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Pentium is a registered trademark of Intel Corporation. Athlon is a registered trademark of AMD. Programmable Active Droop is a trademark of Fairchild Semiconductor.

REV. 1.0.0 5/10/01

Block Diagram

2 3

VID0

OSC

Digital

Control

Power

Good

5-Bit
DAC

VID1

VID2

VID3

VID4

+5V

PWRGD

ENABLE/SS

GNDA

DROOP/E*

+12V

5V Reg

Bypass

+5V

+12V

Digital

Control

-
+

ILIM

FAN5091

Two Slice Interleaved Synchronous Buck Converter

Features

· Programmable output from 1.10V to 1.85V in 25mV steps

using an integrated 5-bit DAC

· Two interleaved synchronous slices for maximum

performance

· 100nsec response time
· Built-in current sharing between slices
· Remote sense
· Programmable Active Droop

(Voltage Positioning)

· Programmable frequency from 200KHz to 2MHz
· Adaptive delay gate switching
· Integrated high-current gate drivers
· Integrated Power Good, OV, UV, Enable/Soft Start

functions

· Drives N-channel MOSFETs
· Operation optimized for 5V operation
· High efficiency mode (E*) at light load
· Overcurrent protection using MOSFET sensing
· 24 pin TSSOP package

Applications

· Power supply for Pentium

· Power supply for Athlon

· VRM for Pentium IV processor
· Programmable step-down power supply

Description

The FAN5091 is a synchronous multi-slice DC-DC controller
IC which provides a highly accurate, programmable output
voltage for all high-performance processors. Two interleaved
synchronous buck regulator slices with built-in current shar-
ing operate 180° out of phase to provide the fast transient
response needed to satisfy high current applications while
minimizing external components. The FAN5091 features
remote voltage sensing and Programmable Active Droop

for 100nsec converter transient response with minimum out-
put capacitance. It has integrated high-current gate drivers,
with adaptive delay gate switching, eliminating the need for
external drive devices. The FAN5091 uses a 5-bit D/A con-
verter to program the output voltage from 1.10V to 1.85V in
25mV steps with an accuracy of 1%. The FAN5091 uses a
high level of integration to deliver load currents in excess of
50A from a 5V source with minimal external circuitry. The
FAN5091 also offers integrated functions including Power
Good, Output Enable/Soft Start, under- voltage lockout,
over-voltage protection, and adjustable current limiting with
independent current sense on each slice. It is available in a
24 pin TSSOP package.

FAN5091

PRODUCT SPECIFICATION

REV. 1.0.0 5/10/01

Pin Assignments

Pin Definitions

Pin Number

Pin Name

Pin Function Description

1-5

VID0-4

Voltage Identification Code Inputs.

These open collector/TTL compatible

inputs will program the output voltage over the ranges specified in Table 1.
Pull-ups are internal to the controller.

BYPASS

5V Rail.

Bypass this pin with a 1

F ceramic capacitor to AGND.

AGND

Analog Ground.

Return path for low power analog circuitry. This pin should be

connected to a low impedance system ground plane to minimize ground loops.

LDRVB

Low Side FET Driver for B.

Connect this pin to the gate of an N-channel

MOSFET for synchronous operation. The trace from this pin to the MOSFET gate
should be <0.5

PGNDB

Power Ground B.

Return pin for high currents flowing in low-side MOSFET.

Connect directly to low-side MOSFET source.

SWB

High side driver source and low side driver drain switching node B.

Gate

drive return for high side MOSFET, and negative input for low-side MOSFET
current sense.

HDRVB

High Side FET Driver B.

Connect this pin to the gate of an N-channel MOSFET.

The trace from this pin to the MOSFET gate should be <0.5

BOOTB

Bootstrap B.

Input supply for high-side MOSFET.

BOOTA

Bootstrap A.

Input supply for high-side MOSFET.

HDRVA

High Side FET Driver A.

Connect this pin to the gate of an N-channel MOSFET.

The trace from this pin to the MOSFET gate should be <0.5

SWA

High side driver source and low side driver drain switching node A.

Gate

drive return for high side MOSFET, and negative input for low-side MOSFET
current sense.

PGNDA

Power Ground A.

Return pin for high currents flowing in low-side MOSFET.

Connect directly to low-side MOSFET source.

LDRVA

Low Side FET Driver for A.

Connect this pin to the gate of an N-channel

MOSFET for synchronous operation. The trace from this pin to the MOSFET gate
should be <0.5

VCC

VCC.

Internal IC supply. Connect to system 12V supply, and decouple with a

0.1

F ceramic capacitor.

PWRGD

Power Good Flag.

An open collector output that will be logic LOW if the output

voltage is not within +10/-15% of the nominal output voltage setpoint.

