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Электронный компонент: FAN5068

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www.fairchildsemi.com
REV. 1.0.1 9/9/04
Features
PWM regulator for VDDQ (2.5V or 1.8V)
Linear LDO regulator generates VTT = VDDQ/2, 1.5A
Peak sink/source capability
1 independent programable ULDO controllers driving
external N-Channel MOSFET
ACPI drive and control for 5V DUAL generation
3.3V Internal LDO for 3V-ALW generation
300kHz fixed frequency switching
R
DS(ON)
current sensing or optional current sense resistor
for precision over-current detect
Internal Synchronous Boot diode
Power Good signal for all voltages
Input Under-Voltage Lock-Out (UVLO)
Thermal Shutdown
Latched Multi-Fault Protection
24-pin 5x5mm MLP package
Applications
DDR-1/DDR-2 VDDQ and VTT voltage generation with
ACPI support
Desktop PC's
Servers
General Description
The FAN5068 DDR memory regulator combines a high-
efficiency PWM controller to generate the supply voltage,
VDDQ, and a linear regulator to generate VTT, the termina-
tion voltage. Synchronous rectification provides high-
efficiency over a wide range of load currents. Efficiency is
further enhanced by using the low-side MOSFET's R
DS(ON)
to sense current instead of a series sense resistor.
In S3 mode, only the VDDQ switcher and the 3.3V regula-
tors remain on while the VTT and ULDO regulators are shut
off. To avoid "glitching" the VDDQ output during the transi-
tion from S3 to S0, the three linear regulators use the SS
capacitor to limit their slew rates, thereby limiting the surge
current from the VDDQ output. PGOOD becomes true in S0
only when all 3 regulators have achieved stable outputs.
In S5 (EN = 0), the 3.3V internal LDO stays on, while the
other regulators are powered down.
The VDDQ PWM regulator is a sampled current mode con-
trol with external compensation to achieve fast load transient
response and provide system design optimization.
The VTT regulator derives its reference and takes its power
from the VDDQ PWM regulator output using a precision
internal voltage divider to set its output at 1/2 of VDDQ. The
VTT termination regulator is capable of sourcing or sinking
at least 1.5A peak current.
FAN5068
DDR-1/DDR-2 plus ACPI Regulator Combo
PRODUCT SPECIFICATION
FAN5068
2
REV. 1.0.1 9/9/04
Block Diagrams
Figure 1. DDR/ACPI System Regulation Schematic
The components selected are for a 15A ouput on VDDQ.
Table 1. BOM for Figure 1
Description
Qty
Ref.
Vendor
Part Number
See notes below
COUT
See notes below
C1, C12,
C17
Capacitor 1uF, 10%, 16VDC, X7R, 1206
2
C2, C4
KEMET
C1206C105K4RACTU
Capacitor 10nF, 10%, 100VDC, X7R, 0805
1
C3
Panasonic
ECJ-2VB2A103K
Capacitor 2.2nF, 10%, 50V, X7R, 0805
1
C6
AVX
08055C222KAT2A
Capacitor 33pF, 10%, 50VDC, NPO, 0805
1
C9
Panasonic
ECJ-2VC1H330J
Capacitor 10nF, 10%, 100VDC, X7R, 0805
2
C10, C11
Panasonic
ECJ-2VB2A103K
Capacitor 220nF, 20%, 10VDC, X5R, 0603
1
C5
AVX
06033D224MAT
Capacitor 100nF, 10%, 25VDC, X7R, 0805
1
C8
Kemet
C0805C104K3RACTU
Inductor 1.8uH, 3.24m
, 16 Amps, 20%, 0.5"
1
L1
Inter-Technical
SC5018-1R8M
Inductor 0.39uH, 2.8m
, 15 Amps, 20%, 0.25"
1
L2
Inter-Technical
SC7232-R39M
MOSFET N-CH, 8.8m
, 30V, 50A, D-PAK, FSID:
FDD6296
1
Q1
Fairchild
FDD6296
Q1
C5
C2
C13
C1
C3
C17
C4
C12
L1
L2
R3
R8
R7
R2
R5
R9
R4
+12
+5MAIN
+5VSB
+5VSB
5V DUAL
R10
R1
R6
C6
C11
C10
C8
C7
S3
PWM
SYS
LDOs
VTT
LDO
3.3 MAIN
VDDQ
3.3 ALW
VCC
C9
VDDQ
C
OUT
5V MAIN
BOOT
SBSW
S3#O
S3#I
SS
ILIM
G1.2
1.2 OUT
FB1.2
PGOOD
VCC
EN
HDRV
SW
ISNS
LDRV
GND
FB
COMP
VDDQ IN
REF IN
VTT SNS
VTT OUT
Q3
Q4
Q5
Q6
Q2
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
3
MOSFET N-CH, 6m
, 30V, 75A, D-PAK, FSID:
FDD6606
1
Q2
Fairchild
FDD6606
MOSFET N-CH, 32m
, 20V, 21A, D-PAK, FSID:
FDD6530A
3
Q3, Q6,
Q5
Fairchild
FDD6530A
MOSFET P-CH, 35m
, -20V, -5.5A, SSOT-6,
FSID: FDC602P
1
Q4
Fairchild
FDC602P
Resistor 1.82k
, 1%, 0805
4
R1, R2,
R9, R10
Yageo
9C08052A1821FKHFT
Resistor 56k
, 1%, 0805
1
R5
Any
Resistor 60.4k
, 1%, 0805
1
R6
Any
Resistor 3.01k
, 1%, 0603
1
R7
Resistor 9.09k
, 1%, 0603
1
R8
Resistor 10k
, 1%, 0805
1
R4
Any
Resistor 1k
, 1%, 0805
1
R3
Any
IC System Regulator, MLP 24-Pin 5X5mm
1
U2
Fairchild
FAN5068
Table 1. BOM for Figure 1
(continued)
Description
Qty
Ref.
