1K for the components in Figure 1.

Active Droop

The core converter incorporates a proprietary output voltage
droop method for optimum handling of fast load transients
found in modern processors.

"Active droop" or voltage positioning is now widely used in
the computer power applications. The technique is based on
raising the converter voltage at light load in anticipation of a
step increase in load current, and conversely, lowering
VCORE in anticipation of a step decrease in load current.

With Active Droop, the output voltage varies with the load as
if a resistor were connected in series with the converter's out-
put, in other words, it's effect is to raise the output resistance
of the converter.

Figure 5. Active Droop

To get the most from the Active Droop, its magnitude should
be scaled to match the output capacitor's ESR voltage drop.

Active Droop allows the size and cost of the output capaci-
tors required to handle CPU current transients to be reduced.
The reduction may be almost a factor of 2 when compared to
a system without Active Droop.

SENSE

DS ON

(

)

MAX

µA

-----------------------------------------

1.2

CORE

LOAD

DROOP

MAX

DROOP

MAX

ESR

(6)

Figure 6. Current Limit and Active Droop Circuits

LDRV1

PGND1

ISNS1

in +

in D

2.5V

ILIM det. 1

SENSE

V to I

ISNS1

I2 =

ILIM

0.9V

ILIM

ILIM mirror

S/H

ISNS2

To A1 (+)

ISNS1

ISNS2

A B

A-B

B-A

ISNS2-ISNS1

ISNS1-ISNS2

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

Additionally, the CPU power dissipation is also slightly
reduced as it is proportional to the applied voltage squared
and even slight voltage decrease translates to a measurable
reduction in power dissipated.

Figure 7. Effect of Active Droop on ESR

The processor regulation window including transients is
specified as +100mV..50mV. To accommodate the droop,
the output voltage of the converter is raised by about 30mV
at no load.

The converter response to the load step is shown in Figure 8.
At zero load current, the output voltage is raised ~30mV
above nominal value of 1.5V. When the load current
increases, the output voltage droops down approximately
55mV. Due to use of Active Droop, the converter's output
voltage adaptively changes with the load current allowing
better utilization of the regulation window.

Figure 8. Converter response to 5A load step

The current through each R

SENSE

resistor (ISNS) is sam-

pled shortly after LDRV is turned on. That current is held for
the remainder of the cycle, and then injected to produce an
offset to VCORE+ through the external 1K resistor (R6 in
Figure 1). This creates a voltage at the input to the error
amplifier that rises with increasing current, causing the regu-
lator's output to droop as the current increases.

Gate Driver section

The gate control logic translates the internal PWM control
signal into the MOSFET gate drive signals providing
necessary amplification, level shifting and shoot-through
protection. Also, it has functions that help optimize the IC
performance over a wide range of operating conditions.
Since MOSFET switching time can vary dramatically from
type to type and with the input voltage, the gate control logic
provides adaptive dead time by monitoring the gate-to-
source voltages of both upper and lower MOSFETs. The
lower MOSFET drive is not turned on until the gate-to-
source voltage of the upper MOSFET has decreased to less
than approximately 1 volt. Similarly, the upper MOSFET is
not turned on until the gate-to-source voltage of the lower
MOSFET has decreased to less than approximately 1 volt.
This allows a wide variety of upper and lower MOSFETs to
be used without a concern for simultaneous conduction, or
shoot-through.

There must be a low resistance, low inductance path
between the driver pin and the MOSFET gate for the adap-
tive dead-time circuit to work properly. Any delay along that
path will subtract from the delay generated by the adaptive
dead-time circit and a shoot-through condition may occur.

Frequency Loop Compensation

Due to the implemented current mode control, the modulator
has a single pole response with -1 slope at frequency deter-
mined by load

where R

is load resistance, C

is load capacitance. For this

type of modulator Type 2 compensation circuit is usually
sufficient. To reduce the number of external components and
simplify the design task, the PWM controller has an inter-
nally compensated error amplifier. Figure 9 shows a Type 2
amplifier and its response along with the responses of a cur-
rent mode modulator and of the converter. The Type 2 ampli-
fier, in addition to the pole at the origin, has a zero-pole pair
that causes a flat gain region at frequencies between the zero
and the pole.