FAN5091

VID0
VID1
VID2
VID3

VID4

AGND

BYPASS

LDRVB

PGNDB

SWB

HDRVB

BOOTB

VFB
RT
ENABLE/SS

DROOP/E*

ILIM

PWRGD
VCC

LDRVA
PGNDA

SWA

HDRVA

BOOTA

2
3

4
5

6
7
8
9
10
11
12

23
22

21
20
19
18

17
16
15
14
13

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

Absolute Maximum Ratings

Recommended Operating Conditions

ILIM

Current Limit.

A resistor from this pin to ground sets the over current trip level.

DROOP/E*

Droop Control/Energy Star Mode Control.

A resistor from this pin to ground

sets the amount of droop by controlling the gain of the current sense amplifier.
When this pin is pulled high to BYPASS, the slice A drivers are turned off for
Energy-star operation.

ENABLE/SS

Output Enable/Softstart.

A logic LOW on this pin will disable the output. An

internal current source allows for open collector control. This pin also doubles as
soft start.

Frequency Set.

A resistor from this pin to ground sets the switching frequency.

VFB

Voltage Feedback.

Connect to the desired regulation point at the output of the

converter.

Parameter

Min.

Typ.

Max.

Unit

Supply Voltage VCC

Supply Voltages BOOTA, BOOTB

Voltage Identification Code Inputs, VID0-VID4

VFB, ENABLE/SS, PWRGD, DROOP/E*

SWA, SWB

-3

PGNDA, PGNDB to AGND

-0.5

0.5

Gate Drive Current, peak pulse

Junction Temperature, T

-55

150

Storage Temperature

-65

150

Lead Soldering Temperature, 10 seconds

300

Power Dissipation, P

950

Thermal Resistance Junction-to-Case,

C/W

Parameter

Conditions

Min.

Typ.

Max.

Units

Output Driver Supply, BOOT

10.8

13.2

Input Logic HIGH

2.4

Input Logic LOW

0.8

Ambient Operating Temperature

Pin Number

Pin Name

Pin Function Description

FAN5091

PRODUCT SPECIFICATION

REV. 1.0.0 5/10/01

Electrical Specifications

= 12V,V

OUT

= 1.500V, and T

= +25°C using circuit in Figure 1, unless otherwise noted.)

The

denotes specifications which apply over the full operating temperature range.

Parameter

Conditions

Min.

Typ.

Max.

Units

Output Voltage

See Table 1

1.100

1.850

Output Current

Initial Voltage Setpoint

LOAD

= 5A

1.485

1.500

1.515

Output Temperature Drift

= 0 to 70°C

Line Regulation

= 11.4V to 12.6V

+130

µV

Droop

LOAD

= 0.8A to I

max

-90

-100

-110

Programmable Droop Range

-10

%Vout

Total Output Variation,
Steady State

LOAD

= 0.8A to I

max

1.430

1.570

Total Output Variation, Transient

LOAD

= 0.8A to I

max

1.430

1.570

Response Time

out

= 10mV

100

nsec

Gate Drive On-Resistance

1.0

Upper Drive Low Voltage

HDRV

at I

sink

= 10µA

0.2

Upper Drive High Voltage

BOOT

HDRV

at I

source

= 10µA

0.5

Lower Drive Low Voltage

sink

= 10µA

0.2

Lower Drive High Voltage

LDRV

at I

source

= 10µA

0.5

Output Driver Rise & Fall Time

See Figure 3

nsec

Current Mismatch

DS,on

(A) = R

DS,on

(B), I

LOAD

= I

max

Output Overvoltage Detect

2.1

2.3

Efficiency

LOAD

= I

max

LOAD

= 2A (E*-mode)

85
70

Oscillator Frequency

RT = 41.2K

450

600

750

KHz

Oscillator Range

RT = 125K

to 12.5 K

200

2000

KHz

Maximum Duty Cycle

RT = 125K

Minimum LDRV on-time

RT = 12.5K

330

nsec

Input LOW current, VID pins

VID

= 0.4V

µA

Soft Start Current

µA

Enable Threshold

ON
OFF

0.4

1.0

BYPASS Voltage

4.75

5.25

BYPASS Capacitor

100

PWRGD Threshold

Logic LOW, minimum
Logic LOW, maximum

106

110

114

out

PWRGD Hysteresis

PWRGD Output Voltage

sink

= 4mA

0.4

PWRGD Delay

High

Low

500

µsec

12V UVLO

8.5

9.5

10.5

UVLO Hysteresis

0.5

12V Supply Current

HDRV and LDRV Open

Over Temperature Shutdown

150

°C

Over Temperature Hysteresis

°C

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

Notes:

1. Steady State Voltage Regulation includes Initial Voltage Setpoint, Output Ripple and Output Temperature Drift and is

measured at the converter's VFB sense point.

2. As measured at the converter's VFB sense point. For motherboard applications, the PCB layout should exhibit no more than

0.5m

trace resistance between the converter's output capacitors and the CPU. Remote sensing should be used for optimal

performance.