Vendor
Part Number
Bypass Capacitor Notes:
Input capacitor C1 is typically chosen based on the ripple
current requirements. COUT is typically selected based on
both current ripple rating and ESR requirement. See AN-
6006 for these calculations.
C17 and C12 selection will be largely determined by ESR
and load transient response requirements. In each case, the
number of capacitors required depends on the capacitor tech-
nology chosen. Oscons can meet the requirements with less
space, but higher cost than using low ESR electrolytics (like
Rubycon MBZ).
Figure 2. PWM Modulator Block Diagram
VDDQ IN
S3
PGOOD
EN
SS
C
BOOT
Q1
Q2
5V
VDD
ADAPTIVE
GATE
CONTROL LOGIC
CURRENT PROCESSING
HDRV
SW
LDRV
PGND
BOOT
ISNS
VIN
C
OUT
VDDQ
PWM
S/H
ILIM
R
ILIM
R
SENSE
ILIM det.
PWM
S
R
Q
RAMP
RAMP
CLK
OVP
POR/UVLO
Reference and
Soft Start
S3
L
OUT
VREF
D2
4.41K
FB
COMP
ISNS
FB
VIN
OSC
PRODUCT SPECIFICATION
FAN5068
4
REV. 1.0.1 9/9/04
Figure 3. VTT Regulator Block Diagram
Pin Configuration
FAN5068MP 5x5 mm MLP-24 Package (
JA
= 38C/W,
JC
= 1.4C/W)*
Note: Connect P1 pad to GND.
Pin Definitions
Pin #
Pin Name
Pin Function Description
1
G1.2
Gate Drive
for the 1.2V LDO
. Turned off (low) in S3 and S5 modes.
2
FB1.2
Feedback for the 1.2V LDO Output
. Tie to a voltage higher than 0.9V to disable this
regulator.
3
SBSW
Standby Switch
. Drives the P-Channel MOSFET to power 5V DUAL from 5VSB when in
S3. Goes high in S0 and S5.
4
5V MAIN
5V MAIN
. When this pin is below 4.5V, transition from S3 to S0 is inhibited.
5
VTT SNS
VTT Remote Sense Input
.
6
VTT OUT
VTT Regulator Power Output
.
7
VDDQ IN
VDDQ Input from PWM
. Connect to PWM output voltage. This is the VTT Regulator power
input.
VTT SNS
VTT OUT
+
EN
S3
VDDQ IN
PGND
REF IN
VDDQ IN
50K
50K
R9
R10
7
8
9
10
11
12
24
23
22
21
20
19
EN
S3#I
S3#O
3.3 ALW
VCC
PGOOD
1
2
3
4
5
6
18
17
16
15
14
13
G1.2
FB1.2
SBSW
5V MAIN
VTT SNS
VTT OUT
P1 = GND
VDDQ IN
BOOT
HDRV
SW
ISNS
LDRV
REF IN
FB
COMP
SS
ILIM
GND
*Test method as per JEDEC Specification JESD51-5
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
5
8
BOOT
Boot
. Positive supply for the upper MOSFET driver. Connect as shown in Figure 1. IC
contains a boot diode to VCC.
9
HDRV
High-Side Drive
. High-side (upper) MOSFET driver output. Connect to gate of high-side
MOSFET.
10
SW
Switching Node
. Return for the high-side MOSFET driver and a current sense input.
Connect to source of high-side MOSFET and low-side MOSFET drain.
11
ISNS
Current Sense Input
. Monitors the voltage drop across the lower MOSFET or external
sense resistor for current feedback.
12
LDRV
Low-Side Drive
. The low-side (lower) MOSFET driver output. Connect to gate of low-side
MOSFET.
13
PGOOD
Power Good Flag
. An open-drain output that will pull LOW when FB is outside of a 10%
range of the 0.9V reference and the LDO outputs are > 80% or < 110% of its reference.
PGOOD goes low when S3 is high. The power good signal from the PWM regulator enables
the VTT regulator and the LDO controller.
14
VCC
VCC
. The IC takes its bias power from this pin. Also used for gate drive power. The IC is held
in standby until this pin is above 4.35V (UVLO threshold).
15
3.3 ALW
3.3V LDO Output
. Internal LDO output. Turned off in S0, on in S5 or S3 modes.
16
S3#O
S3# Output
. Open-drain output which pulls the gates of two N-Channel blocking MOSFETs
low in S5 and S3. This pin goes high (open) in S0 mode.
17
S3#I
S3 Input
. When LOW, turns off the VTT and 1.2V LDO regulators and turns on the 3.3V
regulator. Also causes S3#O to pull low to turn off blocking switch Q3 as shown in Figure 1.
PGOOD is low when S3#I is LOW.
18
EN
ENABLE
. Typically tied to S5#. When this pin is low, the IC is in a low quiescent current
state, all regulators are off and S3#O is low.
19
GND
GROUND
for the IC are tied to this pin and also connected to P1.
20
ILIM
Current Limit
. A resistor from this pin to GND sets the current limit.
21
SS
Soft Start
. A capacitor from this pin to GND programs the slew rate of the converter during
initialization as well as sets the initial slew rate of the LDO controllers when transitioning from
S3 to S0. This pin is charged/discharged with a 5A current source during initialization, and
charged with 50A during PWM soft-start.