ILOAD

Vout

(no droop)

Vout

droop

ESR

upper lim

lower lim

upper lim

DROOP

LOAD

DS ON

(

)

SENSE

--------------------------------------------

(7)

------------------------

(8)

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Figure 9. Compensation

This region is also associated with phase `bump' or reduced
phase shift. The amount of phase shift reduction depends on
how wide the region of flat gain is and has a maximum value
of 90 degrees. To further simplify the converter compensa-
tion, the modulator gain is kept independent of the input
voltage variation by providing feed-forward of VIN to the
oscillator ramp.

The zero frequency, the amplifier high frequency gain and
the modulator gain are chosen to satisfy most typical appli-
cations. The crossover frequency will appear at the point
where the modulator attenuation equals the amplifier high
frequency gain. The only task that the system designer has to
complete is to specify the output filter capacitors to position
the load main pole somewhere within one decade lower than
the amplifier zero frequency. With this type of compensation
plenty of phase margin is easily achieved due to zero-pole
pair phase `boost'.

Conditional stability may occur only when the main load
pole is positioned too much to the left side on the frequency
axis due to excessive output filter capacitance. In this case,
the ESR zero placed within the 10kHz...50kHz range gives
some additional phase `boost'. Fortunately, there is an oppo-
site trend in mobile applications to keep the output capacitor
as small as possible.

Protection

The converter output is monitored and protected against
short circuit (over-current), and over-voltage conditions.

Over-Current sensing

(see Figure 10)

When the circuit's current limit signal ("ILIM det" as shown
in Figure 6) goes high, a pulse-skipping circuit is activated
and a 16-clock cycle counter is started. HDRV will be inhib-
ited as long as the sensed current is higher than the ILIM
value. This limits the current supplied by the DC input.

Figure 10. Over-current shut-down delay logic

If ILIM det goes high during counts 9-16 of the counter, the
overcurrent delay timer is started and the 16-clock counter
starts again. This timer delays the shut-down of the chip and
its time is a function of the value of C

DELAY

Over-current must detected at least once during the first 8
clock cycles and once during the 2

8 clock cycles of the

16-cycle counter for the timer to continue timing. If the over-
current condition does not occur at least once per 8 clock
counts during any clock counter cycle while the timer is
high, the timer and the over-current detection circuit are
reset, preventing shutdown. The clock counter coutinues to
count and look for ILIM det pulses in this manner until
either:

the IC is shut-down because the timer timed out:
If the timer pulse is allowed to finish by timing out, the
IC is shut-down and can only be restarted by removing
power or toggling the EN pin.

ILIM det does not go high at least once per 8 clock
counts. In this case, the timer and over-current shutdown
logic are reset, and a chip shut-down is averted.

PGOOD will go LOW if the IC shuts down from over-
current.

Setting the Current Limit

ISNS is compared to the current established when a 0.9 V
internal reference drives the ILIM pin. The threshold is
determined at the point when the

. Since

therefore,

REF

Converter

EAOut

Error Amp

Modulator

----------------------

6 kHz

(9a)

----------------------

600 kHz

(9b)

Clock

Shut-down

RESET

16 Clock

Counter and

Logic

START

DELAY

Q TIMER

ILIM det. 1
ILIM det. 2

HOLDOFF

in mS

(

)

DELAY

in nF

(

)

---------------------------------------

(10)

ISNS

---------------

0.9V

ILIM

--------------

ISNS

LOAD

DS ON

(

)

SENSE

--------------------------------------------

ILIM

0.9V

LIMIT

---------------

SENSE

(

)

DS ON

(

)

-----------------------------------

(11)

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

Since the tolerance on the current limit is largely dependent
on the ratio of the external resistors it is fairly accurate if the
voltage drop on the Switching Node side of R

SENSE

is an

accurate representation of the load current. When using the
MOSFET as the sensing element, the variation of R

DS(ON)

causes proportional variation in the ISNS. This value not
only varies from device to device, but also has a typical
junction temperature coefficient of about 0.4% / °C (consult
the MOSFET datasheet for actual values), so the actual
current limit set point will decrease propotional to increasing
MOSFET die temperature. The same discussion applies to
the V

DROOP

calculation.

Figure 11. Improving current sensing accuracy

More accurate sensing can be achieved by using a resistor
(R1) instead of the R

DS(ON)

of the FET as shown in Figure

11. This approach causes higher losses, but yields greater
accuracy in both V

DROOP

and I

LIMIT

. R1 is a low value

(e.g. 10m

) resistor.