3. Using the VFB pin for remote sensing of the converter's output at the load, the converter will be in compliance with Intel's

VRM 9.0 specification of +70, -70mV.

Table 1. Output Voltage Programming Codes

Note:

1. 0 = VID pin is tied to GND.
1 = VID pin is open.

VID4

VID3

VID2

VID1

VID0

OUT

to CPU

OFF

1.100V

1.125V

1.150V

1.175V

1.200V

1.225V

1.250V

1.275V

1.300V

1.325V

1.350V

1.375V

1.400V

1.425V

1.450V

1.475V

1.500V

1.525V

1.550V

1.575V

1.600V

1.625V

1.650V

1.675V

1.700V

1.725V

1.750V

1.775V

1.800V

1.825V

1.850V

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

Table 2. FAN5091 Application Bill of Materials for Figure 1

Notes:
1. Inductor L1 is recommended to isolate the 5V input supply from noise generated by the MOSFET switching, and to comply

with Intel dI/dt requirements. L1 may be omitted if desired.

2. For designs using the TO-220 MOSFETs, heatsinks with thermal resistance

< 20°C/W should be used. For details and a

spreadsheet on MOSFET selections, refer to Applications Bulletin AB-8.

*Output capacitance requirements depend critically on layout and processor type. Consult Application Bulletin AB-14 for details.
See the Appendix to this datasheet for the method of calculation of these components. Pin 5 must be used to remote sense the
voltage at the processor to achieve the specified performance.

Reference

Manufacturer Part #

Quantity

Description

Requirements/Comments

C1-3

Panasonic
ECU-V1H104ZFX

100nF, 50V Capacitor

Rubycon
16ZL1000M

1000µF, 16V Electrolytic

RMS

= 3.8A @ 65

°C

OUT

Rubycon
6.3ZL1500M

1500µF, 6.3V Electrolytic

ESR

23m

D1-2

Motorola
MBRD835L

8A, 35V Schottky Diode

Any

Optional

1.3µH, 14A Inductor

DCR ~ 4m

See Note 1.

L2-3

Any

500nH, 20A Inductor

DCR ~ 1.5m

Q1, Q3

Fairchild
FDB7030B

N-Channel MOSFET

DS(ON)

= 9m

@ V

= 4.5V

See Note 2.

Q2, Q4

Fairchild
FDB7045L

N-Channel MOSFET

DS(ON)

= 4.5m

@ V

= 4.5V

See Note 2.

Any

10K

Any

62K

Any

2.0K

Any

24.9K

Any

R6-7, R9-10

Any

4.7

Fairchild
FAN5091M

DC/DC Controller

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

Table 3. FAN5091 Application Bill of Materials for Figure 2

Notes:
1. Inductor L1 is recommended to isolate the 5V input supply from noise generated by the MOSFET switching. L1 may be

omitted if desired.

2. For a spreadsheet on MOSFET selections, refer to Applications Bulletin AB-8.

Reference

Manufacturer Part #

Quantity

Description

Requirements/Comments

C1-3

Panasonic
ECU-V1H104ZFX

100nF, 50V Capacitor

Rubycon
16ZL1000M

1000µF, 16V Electrolytic

RMS

= 3.8A @ 65

°C

OUT

Rubycon
6.3ZL1500M

1500µF, 6.3V Electrolytic

ESR

23m

D1-2 Motorola

MBRB154SCT

15A, 45V Schottky Diode

Any

Optional

1.3µH, 25A Inductor

DCR ~ 1m

See Note 1.

L2-3

Coiltronics
HC2-1R0

1µH, 33A Inductor

DCR ~ 600µ

Q1, Q4

Fairchild
FDB7030BL

N-Channel MOSFET

DS(ON)

= 9m

. See Note 2.

Q2-3, Q5-6

Fairchild
FDB7045L

N-Channel MOSFET

DS(ON)

= 4.5m

.See Note 2.

Any

10K

Any

150K

Any

2.0K

Any

61.9K

Any

R6-11

Any

4.7

Fairchild
FAN5091M

DC/DC Controller

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

Table 4. FAN5091 Application Bill of Materials for Figure 3

Notes:
1. Inductor L1 is recommended to isolate the 5V input supply from noise generated by the MOSFET switching. L1 may be

omitted if desired.

2. For a spreadsheet on MOSFET selections, refer to Applications Bulletin AB-8.

Test Parameters

Figure 4. Output Drive Timing Diagram

Reference

Manufacturer Part #

Quantity

Description

Requirements/Comments

C1-3

Panasonic
ECU-V1H104ZFX

100nF, 50V Capacitor

Rubycon
16ZL1000M

1000µF, 16V Electrolytic

RMS

= 3.8A @ 65

°C

OUT

Rubycon
6.3ZL1500M

1500µF, 6.3V Electrolytic

ESR

23m

D1-2

Motorola
MBRD835L

16A, 35V Schottky Diode

Any

Optional

1.3µH, 14A Inductor

DCR ~ 4m

See Note 1.