22
COMP
Output of the PWM error amplifier
. Connect compensation network between this pin and
FB.
23
FB
VDDQ Feedback
. The feedback from PWM output. Used for regulation as well as PGOOD,
under-voltage, and over-voltage protection and monitoring.
24
REF IN
VTT Reference
. Input which provides the reference for the VTT regulator. A precision
internal divider from VDDQ IN is provided.
Pin Definitions
(continued)
Pin #
Pin Name
Pin Function Description
PRODUCT SPECIFICATION
FAN5068
6
REV. 1.0.1 9/9/04
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
Parameter
Min.
Typ.
Max.
Units
VCC
6.5
V
BOOT, SW, ISNS, HDRV, S3#O
28
V
BOOT to SW
6.5
V
All Other Pins
0.3
VCC+0.3
V
Junction Temperature (T
J
)
20
150
C
Storage Temperature
65
150
C
Lead Soldering Temperature, 10 seconds
300
C
I(VTT) Peak (Duration < 2ms)
1.5
A
I(VTT) RMS
1
A
Parameter
Conditions
Min.
Typ.
Max.
Units
Supply Voltage VCC
4.5
5
5.5
V
I(3.3 ALW)
1.25
A
Ambient Temperature (T
A
)
10
85
C
Electrical Specifications
Recommended operating conditions, component values per Figure 1 unless otherwise noted.
Parameter
Conditions
Min.
Typ.
Max.
Units
Power Supplies
VCC Current:
S0
LDRV, HDRV Open, FB forced above
regulation point, I(VTT) = 0, EN = 1, S3#I = 1
15
24
mA
S3
EN = 1, S3#I = LOW, I(3.3) < 10mA
15
24
mA
S5
EN = 0, I(3.3) = 0
2
4
mA
VCC UVLO Threshold
Rising VCC
4.0
4.2
4.4
V
Falling
3.9
4.05
4.2
V
Hysteresis
0.150
V
5V MAIN UVLO Threshold
Rising
4.3
4.4
4.6
V
Falling
3.9
4.1
4.2
V
Hysteresis
0.300
V
Oscillator
Frequency
255
300
345
kHz
Ramp Amplitude, pkpk
1.8
V
Ramp Offset
0.5
V
Reference and Soft Start
Internal Reference Voltage
0.891
0.900
0.909
V
Soft Start current (I
SS
)
Initial ramp after power-up
4.5
A
During PWM / LDO soft start
48
SS Discharge on-resistance
EN = 0
150
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
7
SS Complete Threshold
1.5
V
SS Complete Hysteresis
50
mV
PWM Converter
Load Regulation
I
OUT
from 0 to 16A
2
+2
%
FB Bias Current
0.75
1
1.25
A
Under-Voltage Shutdown
As % of set point.
2s noise filter
65
75
80
%
I
SNS
Over-Current threshold
R
ILIM
= 56k
145
170
195
A
Over-Voltage threshold
As % of set point
110
115
120
%
PWM Output Driver
HDRV Output Resistance
Sourcing
1.8
3
Sinking
1.8
3
LDRV Output Resistance
Sourcing
1.8
3
Sinking
1.2
2
PGOOD (Power Good Output) and Control pins. VDDQ output
Lower Threshold
As % of set point, 2s noise filter
86
92
%
Upper Threshold
As % of set point, 2s noise filter
108
115
%
PGOOD Output Low
I
PGOOD
= 1.5mA
0.5
V
Leakage Current
V
PULLUP
= 5V
1
A
VTT Regulator
VDDQ IN Current
S0 mode, I
VTT
= 0
35
70
mA
VREF IN to VTT
Differential Output Voltage
I
VTT
= 0, T
A
= 25C
I
VTT
= 1.25A (pulsed)
20
40
20
40
mV
Internal Divider Gain
0.493
0.498
0.503
V/V
VTT Current limit
Pulsed (300ms max.), T
A
= 25C
1.5
3
4
A
VTT Leakage Current
S3#I = LOW
20
20
mA
VTT SNS input resistance
VTT = 0.9V
110
k
VTT PGOOD
Measured at VTT SNS
80
110
% VTT
REF
Drop-Out Voltage
ITT = 1.5A
0.8
0.8
V
1.2V LDO
Regulation
I(1.2) from 0 to 5A
1.17
1.2
1.23
V
Drop-Out Voltage
I(1.2)
5A, R
DS(ON)
< 50m
0.3
V
External Gate Drive
VCC = 4.75V
4.5
V
Gate Drive Source Current
1.2
mA
Gate Drive Sink Current
1.2
mA
External Gate Drive
VCC = 4.75V
4.5
V
FB1.2 PGOOD threshold
0.80
V
3.3V LDO
Regulation
I(3.3) from 0 to 1.25A, VCC > 4.75V
3.2
3.3
3.4
V
Drop-Out Voltage
I(3.3)
1.25A
1.5
V
Electrical Specifications
(continued)
Recommended operating conditions, component values per Figure 1 unless otherwise noted.
Parameter
Conditions
Min.
Typ.
Max.
Units
PRODUCT SPECIFICATION
FAN5068
8
REV. 1.0.1 9/9/04
Circuit Description
Overview
Control Functions
S3#I, EN Input Threshold
1.0
1.25
1.55
V
S3#I EN Input Current
1
1
A
Over-Temperature Shutdown
150
C
Over-Temperature Hysteresis
25
C
S3#O Output Low R
DS(ON)
170
300
S3#O Output High Leakage
V(S3#O) = 12V
4
10
A
SBSW Output Low Resistance
5V MAIN OK
125
200
SBSW Sink current (note 1)
5V MAIN < UVLO
500
nA
SBSW Output High (note 2)
820
1200
Electrical Specifications
(continued)
Recommended operating conditions, component values per Figure 1 unless otherwise noted.