The current limit (I

LIMIT

) set point chosen needs to accom-

modate ripple current, slew current, and variability in the
MOSFET's R

DS(ON)

Slew current (

) is the current required for the

output voltage to slew upwards during VID code changes,
since the circuit will limit the regulator's output current by

pulse skipping when I

LIMIT

is reached. The

term we

used earlier in the discussion (set up by the C

) was

500mV/100

µS or 5V/mS. Assuming C

OUT

of 4000

µF, the

current required to slew C

OUT

at this rate is:

which is contributed roughly equally from each phase,
therefore, 1/2 of the slew current comes from a single phase.

The over-current comparator is sampled just after LDRV is
turned on, when the current is near its peak in the cycle.
Assuming 20% inductor ripple current, we can then add 1/2
of the ripple current, or 10%. An additional factor of 1.2
accounts for the inaccuracy in the initial (room temperature)
R

DS(ON)

of the MOSFETs with an additional factor of 1.4 to

accommodate the rise of the MOSFET R

DS(ON)

when oper-

ating with T

@ 125°C. With a maximum load current of

12.5A/phase, the target for I

LIMIT

(per phase) would be:

so using equation 11, with R

DS(ON)

= 3m

for the 2 parallel

FDS6688 MOSFETs, R

ILIM

56K:

Over-Voltage Protection

Should the output voltage exceed 2.35V due to an upper
MOSFET failure, or for other reasons, the overvoltage
protection comparator will force the LDRV high. This action
actively pulls down the output voltage and, in the event of
the upper MOSFET failure, will eventually blow the battery
fuse. As soon as the output voltage drops below the thresh-
old, the OVP comparator is disengaged.

This OVP scheme provides a `soft' crowbar function which
helps to tackle severe load transients and does not invert the
output voltage when activated -- a common problem for
OVP schemes with a latch.

Over-Temperature Protection

The chip incorporates an over temperature protection circuit
that shuts the chip down when a die temperature of 150°C is
reached. Normal operation is restored at die temperature
below 125°C with internal Power On Reset asserted, result-
ing in a full soft-start cycle.

LDRV

PGND

ISNS

SENSE

LIMIT

LOAD

OUT

-------

(12a)

OUT

-------

OUT

-------

4mF

5V/mS

20A

(12b)

LIMIT

1.1

1.2

1.4

12.5A

20A

-----------

42A

(12c)

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Design and Component Selection
Guidelines

As an initial step, define operating voltage range and mini-
mum and maximum load currents for the controller. For this
discussion,

Output Inductor Selection

The minimum practical output inductor value is the one that
keeps inductor current just on the boundary of continuous
conduction at some minimum load. The industry standard
practice is to choose the ripple current to be somewhere from
15% to 35% of the nominal current. At light load, the ripple
current also determines the point where the converter will
automatically switch to hysteretic mode of operation (I

MIN

)

to sustain high efficiency. The following equations help to
choose the proper value of the output filter inductor.

where

I is the inductor ripple current, which we will choose

for 20% of the full load current (12.5A in each phase) and
V

OUT

is the maximum output ripple voltage allowed.

for this example we'll use:

= 20V, V

OUT

= 1.5V

I = 20% *12.5A (per phase) = 2.5A
F

= 300KHz.

Therefore,
L

1.8µH

The inductor's current rating should be chosen per the
I

LIMIT

calculated above. Some transient currents over the

inductor current rating may be tolerable if the inductor's

saturation characteristic

is sufficiently "soft".

Output Capacitor Selection

The output capacitor serves two major functions in a switch-
ing power supply. Along with the inductor it filters the
sequence of pulses produced by the switcher, and it supplies
the load transient currents. The filtering requirements are a
function of the switching frequency and the ripple current
allowed, and are usually easy to satisfy in high frequency
converters.

The load transient requirements are a function of the slew
rate (di/dt) and the magnitude of the transient load current.
Modern microprocessors produce transient load rates in
excess of 10A/

µs. High frequency ceramic capacitors placed

beneath the processor socket initially supply the transient
and reduce the slew rate seen by the bulk capacitors. The
bulk capacitor values are generally determined by the total
allowable ESR rather than actual capacitance requirements.