L2-3

Any

250nH, 20A Inductor

DCR ~ 1.5m

Q1-4

Fairchild
FDB6690A

N-Channel MOSFET

DS(ON)

= 16m

, Q

= 17nC.

See Note 2.

Any

10K

Any

43.2K

Any

82.5K

Any

11K

Any

R6-7, R9-10 Any

4.7

Fairchild
FAN5091M

DC/DC Controller

tDT

HIDRV

LODRV

10%

90%

10%

FAN5091

PRODUCT SPECIFICATION

REV. 1.0.0 5/10/01

Application Information

Operation

The FAN5091 Controller

The FAN5091 is a programmable synchronous multi-slice
DC-DC controller IC. When designed around the appropriate
external components, the FAN5091 can be configured to
deliver more than 50A of output current, as appropriate for
the new generation of high-current processors. The
FAN5091 functions as a fixed frequency PWM step down
regulator, with a high efficiency mode (E*) at light load.

Main Control Loop

Refer to the FAN5091 Block Diagram on page 1. The
FAN5091 consists of two interleaved synchronous buck con-
verters, implemented with summing-mode control. Each
slice has its own current feedback, and there is a common
voltage feedback.

The two buck converters controlled by the FAN5091 are
interleaved, that is, they run 180

° out of phase with each

other. This minimizes the RMS input ripple current, mini-
mizing the number of input capacitors required. It also
doubles the effective switching frequency, improving
transient response.

The FAN5091 implements "summing mode control", which
is different from both classical voltage-mode and current-
mode control. It provides superior performance to either by
allowing a large converter bandwidth over a wide range of
output loads and external components.

The control loop of the regulator contains two main sections:
the analog control block and the digital control block. The
analog section consists of signal conditioning amplifiers
feeding into a comparator which provides the input to the
digital control block. The signal conditioning section accepts
inputs from a current sensor and a voltage sensor, with the
voltage sensor being common to both slices, and the current
sensor separate for each. The voltage sensor amplifies the
difference between the VFB signal and the reference voltage
from the DAC and presents the output to each of the two
comparators. The current control path for each slice takes the
difference between its PGND and SW pins when the low-
side MOSFET is on, reproducing the voltage across the
MOSFET and thus the input current; it presents the resulting
signal to the same input of its summing amplifier, adding its
signal to the voltage amplifier's with a certain gain. These
two signals are thus summed together. This sum is then pre-
sented to a comparator looking at the oscillator ramp, which
provides the main PWM control signal to the digital control
block. The oscillator ramps are 180

° out of phase with each

other, so that the two slices are on alternately.

The digital control block takes the analog comparator input
to provide the appropriate pulses to the HDRV and LDRV
output pins for each slice. These outputs control the external
power MOSFETs.

Remote Voltage Sense

The FAN5091 has true remote voltage sense capability, elim-
inating errors due to trace resistance. To utilize remote sense,
the VFB and AGND pins should be connected as a Kelvin
trace pair to the point of regulation, such as the processor
pins. The converter will maintain the voltage in regulation at
that point. Care is required in layout of these grounds; see
the layout guidelines in this datasheet.

High Current Output Drivers

The FAN5091 contains four high current output drivers that
utilize MOSFETs in a push-pull configuration. The drivers
for the high-side MOSFETs use the BOOT pin for input
power and the SW pin for return. The drivers for the low-side
MOSFETs use the VCC pin for input power and the PGND
pin for return. Typically, the BOOT pin will use 12V directly.
Note that the BOOT and VCC pins are separated from the
chip's internal power and ground, BYPASS and AGND, for
switching noise immunity.

Adaptive Delay Gate Drive

The FAN5091 embodies an advanced design that ensures
minimum MOSFET transition times while eliminating
shoot-through current. It senses the state of the MOSFETs
and adjusts the gate drive adaptively to ensure that they are
never on simultaneously. When the high-side MOSFET turns
off, the voltage on its source begins to fall. When the voltage
there reaches approximately 2.5V, the low-side MOSFETs
gate drive is applied with approximately 50nsec delay. When
the low-side MOSFET turns off, the voltage at the LDRV pin
is sensed. When it drops below approximately 2V, the high-
side MOSFET's gate drive is applied.

Maximum Duty Cycle

In order to ensure that the current-sensing and charge-
pumping work, the FAN5091 guarantees that the low-side
MOSFET will be on a certain portion of each period. For low
frequencies, this occurs as a maximum duty cycle of approxi-
mately 90%. Thus at 500KHz, with a period of 2µsec, the
low-side will be on at least 2µsec · 10% = 200nsec. At higher
frequencies, this time might fall so low as to be ineffective.
The FAN5091 guarantees a minimum low-side on-time of
approximately 330nsec, regardless of what duty cycle this
corresponds to.