Parameter
Conditions
Min.
Typ.
Max.
Units
The FAN5068 provides 5 functions:
1.
A general purpose PWM regulator, typically used to gen-
erate VDDQ for DDR memory.
2.
A low-dropout linear VTT regulator capable of sinking
and sourcing 1.5A peak.
3.
An adjustable Ultra Low Drop Out (ULDO) controller
which, in conjunction with an external N-Channel power
MOSFET, provides a programmable low voltage output.
The power source for this output is typically the VDDQ
output and is used to provide the 1.2V GTL
processor FSB termination voltage.
4.
Control for generating a 5V DUAL voltage using an
external N-channel to supply power from 5V MAIN in
S0, and an external P-Channel to provide power from 5V
Standby (5VSB) in S3.
5.
An internal LDO which regulates "3.3V Always" in S3
mode from VCC(VSB). In S3, this regulator is capable
of 1.25A peak currents with average currents limited by
the thermal design of the PCB.
At initial power-up, or when transitioning from S5, the PWM
regulator will be disabled until 5V MAIN is above its UVLO
threshold.
Table 2. ACPI States
STATE
S3#I
EN (S5#)
S3#O
SBSW
VDDQ
VTT
G1.2 LDO 3.3 ALW LDO
S5
X
L
L
H
OFF
OFF
OFF
ON
S3
L
H
L
L
ON
OFF
OFF
ON
S0
H
H
H
H
ON
ON
ON
OFF
STATE
3.3 ALW
5V Dual
S5
LDO
OFF
S3
LDO
+5VSB
S0
Q5 (MAIN)
+5 MAIN
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
9
Regulator Sequencing
The VCC pin provides power to all logic and analog control
functions of the regulator including:
1.
Power for the 3.3V regulator
2.
LDRV gate driver current
3.
HDRV boot diode charging current.
4.
The regulator analog control and logic
This pin must be decoupled with a ceramic capacitor (4.7F
or larger recommended) as close as possible to the VCC pin.
After VCC is above UVLO, the start-up sequence begins as
shown in Figure 6.
T1 to T3: After initial power-up, the IC will ignore all logic
inputs for a time period (T3-T1) of about:
where T3-T1 is in ms if C
SS
is in nF. At T2 (about 2/3 of the
way from T1 to T3), the 3.3V-ALW LDO is in regulation.
The 3.3V LDO's slew rate is limited by the discharge slope
of CSS. If 3.3V MAIN has come up prior to this time, the
3.3V-ALW node will already be pre-charged through the
body diode of Q5 (see Figure 1).
T3 to T4: The IC will start VDDQ only if 5V MAIN is
above its UVLO threshold (5V MAIN OK). Provided 5V
MAIN is up before T3, the IC waits about 100s before
initiating soft-start on VDDQ to allow CSS time to fully
discharge. The IC is in "SLEEP" or S5 state when EN is low.
In S5, only the 3.3V LDO is on. If the IC is in S5 at T4, CSS
will be held to 0V.
T4 to T5: After VDDQ is stablized (when CSS is at about
1.3V) an internal VDDQ OK is generated which will allow
the 1.2V LDO and the VTT LDO to start. To ensure that the
VDDQ output is not subjected to large transient currents dur-
ing S3 to S0 transition, the VTT and 1.2V LDO slew rates
are limited by the slew rate of the SS cap until the LDO is in
regulation. In addition, the VTT regulator is current limited.
After VDDQ OK becomes true, CSS will be held to 1.2V
until S3#I goes high.
S0 to S3: The system signals this transition by dropping the
S3#I signal. When this occurs, S3#O goes low, and the 3.3V
LDO turns on. The 1.2V LDO and the VTT LDO are turned
off, and CSS is discharged to 2V. SBSW pulls low to turn on
the P-Channel 5V DUAL switch.
S3 to S0: The system signals this transition by raising the
S3#I signal. S0 mode is not entered until 5V MAIN OK.
Then the following occurs:
S3#O releases
SBSW pulls high to turn off the P-Channel switch
The 3.3V LDO turns off
The 1.2V LDO and the VTT LDO are turned on and, and
CSS is allowed to charge up
In most systems, the ATX power supply is enabled when
S3#I goes from high. At that point, 5V and 3.3V MAIN will
start to rise. The FAN5068 waits until 5V MAIN is above
4.5V to turn on Q3 and Q5. This can cause about a 10%
"bump" in both 5V DUAL and 3.3V ALW when Q3 and Q5
turn on, since at that point, 5V MAIN and 3.3V MAIN are at
90% of their regulation value.
Figure 4. S3 to S0 Transition: 5V DUAL
To eliminate the "bump", add delay to the 5V MAIN pin as
shown below. The 5V MAIN pin on the FAN5068 does not
supply power to the IC, it is only used to monitor the voltage
level of the 5V MAIN supply, and therefore is a high imped-
ance input.
Figure 5. Adding Delay to 5V MAIN
Another method to eliminate the potential for this "bump" is
to use the PWR_OK to drive the 5V MAIN pin. Some sys-
tems cannot tolerate the long delay for PWR_OK (>100ms)
to assert, hence the solution in figure 5 may be preferrable.
S5 to S3: During S5 to S3 transition, the IC will pull SBSW
low with a 500nA current sink to limit inrush in Q4 if 5V
MAIN is below its UVLO threshold. At that time, 5V DUAL
is discharged. The limited gate drive controls the inrush
current through Q4 as it charges C1 (capacitance on 5V
Dual). Depending on the C
GD
of Q4, the current available
from 5VSB, and the size of C1, C13 may be omitted.
