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Электронный компонент: US3011CW

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US3011
4-1
Rev. 1.3
12/8/00
TYPICAL APPLICATION
TYPICAL APPLICATION
DESCRIPTION
DESCRIPTION
The US3011controller IC is specifically designed to meet
Intel specification for latest Pentium III
TM
microproces-
sor applications as well as the next generation P6 fam-
ily processors. These products feature a patented topol-
ogy that in combination with a few external components
as shown in the typical application circuit ,will provide in
excess of 20A of output current for an on- board DC/DC
converter while automatically providing the right output
voltage via the 5 bit internal DAC.These devices also
features, loss less current sensing by using the Rds-
on of the high side Power MOSFET as the sensing
resistor
, a Power Good window comparator that switches
its open collector output low when the output is outside
of a
10% window and an OVP output. Other features of
the device are ; Undervoltage lockout for both 5V and
12V supplies , an external programmable soft start func-
tion as well as programming the oscillator frequency by
using an external capacitor.
Dual Layout Compatible with HIP6004A
Designed to meet Intel specification of VRM8.4
for Pentium III
TM
On board DAC programs the output voltage
from 1.3V to 3.5V. The US3011 remains on for
VID code of (11111).
Loss less Short Circuit Protection
Synchronous operation allows maximum
efficiency
Patented architecture allows fixed frequency
operation as well as 100% duty cycle during
dynamic load
Over Voltage Protection Output
Soft Start
High current totem pole driver for direct
driving of the external Power MOSFET
Power Good function
PACKAGE ORDER INFORMATION
PACKAGE ORDER INFORMATION
PRELIMINARY DATASHEET
FEATURES
FEATURES
5 BIT PROGRAMMABLE SYNCHRONOUS BUCK
CONTROLLER IC
APPLICATIONS
APPLICATIONS
Pentium III & Pentium II
TM
processor DC to DC con-
verter application
Low cost Pentium with AGP
Notes: Pentium II and Pentium III are
trade marks of Intel Corp.
5V
12V
VID0
Power Good
3011app1-1.1
Q2
VID1
VID2
VID3
VID4
L1
L2
C5
R1
C3
C6
Q1
R2
R3
C8
R4
C10
C14
C1
C7
OVP
C2
R6
14
HDrv
LDrv
NC/Gnd
SS
CS+
Gnd
NC/Sen
D3
D2
D4
D1
D0
US3011
Vfb
V5/Comp
CS-
V12
NC/
Boot
6
15
5
13
4
17
12
16
20
1
19
9
10
3
11
18
8
2
7
OVP
PGd
Ct/Rt
C12
R8
C13
C11
R7
R9
R5
C9
D1
C4
Ta (
C)
Device Package VID Voltage Range
0 TO 70
US3011CW 20 pin Plastic SOIC WB 1.3V to 3.5V
4-2
Rev. 1.3
12/8/00
US3011
ELECTRICAL SPECIFICATIONS
ELECTRICAL SPECIFICATIONS
Unless otherwise specified ,these specifications apply over ,V
12
= 12V, V
5
= 5V and Ta=0 to 70
C. Typical values
refer to Ta =25
C. Low duty cycle pulse testing are used which keeps junction and case temperatures equal to the
ambient temperature.