High frequency decoupling capacitors should be placed as
close to the processor power pins as physically possible.
Consult with the processor manufacturer for specific
decoupling requirements. Use only specialized low-ESR
electrolytic capacitors intended for switching-regulator
applications for the bulk capacitors. The bulk capacitor's
ESR will determine the output ripple voltage and the initial
voltage drop after a transient. In most cases, multiple electro-
lytic capacitors of small case size perform better than a
single large case capacitor.

Input Capacitor Selection

The input capacitor should be selected by its ripple current
rating. For a 2 phase converter, the RMS currents is calcu-
lated:

This equation produces the worst case value at maximum
duty cycle. For our example, that occurs when VIN = 5.5V
and VOUT = 2V. For 25A maximum output the maximum
RMS current at C

Power MOSFET Selection

For the example in the following discussion, we will be
selecting components for:

VIN from 5V to 20V
V

OUT

= 1.5V @ I

LOAD(MAX)

= 12.5A/phase

The FAN5240 converter's output voltage is very low with
respect to the input voltage, therefore the Lower MOSFET
(Q2) is conducting the full load current for most of the cycle.
Therefore, Q2 should be selected to be a MOSFET with low
R

DS(ON)

to minimize conduction losses.

In contrast, Q1 is on for a maximum of 20% (when VIN =
5V) of the cycle, and its conduction loss will have less of an
impact. Q1, however, sees most of the switching losses, so
Q1's primary selection criteria should be gate charge
(Q

G(SW)

IOUT Max

25A

VIN

5.5 to 21 V

VOUT

0.925 to 2 V

MIN

OUT

ESR

------------------

OUT

------------------------------

OUT

--------------

(13)

-------

RMS

-------- 2D

(14)

RMS MAX

(

)

5.6A

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

High-Side Losses:

Figure 12. Switching losses and Q

Figure 13. Drive Equivalent Circuit

Assuming switching losses are about the same for both the
rising edge and falling edge, Q1's switching losses, as can be
seen by Figure 12, are given by:

where R

DS(ON)

is @T

J(MAX)

and:

is the switching period (rise or fall time) and is predomi-

nantly the sum of t2, t3 (Figure 12), a function of the imped-
ance of the driver and the Q

G(SW)

of the MOSFET. Since

most of t

occurs when V

= V

we can use a constant

current assumption for the driver to simplify the calculation
of t

For the high-side MOSFET, V

= VIN, which can be as

high as 20V in a typical portable application. Q2, however,
switches on or off with its parallel shottky diode conducting,
therefore V

0.5V. Since P

is proportional to V

Q2's switching losses are negligible and we can select Q2
based on R

DS(ON)

only.

Care should also be taken to include the delivery of the
MOSFET's gate power ( P

GATE

) in calculating the power

dissipation required for the FAN5240:

Low-Side Losses

Conduction losses for Q2 are given by:

where R

DS(ON)

is the R

DS(ON)

of the MOSFET at the

highest operating junction temperature and

the minimum duty cycle for the converter. Since D

MIN

is 5%

for portable computers, (1-D)

1, further simplifying the

calculation.

The maximum power dissipation (P

D(MAX)

) is a function of

the maximum allowable die temperature of the low-side
MOSFET, the

J-A

, and the maximum allowable ambient

temperature rise:

J-A

, depends primarily on the amount of PCB area that can

be devoted to heat sinking (see FSC app note AN-1029 for
SO-8 MOSFET thermal information).

4.5V

G(SW)

ISS

RSS

ISS

ISS

= C

|| C

GATE

HDRV

VIN

UPPER

COND

(15a)

---------------------

(15b)

COND

OUT

--------------

OUT

DS ON

(

)

(15c)

G SW

(

)

DRIVER

---------------------

G SW

(

)

VDD

DRIVER

GATE

------------------------------------------------

------------------------------------------------------

(16)

GATE

VDD

(17)

COND

(

) I

OUT

DS ON

(

)

(18)

OUT

--------------

D MAX

(

)

J MAX

(

)

A MAX

(

)

-------------------------------------------------

FAN5240

PRODUCT SPECIFICATION

REV. 1.1.7 8/29/02

Layout Considerations

Switching converters, even during normal operation,
produce short pulses of current which could cause substan-
tial ringing and be a source of EMI if layout constrains are
not observed.

There are two sets of critical components in a DC-DC
converter. The switching power components process large
amounts of energy at high rate and are noise generators.
The low power components responsible for bias and feed-
back functions are sensitive to noise.