Current Sensing

The FAN5091 has two independent current sensors, one for
each slice. Current sensing is accomplished by measuring
the source-to-drain voltage of the low-side MOSFET during

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

its on-time. Each slice has its own power ground pin, to per-
mit the slices to be placed in different locations without
affecting measurement accuracy. For best results, it is impor-
tant to connect the PGND and SW pins for each slice as a
Kelvin trace pair directly to the source and drain, respec-
tively, of the appropriate low-side MOSFET. Care is required
in the layout of these grounds; see the layout guidelines in
this datasheet.

Current Sharing

The two independent current sensors of the FAN5091 operate
with their independent current control loops to guarantee that
the two slices each deliver half of the total output current.
The only mismatch between the two slices occurs if there is a
mismatch between the R

DS,on

of the low-side MOSFETs.

Short Circuit Current Characteristics

The FAN5091 short circuit current characteristic includes a
function that protects the DC-DC converter from damage in
the event of a short circuit. The short circuit limit is set with
the R

resistor, as given by the formula

with I

the desired current limit, RT the oscillator resistor

and R

DS,on

one slice's low-side MOSFET's on resistance.

Remember to make the R

large enough to include the

effects of initial tolerance and temperature variation on the
MOSFETs' R

DS,on

. It is recommended to set I

substan-

tially above maximum operating current, to avoid nuisance
trips.

Important Note! The oscillator frequency must be selected
before selecting the current limit resistor, because the value
of RT is used in the calculation of R

When an overcurrent is detected, the high-side MOSFETs
are turned off, and the low-side MOSFETs are turned on,
and they remain in this state until the measured current
through the low-side MOSFET has returned to zero amps.
After reaching zero, the FAN5091 re-soft-starts, ensuring
that it can also safely turn on into a short.

A limitation on the current sense circuit is that I

· R

DS,on

must be less that 375mV. To ensure correct operation, use
I

· R

DS,on

300mV; between 300mV and 375mV, there

will be some non-linearity in the short-circuit current not
accounted for in the equation.

As an example, consider the typical characteristic of the
DC-DC converter circuit with two FDP6670AL low-side
MOSFETs (R

= 6.5m

maximum at 25°C · 1.2 at 75°C

= 7.8m

each, or 3.9m total) in each slice, RT = 42.1K

(600KHz oscillator) and a 50K

The converter exhibits a normal load regulation characteris-
tic until the voltage across the MOSFETs exceeds the inter-
nal short circuit threshold of 50K

/(3.9m · 41.2K · 6.66)

= 47A. [Note that this current limit level can be as high as
50K

/(3.5m · 41.2K · 6.66) = 52A, if the MOSFETs

have typical R

DS,on

rather than maximum, and are at 25

°C.]

At this point, the internal comparator trips and signals the
controller to leave on the low-side MOSFETs and keep off
the high-side MOSFETs. The inductor current decreases,
and power is not applied again until the inductor current
reaches 0A and the converter attempts to re-softstart.

Precision Current Sensing

The tolerances associated with the use of MOSFET current
sensing can be circumvented by the use of a current sense
resistor, as provided for by the FAN5092.

Light Load Efficiency

At light load, the FAN5091 uses a number of techniques to
improve efficiency. Because a synchronous buck converter is
two quadrant, able to both source and sink current, during
light load the inductor current will flow away from the out-
put and towards the input during a portion of the switching
cycle. This reverse current flow is detected by the FAN5091
as a positive voltage appearing on the low-side MOSFET
during its on-time. When reverse current flow is detected, the
low-side MOSFET is turned off for the rest of the cycle, and
the current instead flows through the body diode of the
high-side MOSFET, returning the power to the source. This
technique substantially enhances light load efficiency.

E*-mode

In addition, further enhancement in efficiency can be obtained
by putting the FAN5091 into E*-mode. When the Droop pin
is pulled to the 5V BYPASS voltage, the "A" slice of the
FAN5091 is completely turned off, reducing in half the
amount of gate charge power being consumed. E*-mode can
be implemented with the circuit shown in Figure 5:

Figure 5. Implementing E*-mode Control

Internal Voltage Reference

The reference included in the FAN5091 is a precision band-
gap voltage reference. Its internal resistors are precisely
trimmed to provide a near zero temperature coefficient (TC).
Based on the reference is the output from an integrated 5-bit
DAC. The DAC monitors the 5 voltage identification pins,
VID0-4, and scales the reference voltage from 1.100V to
1.850V in 25mV steps.

( )

DS on

6.66

BYPASS

10K

2N2222

2N2907

DROOP

FAN5091
pin 21

HI = E*-
mode on

FAN5091

PRODUCT SPECIFICATION

REV. 1.0.0 5/10/01

BYPASS Reference

The internal logic of the FAN5091 runs on 5V. To permit the
IC to run with 12V only, it produces 5V internally with a
linear regulator, whose output is present on the BYPASS pin.
This pin should be bypassed with a 1µF capacitor for noise
suppression. The BYPASS pin should not have any external
load attached to it.