ABSOLUTE MAXIMUM RATINGS
ABSOLUTE MAXIMUM RATINGS
V5 supply Voltage ........................................... 7V
V12 Supply Voltage ............................................ 20V
Storage Temperature Range ................................. -65 TO 150
C
Operating Junction Temperature Range .......... 0 TO 125
C
PACKAGE INFORMATION
PACKAGE INFORMATION
20 PIN WIDE BODY PLASTIC SOIC (W)
JA
=85
C/W
PARAMETER
SYM TEST CONDITION
MIN
TYP
MAX
UNITS
VID Section
DAC output voltage
0.99Vs
Vs
1.01Vs
V
(note 1)
DAC Output Line Regulation
0.1
%
DAC Output Temp Variation
0.5
%
VID Input LO
0.4
V
VID Input HI
2
V
VID input internal pull-up
27
k
resistor to V5
Power Good Section
Under voltage lower trip point
Vout ramping down
0.89Vs
0.90Vs
0.91Vs
V
Under voltage upper trip point
Vout ramping up
0.92Vs
V
UV Hysterises
.015Vs
.02Vs
.025Vs
V
Over voltage upper trip point
Vout ramping up
1.09Vs
1.10Vs
1.11Vs
V
Over voltage lower trip point
Vout ramping down
1.08Vs
V
OV Hysterises
.015Vs
.02Vs
.025Vs
V
Power Good Output LO
RL=3mA
0.4
V
Power Good Output HI
RL=5K pull up to 5V
4.8
V
Soft Start Section
Soft Start Current
CS+ =0V , CS- =5V
10
uA
4
3
2
1
7
6
5
18
19
20
TOP VIEW
11
13
12
14
10
15
9
16
8
17
Ct
OVP
V12
LDrv
Gnd
NC
HDrv
CS-
PGd
NC
NC
CS+
SS
D0
D1
D2
D3
D4
V5
Vfb
US3011
4-3
Rev. 1.3
12/8/00
UVLO Section
UVLO Threshold-12V
Supply ramping up
9.2
10
10.8
V
UVLO Hysterises-12V
0.3
0.4
0.5
V
UVLO Threshold-5V
Supply ramping up
4.1
4.3
4.5
V
UVLO Hysterises-5V
0.2
0.3
0.4
V
Error Comparator Section
Input bias current
2
uA
Input Offset Voltage
-2
+2
mV
Delay to Output
Vdiff=10mV
100
nS
Current Limit Section
C.S Threshold Set Current
160
200
240
uA
C.S Comp Offset Voltage
-5
+5
mV
Hiccup Duty Cycle
Css=0.1 uF
2
%
Supply Current
Operating Supply Current
CL=3000pF
V5
20
V12
14
mA
Output Drivers Section
Rise Time
CL=3000pF
70
100
nS
Fall Time
CL=3000pF
70
130
nS
Dead band Time
CL=3000pF
100
200
300
nS
Oscillator Section
Osc Frequency
Ct=150pF
190
220
250
Khz
Osc Valley
0.2
V
Osc Peak
V5
V
Over Voltage Section
OVP Drive Current
50
mA
Note 1: Vs refers to the set point voltage given in Table 1.
D4
D3
D2
D1
D0
Vs
D4
D3
D2
D1
D0
Vs
0
1
1
1
1
1.30
1
1
1
1
1
2.0
0
1
1
1
0
1.35
1
1
1
1
0
2.1
0
1
1
0
1
1.40
1
1
1
0
1
2.2
0
1
1
0
0
1.45
1
1
1
0
0
2.3
0
1
0
1
1
1.50
1
1
0
1
1
2.4
0
1
0
1
0
1.55
1
1
0
1
0
2.5
0
1
0
0
1
1.60
1
1
0
0
1
2.6
0
1
0
0
0
1.65
1
1
0
0
0
2.7
0
0
1
1
1
1.70
1
0
1
1
1
2.8
0
0
1
1
0
1.75
1
0
1
1
0
2.9
0
0
1
0
1
1.80
1
0
1
0
1
3.0
0
0
1
0
0
1.85
1
0
1
0
0
3.1
0
0
0
1
1
1.90
1
0
0
1
1
3.2
0
0
0
1
0
1.95
1
0
0
1
0
3.3
0
0
0
0
1
2.00
1
0
0
0
1
3.4
0
0
0
0
0
2.05
1
0
0
0
0
3.5
Table 1 - Set point voltage vs. VID codes
4-4
Rev. 1.3
12/8/00
US3011
PIN DESCRIPTIONS
PIN DESCRIPTIONS
Pin Description
LSB input to the DAC that programs the output voltage. This pin can be pulled up exter-
nally by a 10k resistor to either 3.3V or 5V supply.