If 5V MAIN is above its UVLO threshold, SBSW will be
pulled down with an impedance of about 2K. VDDQ will not
start until 5V MAIN OK is true.
T3
T1
6.4
C
SS
5
-----------------------
=
(1)
S3#I
5V MAIN
4.5V
5V DUAL
5V Dual "bump"
S3#O
5V MAIN
4
+5MAIN
FROM
ATX
R
DLY
C
DLY
I
Q4(INRUSH)
C1
5
10
7
C13
C
GD(Q4)
+
------------------------------------
=
(2)
PRODUCT SPECIFICATION
FAN5068
10
REV. 1.0.1 9/9/04
Figure 6. Start-up Sequence into S0.
VDDQ
5V SB
V(UVLO)
4V
1V
3.3V LDO
T0
3.8V
T1
T2
T3 T4
T5
PWM Regulator
A PSPICE model and spreadsheet calculator are available for
the VDDQ PWM regulator to select external components
and verify loop stability. The topics covered below provide
the explanation behind the calculations in the spreadsheet.
Setting the output voltage
The output voltage of the PWM regulator can be set in the
range of 0.9V to 90% of its power input by an external resis-
tor divider.
The internal reference is 0.9V. The output is divided down by
an external voltage divider to the FB pin (for example, R1
and R2 in Figure 1). There is also a 1A precision (5%)
current sourced out of FB to ensure that if the pin is open,
VDDQ will remain low. The output voltage therefore is:
To minimize noise pickup on this node, keep the resistor to
GND (R2) below 2k. We selected R2 at 1.82k and solved for
R1.
The synchronous buck converter is optimized for 5V opera-
tion. The PWM modulator uses an average current mode
control for simplified feedback loop compensation.
Oscillator
The oscillator frequency is 300kHz. The internal PWM ramp
is reset on the rising clock edge.
PWM Soft Start
When the PWM regulator is enabled the circuit will wait
until the VDDQ IN pin is below 100mV to ensure that the
soft-start cycle does not begin with a large residual voltage
on the PWM regulator output.
When the PWM regulator is disabled, 50
is turned on from
VDDQ IN to PGND to discharge the output.
The voltage at the positive input of the error amplifier is
limited to V
CSS
which is charged with a 50A current
source. Once CSS has charged to 0.9V, the output voltage
will be in regulation.
The time it takes SS to reach 0.9V is:
where T
0.9
is in ms if C
SS
is in nF.
CSS charges another 400mV before the PWM regulator's
latched faults are enabled. When CSS reaches 2.5V, the VTT
and 1.2V LDO will begin their soft-start ramps. After the
VTT and 1.2V LDO regulators are in regulation, PGOOD is
then allowed to go HIGH (open). UVLO on VCC will
discharge SS and reset the IC.
To prevent large duty cycles and high currents during the
beginning of the PWM soft-start, the input to the PWM com-
parator is also clamped by CSS. This clamping action has no
practical effect on operation of the circuit after CSS has
passed about 0.4V.
0.9V
R2
------------
V
OUT
0.9V
R1
---------------------------------
1
A
+
=
(3a)
R1
R2
V
OUT
0.9
(
)
0.9
1
A
----------------------------------------------
1.816k
1.82k
=
=
(3b)
T
0.9
0.9
C
SS
50
-----------------------
=
(4)
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
11
Figure 7. SS Clamp and FB Open Protection
Figure 8. Current Limit / Summing Circuits
ISNS
RAMP
SS
FB
COMP
4.41K
1
A
I
SS
+
+
E/A
VREF
+
+
PWM
LDRV
PGND
ISNS
2.5V
ILIM det.
R
SENSE
I2 =
ILIM*9.6
ILIM
0.9V
R
ILIM
ILIM
mirror
in +
in
V to I
ISNS
ISNS
S/H
SS/EN
C
SS
FB
Reference and
Soft Start
TO
PWM
COMP
4.41K
COMP
Current Processing Section
The following discussion refers to Figure 8.
The current through R
SENSE
resistor (ISNS) is sampled
shortly after Q2 is turned on. That current is held, and
summed with the output of the error amplifier. This effec-
tively creates a current mode control loop. R
SENSE
sets the
gain in the current feedback loop. For stable operation, the
voltage induced by the current feedback at the PWM com-
parator input should be set to 30% of the ramp amplitude at
maximum load currrent and line voltage. The following
expression estimates the recommended value of R
SENSE
as a
function of the maximum load current (I
LOAD(MAX
)) and the
value of the MOSFET's R
DS(ON)
.:
RSENSE must, however, be kept higher than:
Setting the Current Limit
An ISNS is compared to the current established when a 0.9 V
internal reference drives the ILIM pin. R
ILIM
, the R
DS(ON)
of
Q2, and R
SENSE
determine the current limit:
Where ILIMIT is the peak inductor current. Since the toler-
ance 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 repre-
sentation of the load current. When using the MOSFET as
the sensing element, the variation of R
DS(ON)
causes propor-
tional 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 tempera-
ture. A factor of 1.6 in the current limit setpoint should
compensate for all MOSFET R
DS(ON)
variations, assuming
the MOSFET's heat sinking will keep its operating die
temperature below 125C.