Dynamic Voltage Adjustment

The FAN5091 has internal pullups on its VID lines. External
pullups should not be used. The FAN5091 can have its output
voltage dynamically adjusted to accommodate low power
modes. The designer must ensure that the transitions on the
VID lines all occur simultaneously (within less than

500nsec)

to avoid false codes generating undesired output voltages.
The Power Good flag tracks the VID codes, but has a
500µsec delay transitioning from high to low; this is long
enough to ensure that there will not be any glitches during
dynamic voltage adjustment.

Power Good (PWRGD)

The FAN5091 Power Good function is designed in accor-
dance with the Pentium IV DC-DC converter specifications
and provides a continuous voltage monitor on the VFB pin.
The circuit compares the VFB signal to the VREF voltage
and outputs an active-low interrupt signal to the CPU should
the power supply voltage deviate more than +14%/-9% of its
nominal setpoint. The output is guaranteed open-collector
high when the power supply voltage is within +6%/-11% of
its nominal setpoint. The Power Good flag provides no
control functions to the FAN5091.

Output Enable/Soft Start (ENABLE/SS)

The FAN5091 will accept an open collector/TTL signal for
controlling the output voltage. The low state disables the
output voltage. When disabled, the PWRGD output is in the
low state.

Even if an enable is not required in the circuit, this pin
should have attached a capacitor (typically 100nF) to soft-
start the switching. A softstart capacitor may be approxi-
mately chosen by the formula:

However, C must be

10nF.

Oscillator

The FAN5091 oscillator section runs at a frequency deter-
mined by a resistor from the RT pin to ground according to
the formula

The oscillator generates two internal sawtooth ramps, each at
one-half the oscillator frequency, and running 180

° out of

phase with each other. These ramps cause the turn-on time of
the two slices to be phased apart. The oscillator frequency of
the FAN5091 can be programmed from 200KHz to 2MHz
with each slice running at 100KHz to 1MHz, respectively.
Selection of a frequency will depend on various system
performance criteria, with higher frequency resulting in
smaller components but lower efficiency.

Programmable Active DroopTM

The FAN5091 features Programmable Active DroopTM: as
the output current increases, the output voltage drops propor-
tionately an amount that can be programmed with an exter-
nal resistor. This feature is offered in order to allow
maximum headroom for transient response of the converter.
The current is sensed losslessly by measuring the voltage
across the low-side MOSFET during its on time. Consult the
section on current sensing for details. Note that this method
makes the droop dependent on the temperature and initial
tolerance of the MOSFET, and the droop must be calculated
taking account of these tolerances. Given a maximum output
current, the amount of droop can be programmed with a
resistor to ground on the droop pin, according to the formula

with V

Droop

the desired droop voltage, RT the oscillator

resistor, I

max

the output current at which the droop is desired,

and R

DS, on

the on-state resistance of one slice's low-side

MOSFET.

Typical response time of the FAN5091 to an output voltage
change is 100nsec.

Important Note! The oscillator frequency must be selected
before selecting the droop resistor, because the value of RT
is used in the calculation of R

Droop

Higher Current Converters

Active droop makes it possible to parallel multiple
FAN5091s for even higher output current requirements.
Synchronized parallelization may be obtained with the
similar FAN5094. Please refer to Application Bulletin
AB-XX for details.

Over-Voltage Protection

The FAN5091 constantly monitors the output voltage for
protection against over-voltage conditions. If the voltage at
the VFB pin exceeds 2.2V, an over-voltage condition is
assumed and the FAN5091 latches on the external low-side
MOSFET and latches off the high-side MOSFET. The
DC-DC converter returns to normal operation only after V

has been recycled.

µA

out

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

( )

f Hz

(

)

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

Droop

( )

Droop

max

DS on

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

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

Thermal Design Considerations

Because of the very large gate capacitances that the
FAN5091 may be driving, the IC may dissipate substantial
power. It is important to provide a path for the IC's heat to be
removed, to avoid overheating. In practice, this means that
each of the pins should be connected to as large a trace as
possible. Use of the heavier weights of copper on the PCB is
also desirable. Since the MOSFETs also generate a lot of
heat, efforts should be made to thermally isolate them from
the IC.

Over Temperature Protection

If the FAN5091 die temperature exceeds approximately
150

°C, the IC shuts itself off. It remains off until the temper-

ature has dropped approximately 40

°C, at which time it

resumes normal operation.

Component Selection

MOSFET Selection

This application requires N-channel Enhancement Mode Field
Effect Transistors. Desired characteristics are as follows:

· Low Drain-Source On-Resistance,
· R

DS,ON

< 10m

(lower is better);

· Power package with low Thermal Resistance;
· Drain-Source voltage rating > 15V;
· Low gate charge, especially for higher frequency

operation.