Input to the DAC that programs the output voltage.This pin can be pulled up externally by
a 10k
resistor to either 3.3V or 5V supply.
Input to the DAC that programs the output voltage.This pin can be pulled up externally by
a 10k resistor to either 3.3V or 5V supply.
MSB input to the DAC that programs the output voltage.This pin can be pulled up exter-
nally by a 10k resistor to either 3.3V or 5V supply.
This pin selects a range of output voltages for the DAC.
This pin is an open collector output that switches LO when the output of the converter is
not within
10% (typ) of the nominal output voltage.When PWRGD pin switches LO the
sat voltage is less than 0.4V at 3mA.
This pin is connected directly to the output of the Core supply to provide feedback to the
Error comparator.
This pin is connected to the Drain of the power MOSFET of the Core supply and it
provides the positive sensing for the internal current sensing circuitry. An external resis-
tor programs the C.S threshold depending on the Rds of the power MOSFET. An external
capacitor is placed in parallel with the programming resistor to provide high frequency
noise filtering.
This pin is connected to the Source of the power MOSFET for the Core supply and it
provides the negative sensing for the internal current sensing circuitry.
This pin provides the soft start for the switching regulator. An internal current source
charges an external capacitor that is conected from this pin to the GND which ramps up
the outputs of the switching regulator, preventing the outputs from overshooting as wellas
limiting the input current. The second function of the Soft Start cap is to provide long off
time for the synchronous MOSFET or the Catch diode (HICCUP) during current limiting.
This pin programs the oscillator frequency in the range of 50 kHZ to 500kHZ with an
external capacitor connected from this pin to the GND.
This pin serves as the ground pin and must be conected directly to the ground plane. A
high frequency capacitor (0.1 to 1 uF) must be connected from V5 and V12 pins to this
pin for noise free operation.
Output driver for the synchronous power MOSFET.
Output driver for the high side power MOSFET.
This pin is connected to the 12 V supply and serves as the power Vcc pin for the output
drivers.A high frequency capacitor (0.1 to 1 uF) must be connected directly from this pin
to GND pin in order to supply the peak current to the power MOSFET during the transi-
tions.
5V supply voltage.
Over voltage comparator output.
No connect
PIN# PIN SYMBOL
4
D0
5
D1
6
D2
7
D3
8
D4
12
PGd
10
Vfb
2
CS+
13
CS-
3
SS
20
Ct
16
Gnd
17
LDrv
14
HDrv
18
V12
9
V5
19
OVP
15,11 NC
US3011
4-5
Rev. 1.3
12/8/00
BLOCK DIAGRAM
BLOCK DIAGRAM
Figure 1 - Simplified block diagram of the US3011.
PWM
Control
V12
V12
3011Ablk1-1.1
Osc
Slope
Comp
+
5Bit
DAC,
Ctrl
Logic
Enable
Soft
Start &
Fault
Logic
200uA
0.9Vset
1.1Vset
Vset
Enable
UVLO
Vset
Enable
D4
V5
V12
SS
PGd
CS-
Ct
CS+
LDrv
HDrv
Vfb
D3
D2
D1
D0
Over
Current
Enable
Gnd
1.18Vset
OVP
4-6
Rev. 1.3
12/8/00
US3011
TYPICAL APPLICATION
TYPICAL APPLICATION
SYNCHRONOUS OPERATION
(Dual Layout with HIP6004B)
Typical application of US3011 in an on board DC-DC converter providing the Core supply for
microprocessor.
Table of components that need to be modified to make the dual layout work for US3011
and HIP6004B.
Part #
R5
R7
R8
R9
C4
C7
C9
C11
C12
C13
D1
HIP6004B O
V
V
V
V
O
O
V
V
V
V
US3011
S
O
O
V
O
V
V
O
O
O
O
S - Short
O - Open
V - See Unisem or Harris parts list for the value.