R
SENSE
I
LOAD MAX
(
)
R
DS ON
(
)
4.1k
30%
0.125
V
IN MAX
(
)
----------------------------------------------------------------------------
100
=
(5)
R
SENSE MIN
(
)
I
LOAD MAX
(
)
R
DS ON
(
)
150
A
-----------------------------------------------------------
100
=
(6)
R
ILIM
9.6
I
LIMIT
----------------
100
R
SENSE
+
(
)
R
DS ON
(
)
----------------------------------------
=
(7)
PRODUCT SPECIFICATION
FAN5068
12
REV. 1.0.1 9/9/04
Figure 9. 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 9.
This approach causes higher losses, but yields greater accu-
racy. R1 is a low value (e.g. 10m
) resistor.
Current limit (I
LIMIT
) should be set sufficiently high as to
allow inductor current to rise in response to an output load
transient. Typically, a factor of 1.3 is sufficient. In addition,
since I
LIMIT
is a peak current cut-off value, we will need to
multiply I
LOAD(MAX)
by the inductor ripple current (we'll
use 20%).
I
LIMIT
> I
LOAD(MAX)
* 1.6 * 1.3 * 1.2
(8)
Gate Driver Section
The Adaptive 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 shoot-through may occur.
Frequency Loop Compensation
The loop is compensated using a feedback network around
the error amplifier, which is a voltage output op amp.
Figure 10. Compensation Network
Figure 10 shows a complete type 3 compensation network. A
type 2 compensation configuration eliminates R4 and C3 and
is shown in Figure 1. Since the FAN5068 architecture
employs summing current mode, type 2 compensation can
be used for most applications. For critical applications that
require wide loop-bandwidth, and use very low ESR output
capacitors, type 3 compensation may be required. The
PSPICE model and spreadsheet calculator can be used to cal-
culate these component values.
PGOOD Signal
PGOOD monitors the status of the PWM output as well as
the VTT and 1.2V LDO regulators. PGOOD remains low
unless all of the conditions below are met:
1.
S3#I is HIGH
2.
SS is above 4V
3.
Fault latch is cleared
4.
FB is between 90% and 110% of VREF
5.
VTT and LDO 1.2 are in regulation
Protection
The converter output is monitored and protected against
extreme overload, short circuit, over-voltage and under-volt-
age conditions.
An internal "Fault Latch" is set for any fault intended to shut
down the IC. When the "Fault Latch" is set, the IC will dis-
charge VOUT by driving LDRV high until VDDQ IN < 0.5V.
LDRV will then go low until VDDQ IN > 0.8V. This behav-
ior will discharge the output without causing undershoot
(negative output voltage).
To discharge the output capacitors, a 50
load resistor is
switched in from VDDQ IN to PGND whenever the IC is in
fault condition, or when EN is low. After a latched fault,
operation can be restored by recycling power or by toggling
the EN pin.
Under-Voltage Shutdown
If FB stays below the under-voltage threshold for 2s, the
"Fault latch" is set. This fault is prevented from setting the
fault latch during PWM soft-start (SS < 1.3V).
LDRV
PGND
ISNS
R
SENSE
R1
Q2
R4
VREF
VDDQ
FB
COMP
R3
R2
R1
C3
C1
C2
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
13
Over-Current Sensing
If the circuit's current limit signal ("ILIM det" as shown in
Figure 8) is high at the beginning of a clock cycle, a pulse-
skipping circuit is activated and HDRV is inhibited. The
circuit continues to pulse skip in this manner for the next 8
clock cycles. If at any time from the 9
th
to the 16
th
clock
cycle, the "ILIM det" is again reached, the fault latch is set.
If "ILIM det" does not occur between cycle 9 and 16, normal
operation is restored and the over-current circuit resets itself.
This fault is prevented from setting the fault latch during
soft-start (SS < 1.3V).
Figure 11. Over-Current Protection Waveforms
OVP / HS Fault / FB short to GND detection:
A HS Fault is detected when there is more than 0.5V from
SW to PGND 350ns after LDRV reaches 4V (same time as
the current sampling time).
OVP Fault Detection occurs if FB > 115% VREF for 16
clock cycles.
During soft-start, the output voltage could potentially "run
away" if either the FB pin is shorted to GND or R1 is open.
This fault will be detected if the following condition persists
for more than 14 s during soft-start.
1.
VDDQ IN (PWM output voltage) > 1V and
2.
FB < 100mV
Any of these 3 faults will set the fault latch. These 3 faults
can set the fault latch during the SS time (SS < 1.3V).
To ensure that FB pin open will not cause a destructive con-
dition, a 1A current source ensures that the FB pin will be
high if open. This will cause the regulator to keep the output
low, and eventually result in an Under-voltage fault shut-
down (after PWM SS complete).
Over-Temperature Protection
The chip incorporates an over temperature protection circuit
that shuts the chip down when a die temperature of about
160C is reached. Normal operation is restored at when the
die temperature falls below 125C with internal Power On
Reset asserted, resulting in a full soft-start cycle. To accom-
plish this, the over-temperature comparator should discharge
the SS pin.
VTT Regulator Section (Figure 3)
The VTT regulator includes an internal resistor divider (50k
for each resistor) from the output of the PWM regulator. If
the REF IN pin is left open, the divider will produce a volt-
age that is 50% of VDDQ IN.
The VTT regulator is enabled when S3#I is HIGH and the
PWM regulator's internal PGOOD signal is true. The VTT
regulator also includes its own PGOOD signal which is high
when VTT SNS > 90% of REF IN.
LDO Controller
The LDO controller is typically used to provide 1.2V for the
Front-side bus GTL termination. Drop-out voltage for this
regulator will depend on the RDS(ON) of the external
N-Channel MOSFET pass element that is selected. Gate
drive comes from VCC and can pull up to within 0.5V of
VCC line. With 1.2V output, the enhancement voltage for
the MOSFET is: VENH = 4.75 - 0.5 - 1.2 = 3.05V. There-
fore, a low enhancement voltage MOSFET should be used
for the pass element.