For the low-side MOSFET, the on-resistance (R

DS,ON

) is the

primary parameter for selection. Because of the small duty
cycle of the high-side, the on-resistance determines the
power dissipation in the low-side MOSFET and therefore
significantly affects the efficiency of the DC-DC converter.
For high current applications, it may be necessary to use two
MOSFETs in parallel for the low-side for each slice.

For the high-side MOSFET, the gate charge is as important
as the on-resistance, especially with a 12V input and with
higher switching frequencies. This is because the speed of
the transition greatly affects the power dissipation. It may be
a good trade-off to select a MOSFET with a somewhat
higher R

DS,on

, if by so doing a much smaller gate charge is

available. For high current applications, it may be necessary
to use two MOSFETs in parallel for the high-side for each
slice.

At the FAN5091's highest operating frequencies, it may be
necessary to limit the total gate charge of both the high-side
and low-side MOSFETs together, to avert excess power dis-
sipation in the IC.

For details and a spreadsheet on MOSFET selection, refer to
Applications Bulletin AB-8.

Gate Resistors

Use of a gate resistor on every MOSFET is mandatory. The
gate resistor prevents high-frequency oscillations caused by
the trace inductance ringing with the MOSFET gate
capacitance. The gate resistors should be located physically
as close to the MOSFET gate as possible.

The gate resistor also limits the power dissipation inside the
IC, which could otherwise be a limiting factor on the switch-
ing frequency. It may thus carry significant power, especially
at higher frequencies. As an example, consider the gate
resistors used for the low-side MOSFETs (Q2 and Q4) in
Figure 1. The FDB7045L has a maximum gate charge of
70nC at 5V, and an input capacitance of 5.4nF. The total
energy used in powering the gate during one cycle is the
energy needed to get it up to 5V, plus the energy to get it up
to 12V:

This power is dissipated every cycle, and is divided between
the internal resistance of the FAN5091 gate driver and the
gate resistor. Thus,

and each gate resistor thus requires a 1/4W resistor to ensure
worst case power dissipation.

The same calculation may be performed for the high-side
MOSFETs, bearing in mind that their gate voltage rises to
only (12V-5V) = 7V.

Inductor Selection

Choosing the value of the inductor is a tradeoff between
allowable ripple voltage and required transient response. A
smaller inductor produces greater ripple while producing
better transient response. In any case, the minimum induc-
tance is determined by the allowable ripple. The first order
equation (close approximation) for minimum inductance for
a two-slice converter is:

where:
Vin = Input Power Supply
Vout = Output Voltage

1
2

---C

70nC

1
2

---

5.4nF

12V

(

)

482nJ

Rgate

E f

gate

internal

(

)

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

482nJ

300KHz

4.7

1.0

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

19mW

min

out

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

out

-----------

ESR

ripple

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

FAN5091

PRODUCT SPECIFICATION

REV. 1.0.0 5/10/01

f = DC/DC converter switching frequency
ESR = Equivalent series resistance of all output capacitors in
parallel
Vripple = Maximum peak to peak output ripple voltage
budget.

One other limitation on the minimum size of the inductor is
caused by the current feedback loop stability criterion. The
inductor must be greater than:

where L is the inductance in Henries, R

DS,on

is the on-state

resistance of one slice's low-side MOSFET, R

Droop

is the

value of the droop resistor in Ohms, V

is either 5V or 12V,

and V

is the output voltage. For most applications, this for-

mula will not present any limitation on the selection of the
inductor value.

A typical value for the inductor is 1.3

µH at an oscillator

frequency of 600KHz (300KHz each slice) and 220nH at an
oscillator frequency of 2MHz (1MHz each slice). For other
frequencies, use the interpolating formula

Schottky Diode Selection

The application circuits of Figures 1-3 show a Schottky
diode, D1 (D2 respectively), one in each slice. They are used
as free-wheeling diodes to ensure that the body-diodes in the
low-side MOSFETs do not conduct when the upper
MOSFET is turning off and the lower MOSFETs are turning
on. It is undesirable for this diode to conduct because its high
forward voltage drop and long reverse recovery time
degrades efficiency, and so the Schottky provides a shunt
path for the current. Since this time duration is extremely
short, being minimized by the adaptive gate delay, the selec-
tion criterion for the diode is that the forward voltage of the
Schottky at the output current should be less than the forward
voltage of the MOSFET's body diode. Power capability is
not a criterion for this device, as its dissipation is very small.

Output Filter Capacitors

The output bulk capacitors of a converter help determine its
output ripple voltage and its transient response. It has
already been seen in the section on selecting an inductor that
the ESR helps set the minimum inductance. For most con-
verters, the number of capacitors required is determined by
the transient response and the output ripple voltage, and
these are determined by the ESR and not the capacitance
value. That is, in order to achieve the necessary ESR to meet
the transient and ripple requirements, the capacitance value
required is already very large.