A multi-layer printed circuit board is recommended.
Dedicate one solid layer for a ground plane. Dedicate
another solid layer as a power plane and break this plane
into smaller islands of common voltage levels.

Notice all the nodes that are subjected to high dV/dt voltage
swing such as SW, HDRV and LDRV, for example. All sur-
rounding circuitry will tend to couple the signals from these
nodes through stray capacitance. Do not oversize copper
traces connected to these nodes. Do not place traces con-
nected to the feedback components adjacent to these traces.
It is not recommended to use High Density Interconnect
Systems, or micro-vias on these signals. The use of blind or
buried vias should be limited to the low current signals only.
The use of normal thermal vias is left to the discretion of the
designer.

Keep the wiring traces from the IC to the MOSFET gate and
source as short as possible and capable of handling peak cur-
rents of 2A. Minimize the area within the gate-source path to
reduce stray inductance and eliminate parasitic ringing at the
gate.

Locate small critical components like the soft-start capacitor
and current sense resistors as close as possible to the respec-
tive pins of the IC.

The FAN5240 utilizes advanced packaging technology that
will have lead pitch of 0.6mm. High performance analog
semiconductors utilizing narrow lead spacing may require
special considerations in PWB design and manufacturing. It
is critical to maintain proper cleanliness of the area sur-
rounding these devices. It is not recommended to use any
type of rosin or acid core solder, or the use of flux in either
the manufacturing or touch up process as these may contrib-
ute to corrosion or enable electromigration and/or eddy cur-
rents near the sensitive low current signals. When chemicals
such as these are used on or near the PWB, it is suggested
that the entire PWB be cleaned and dried completely before
applying power.

PRODUCT SPECIFICATION

FAN5240

REV. 1.1.7 8/29/02

Mechanical Dimensions

28-Pin QSOP

0.069

1.75

Symbol

Inches

Min.

Max.

Min.

Max.

Millimeters

Notes

0.004

0.10

0.061

1.54

0.053

1.35

0.010

0.25

0.008

0.012

0.20

0.30

0.386

0.394

9.81

10.00

0.150

0.157

3.81

3.98

0.016

0.050

0.41

1.27

0.025 BSC

0.635 BSC

0.228

0.244

0.0099

0.0196

5.80

6.19

0.26

0.49

0.007

0.010

0.18

0.25

Notes:

10.

Symbols are defined in the "MO Series Symbol List" in
Section 2.2 of Publication Number 95.

Dimensioning and tolerancing per ANSI Y14.5M-1982.

Dimension "D" does not include mold flash, protrusions
or gate burrs. Mold flash, protrusions shall not exceed
0.25mm (0.010 inch) per side.

Dimension "E" does not include interlead flash or
protrusions. Interlead flash and protrusions shall not
exceed 0.25mm (0.010 inch) per side.

The chamber on the body is optional. If it is not present,
a visual index feature must be located within the
crosshatched area.

"L" is the length of terminal for soldering to a substrate.

"N" is the maximum number of terminals.

Terminal numbers are shown for reference only.

Dimension "B" does not include dambar protrusion.
Allowable dambar protrusion shall be 0.10mm (0.004
inch) total in excess of "B" dimension at maximum
material condition.

Controlling dimension: INCHES. Converted millimeter
dimensions are not necessarily exact.

ccc C

LEAD COPLANARITY

SEATING
PLANE

FAN5240

PRODUCT SPECIFICATION

8/29/02 0.0m 003

Stock#DS30005240

2002 Fairchild Semiconductor Corporation

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FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
CORPORATION. As used herein:

1. Life support devices or systems are devices or systems

which, (a) are intended for surgical implant into the body,
or (b) support or sustain life, and (c) whose failure to
perform when properly used in accordance with
instructions for use provided in the labeling, can be
reasonably expected to result in a significant injury of the
user.

2. A critical component in any component of a life support

device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.

www.fairchildsemi.com

DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO
ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME
ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN;
NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.

Ordering Information

Part Number

Temperature Range

Package

Packing

FAN5240QSC

-10°C to 85°C

QSOP-28 Rails

FAN5240QSCX

-10°C to 85°C

QSOP-28

Tape and Reel

FAN5240MTC

-10°C to 85°C

TSSOP-28

Rails

FAN5240MTCX

-10°C to 85°C

TSSOP-28

Tape and Reel

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