The LDO controller contains a soft-start circuit which limits
its output slew rate when it powers up. The LDO's output
voltage (V
1.2
) is established with the following equation
(reference designators are from Figure 1.):
Design and Component Selection
Guidelines
The spreadsheet calculator, which is part of AN-6006 can be
used to calculate all external component values for the
FAN5068. As an initial step, define:
1.
Output voltage
2.
Maximum VDDQ load current
3.
Maximum load transient current and maximum allow-
able output drop during load transient
4.
RDS(ON) of the low-side MOSFET (Q2)
5.
Maximum allowable output ripple
1
2
3
CH1 5.0V
CH3 2.0A
CH2 100mV
M 10.0
s
IL
SHUTDOWN
PGOOD
8 CLK
VOUT
V
1.2
0.9
1
R7
R8
-------
+
=
(9)
PRODUCT SPECIFICATION
FAN5068
14
REV. 1.0.1 9/9/04
Power MOSFET Selection
For a complete treatment of MOSFET selection and
efficiency calculations, see:
AN-6005: Synchronous buck MOSFET loss calculations
with Excel model.
Losses in a MOSFET are the sum of its switching (P
SW
) and
conduction (P
COND
) losses.
In typical applications, the FAN5068 converter's output
voltage is low with respect to its input voltage, therefore the
Lower MOSFET (Q2) is conducting the full load current for
most of the cycle. Q2 should be therefore be selected to min-
imize conduction losses, thereby selecting a MOSFET with
low R
DS(ON)
.
In contrast, the high-side MOSFET (Q1) has a much shorter
duty cycle, and its conduction loss will therefore have less of
an impact. Q1, however, sees most of the switching losses,
so Q1's primary selection criteria should be gate charge.
High-Side Losses:
Figure 12 shows a MOSFET's switching interval, with the
upper graph being the voltage and current on the Drain to
Source and the lower graph detailing V
GS
vs. time with a
constant current charging the gate. The x-axis therefore is
also representative of gate charge (Q
G
). C
ISS
= C
GD
+ C
GS
,
and it controls t1, t2, and t4 timing. C
GD
receives the current
from the gate driver during t3 (as V
DS
is falling). The gate
charge (Q
G
) parameters on the lower graph are either
specified or can be derived from MOSFET datasheets.
Assuming switching losses are about the same for both the
rising edge and falling edge, Q1's switching losses, occur
during the shaded time when the MOSFET has voltage
across it and current through it.
These losses are given by:
where:
P
UPPER
is the upper MOSFET's total losses, and P
SW
and
P
COND
are the switching and conduction losses for a given
MOSFET. R
DS(ON)
is at the maximum junction temperature
(T
J
). t
S
is the switching period (rise or fall time) and is t2+t3
(Figure 12).
The driver's impedance and C
ISS
determine t2 while t3's
period is controlled by the driver's impedance and Q
GD
.
Since most of t
S
occurs when V
GS
= V
SP
we can use a
constant current assumption for the driver to simplify the
calculation of t
S
:
Figure 12. Switching Losses and QG
Figure 13. Drive Equivalent Circuit
Most MOSFET vendors specify Q
GD
and Q
GS
. Q
G(SW)
can
be determined as: Q
G(SW)
= Q
GD
+ Q
GS
Q
TH
where Q
TH
is the gate charge required to get the MOSFET to it's thresh-
old (V
TH
). For the high-side MOSFET, V
DS
= VIN, which
can be as high as 20V in a typical portable application.
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 FAN5068:
where Q
G
is the total gate charge to reach VCC.
P
UPPER
P
SW
P
COND
+
=
P
SW
V
DS
I
L
2
----------------------
2
t
S
F
SW
=
(10a)
P
COND
V
OUT
V
IN
--------------
I
OUT
2
R
DS ON
(
)
=
(10b)
V
SP
t1
t2
t3
4.5V
t4
t5
Q
G(SW)
V
DS
I
D
Q
GS
Q
GD
V
TH
V
GS
C
ISS
C
GD
C
ISS
C
GD
R
D
R
GATE
C
GS
HDRV
5V
SW
VIN
G
t
S
Q
G SW
(
)
I
DRIVER
---------------------
Q
G SW
(
)
VCC
V
SP
R
DRIVER
R
GATE
+
-----------------------------------------------
-----------------------------------------------------
=
(11)
P
GATE
Q
G
VCC
F
SW
=
(12)
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
15
Low-Side Losses
Q2, however, switches on or off with its parallel shottky
diode conducting, therefore V
DS
0.5V. Since P
SW
is pro-
portional to V
DS
, Q2's switching losses are negligible and
we can select Q2 based on R
DS(ON)
only.
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
is the minimum duty cycle for the converter. Since D
MIN
<
20% for portable computers, (1-D)
1 produces a conserva-
tive result, 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).
PCB Design Guidelines:
Below is a summary of recommendations for PCB layout
when using MLP packages:
1.
PCB lead finger pad should be designed 0.2-0.5mm
longer than the package terminal length for good fillet-
ing.
2.
Non Solder Mask Defined (NSMD) pads are recom-
mended over SMD pads due to the tighter tolerance on
copper etching than solder masking.
3.
For Good thermal performance it is recommended to use
4 layer PCB's with vias to effectively remove heat from
the device.
4.
For a 5X5 die size, it is recommended to use 0.3-
0.33mm size holes in the middle.