The most commonly used choice for output bulk capacitors
is aluminum electrolytics, because of their low cost and low
ESR. The only type of aluminum capacitor used should be
those that have an ESR rated at 100kHz. Consult Application
Bulletin AB-14 for detailed information on output capacitor
selection.

For higher frequency applications, particularly those running
the FAN5091 oscillator at >1MHz, Oscon or ceramic capaci-
tors may be considered. They have much smaller ESR than
comparable electrolytics, but also much smaller capacitance.

The output capacitance should also include a number of
small value ceramic capacitors placed as close as possible to
the processor; 0.1µF and 0.01µF are recommended values.

Input Filter

The DC-DC converter design may include an input inductor
between the system main supply and the converter input as
shown in Figure 6. This inductor serves to isolate the main
supply from the noise in the switching portion of the DC-DC
converter, and to limit the inrush current into the input capac-
itors during power up. A value of 1.3µH is recommended.

It is necessary to have some low ESR capacitors at the input
to the converter. These capacitors deliver current when the
high side MOSFET switches on. Because of the interleaving,
the number of such capacitors required is greatly reduced
from that required for a single-slice buck converter. Figure 6
shows 3 x 1000

µF, but the exact number required will vary

with the output voltage and current, according to the formula

for the two slice FAN5091, where DC is the duty cycle,
DC = Vout / Vin. Capacitor ripple current rating is a function
of temperature, and so the manufacturer should be contacted
to find out the ripple current rating at the expected opera-
tional temperature. For details on the design of an input filter,
refer to Applications Bulletin AB-16.

Figure 6. Input Filter

Design Considerations and Component
Selection

Additional information on design and component selection
may be found in Fairchild's Application Note 59.

DS on

Droop

(

)

L nH

(

)

930,000

f KHz

(

)

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

240

rms

out

--------- 2DC

4DC

1.3

µH

+5V

1000

µF, 16V

Electrolytic

Vin

PRODUCT SPECIFICATION

FAN5091

REV. 1.0.0 5/10/01

PCB Layout Guidelines

· Placement of the MOSFETs relative to the FAN5091 is

critical. Place the MOSFETs such that the trace length of
the HIDRV and LODRV pins of the FAN5091 to the FET
gates is minimized. A long lead length on these pins will
cause high amounts of ringing due to the inductance of the
trace and the gate capacitance of the FET. This noise
radiates throughout the board, and, because it is switching
at such a high voltage and frequency, it is very difficult to
suppress.

· In general, all of the noisy switching lines should be kept

away from the quiet analog section of the FAN5091. That
is, traces that connect to pins 8-17 (LODRV, HIDRV,
PGND and BOOT) should be kept far away from the
traces that connect to pins 1 through 7, and pins 18-24.

· Place the 0.1µF decoupling capacitors as close to the

FAN5091 pins as possible. Extra lead length on these
reduces their ability to suppress noise.

· Each power and ground pin should have its own via to the

appropriate plane. This helps provide isolation between
pins.

· Place the MOSFETs, inductor, and Schottky of a given

slice as close together as possible for the same reasons as
in the first bullet above. Place the input bulk capacitors as
close to the drains of the high side MOSFETs as possible.
In addition, placement of a 0.1

µF decoupling cap right on

the drain of each high side MOSFET helps to suppress
some of the high frequency switching noise on the input
of the DC-DC converter.

· Place the output bulk capacitors as close to the CPU as

possible to optimize their ability to supply instantaneous
current to the load in the event of a current transient.
Additional space between the output capacitors and the
CPU will allow the parasitic resistance of the board traces
to degrade the DC-DC converter's performance under
severe load transient conditions, causing higher voltage
deviation. For more detailed information regarding
capacitor placement, refer to Application Bulletin AB-5.

· A PC Board Layout Checklist is available from Fairchild

Applications. Ask for Application Bulletin AB-11.

PC Motherboard Sample Layout and Gerber File

A reference design for motherboard implementation of the
FAN5091 along with the PCAD layout Gerber file and silk
screen can be obtained through your local Fairchild repre-
sentative.

FAN5091 Evaluation Board

Fairchild provides an evaluation board to verify the system
level performance of the FAN5091. It serves as a guide to
performance expectations when using the supplied external
components and PCB layout. Please contact your local
Fairchild representative for an evaluation board.

Additional Information

For additional information contact your local Fairchild
representative.

FAN5091

PRODUCT SPECIFICATION

5/10/01 0.0m 005

Stock#DS30005091

2001 Fairchild Semiconductor Corporation

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which, (a) are intended for surgical implant into the body,
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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.

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ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME
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Ordering Information

Product Number

Description

Package

FAN5091MTC

24 pin TSSOP

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: FAN5091

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: FAN5091