5.
Vias should be plugged to prevent voids being formed
between the exposed pad and PCB thermal pad due to
solder escaping by the capillary effect. This can be
avoided by tenting the via during the solder mask pro-
cess. The via solder mask diameter should be 100m
larger than the via hole diameter.
PCB Layout General Guidelines
Switching converters, even during normal operation, pro-
duce short pulses of current which could cause substantial
ringing and be a source of EMI if layout constrains are not
observed.
There are two sets of critical components in a DC-DC con-
verter. The switching power components process large
amounts of energy at high rate and are noise generators. The
low power components responsible for bias and feedback
functions are sensitive to noise.
A multi-layer printed circuit board is recommended. Dedi-
cate 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.
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.
Specific Layout recommendations
All component designators reference Figure 1. Layout exam-
ples refer to the FAN5068 EVAL board, available through
your Fairchild Semiconductor representative.
1.
All currents flow in a closed path. All routing should
ensure that the currents are returned to their source of
origin by the shortest possible path without creating
unnecessary loops.
2.
A Multi layer (4 layers or more) PCB facilitates the use
of a complete Ground layer which acts as very low
impedance return path for both the control and power
return currents. This ground plane will also increase the
noise immunity of the control circuit by providing a
shield against radiated disturbances. In the Eval board
layout Layer 2 is reserved for the GND plane.
3.
VCC of the controller should be de-coupled with a
ceramic capacitor (C4) very close to the pin. If it is pos-
sible, the capacitor could be connected across the VCC
(#14) and the GND (#19) pins on top copper. Pin 19
should also connect to the GND plane with a via at the
pin. If a short top-copper connection for C4 is not possi-
ble the capacitor should be close to the VCC pin with
P
COND
1
D
(
)
I
OUT
2
R
DS ON
(
)
=
(13)
D
V
OUT
V
IN
--------------
=
P
D MAX
(
)
T
J MAX
(
)
T
A MAX
(
)
J
A
--------------------------------------------------
=
(14)
PRODUCT SPECIFICATION
FAN5068
16
REV. 1.0.1 9/9/04
the other side connected to the ground plane through a
via. If there is a significant amount of noise on VCC pin,
L-C decoupling at the VCC pin can be used to attenuate
the noise.
4.
The pad under the IC is for power ground return. This
pad needs to be directly connected to the ground plane
by vias under the IC as mentioned above. Do not con-
nect this pad to Pin #19 through the top copper, as this is
the analog ground at the IC.
5.
Use copper planes for power and GND wherever possi-
ble to reduce trace resistance and inductance as well.
6.
Resistors should be as close to the IC pins as possible to
prevent noise pickup due to radiated and adjacent high
dV/dT signals. Avoid routing sense signals near the gate
drive, SW node, boot, or other high dV/dT nodes. Par-
ticular care should be taken to place R3 (ISNS resistor)
as close as possible to pin 11
7.
The closed path (loop) around the feedback components
should be minimized to avoid noise pickup. These com-
ponents will need to be close to the feedback pin. R2,
R10 and C10 should be returned to pin 19 or to the GND
plane as close to pin 19 as possible and away from other
current on the GND plane.
8.
Minimize the path length for the boot cap formed by
SW, C5 and pin 8.
9.
Use thick trace widths for Q1 and Q2 gate drive signals
to minimize their resistance and inductances. Top cop-
per should be used to route the gate drive traces if possi-
ble, especially Q1 Gate to HDRV.
10. Power Plane Routing: The Power Plane routing for
traces carrying higher currents as in the VDDQ output,
need to be routed carefully. The loop from C2 (5V Dual
high frequency bypass cap) to the source of Q2 needs to
be minimized to minimize ringing on SW and Q1 drain.
The loop formed by COUT to the source of the Q2 also
needs to be minimized to keep the ringing on the switch
node low. Basically what we are trying to achieve by
doing this is to reduce the loop inductance and thereby
minimizing the energy in the stray inductance.
This is the
Decoupling Capacitor
Close to VCC Pin
This is the Ground Pin
These are the 5 holes
below the IC for easy
flow soldering
R3 is very
close to ISNS
pin
R5 is very close to ILIM
pin
The Loop
around C6,R6
and C9 is
minimized
The Loop
around
C19,R11 and
R1,R2 is
minimized
- Q2 ON time Current flow path
C13
C14
C16
C20
L1
Q2
Q1
- Q1 ON time Current flow path
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
17
Dimensional Outline Drawing
0.15
5.0
5.0
A
B
C
2X
(0.20)
0.05
0.00
SEATING
PLANE
0.15 C
2X
3.25
3.25
3.25
0.65
0.25~0.35
0.10
0.05
C A B
C
M
M
0.65
PIN #1 IDNET
3.25
0.65 TYP
0.42 TYP
RECOMMENDED LAND PATTERN
TOP VIEW
SIDE VIEW
BOTTOM VIEW
Notes:
A) Conforms to JEDEC registration number MO-220, variation WHHC, dated Aug/2002.
B) Dimensions are in millimeters.
C) Dimensions and tolerances per asme Y14.5-1994.
0.80 MAX
0.08 C
0.10 C
1
1
6
6
24
24
19
19
12
12
7
7
(0.79)
18
18
13
13
0.45
0.35
3.91
5.50
C
PRODUCT SPECIFICATION
FAN5068
LIFE SUPPORT POLICY
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
9/9/04 0.0m 004
Stock#DS30005068
2004 Fairchild Semiconductor Corporation
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
FAN5068MPX
-10C to 85C
MLP-24 5x5 mm
Tape and Reel