Vcore
5V
12V
VID0
Power Good
3011app1-1.3
Q2
VID1
VID2
VID3
VID4
L1
C5
R1
C3
C6
Q1
R2
R3
C8
R4
C10
C14
C1
C7
C2
R6
14
HDrv
LDrv
NC/Gnd
SS
CS+
Gnd
NC/Sen
D3
D2
D4
D1
D0
US3011
Vfb
V5/Comp
CS-
V12
NC/
Boot
6
15
5
13
4
17
12
16
20
1
19
9
10
3
11
18
8
2
7
OVP
PGd
Ct/Rt
C12
R8
C13
C11
R7
R9
R5
C9
D1
C4
Vcc3
R13
L2
R10
R11
C15
R12
US3011
4-7
Rev. 1.3
12/8/00
US3011and HIP6004B Dual Layout Parts List
Ref Desig
Description
Qty
Part #
Manuf
Q1
MOSFET
1
IRL3103s, TO263 package
IR
Q2
MOSFET
1
IRL3103D1S, TO263 package
IR
L1
Inductor
1
L=1uH, 5052 core with 4 turns of
Micro Metal
1.0mm wire
L2
Inductor
1
L=2.7uH, 5052B core with 7 turns of
Micro Metal
1.2mm wire
C1
Capacitor, Electrolytic
1
10MV470GX, 470uF,10V
Sanyo
C2,9
Capacitor, Ceramic
2
1uF, 0603
C3
Capacitor, Electrolytic
2
10MV1200GX, 1200uF,10V
Sanyo
C5
Capacitor, Ceramic
1
220pF, 0603
C6
Capacitor, Ceramic
1
1uF, 0805
C7
Capacitor, Ceramic
1
150pF, 0603
C8
Capacitor, Ceramic
1
1000pF, 0603
C10
Capacitor, Electrolytic
6
6MV1500GX, 1500uF,6.3V
Sanyo
C14
Capacitor, Ceramic
1
0.1uF, 0603
C15
Capacitor, Ceramic
1
4.7uF, 1206
R1
Resistor
1
3.3k
, 5%, 0603
R2,3,4
Resistor
3
4.7
, 5%, 1206
R5
Resistor
1
0
, 0603
R6
Resistor
1
10k
, 5%, 0603
R9
Resistor
1
100
, 1%, 0603
R10
Resistor
1
220
, 1%, 0603
R11
Resistor
1
330
, 1%, 0603
R12
Resistor
1
22k
, 1%, 0603
R13
Resistor
1
10
, 5%, 0603
Note 1: R10, R11, C15, R9, and R12 set the Vcore 2% higher for level shift to reduce CPU Transient Voltage.
4-8
Rev. 1.3
12/8/00
US3011
Application Information
Application Information
An example of how to calculate the components for the
application circuit is given below.
Assuming, two sets of output conditions that this regu-
lator must meet,
a) Vo=2.8V , Io=14.2A,
Vo=185mV,
Io=14.2A
b) Vo=2V , Io=14.2A,
Vo=140mV,
Io=14.2A
The regulator design will be done such that it meets the
worst case requirement of each condition.
Output Capacitor Selection
The first step is to select the output capacitor. This is
done primarily by selecting the maximum ESR value
that meets the transient voltage budget of the total
Vo
specification. Assuming that the regulators DC initial
accuracy plus the output ripple is 2% of the output volt-
age, then the maximum ESR of the output capacitor is
calculated as :
ESR
=
100
14 2
7
.
m
The Sanyo M VGX series is a good choice to achieve
both the price and perform ance goal
s. The 6M V1500GX
, 1500uF, 6.3V has an ESR of less than 36 m
typ .
Selecting 6 of these capacitors in parallel has an ESR
of
6 m
which achieves our low ESR goal.
Other type of Electrolytic capacitors from other manu-
facturers to consider are the Panasonic "FA" series or
the Nichicon "PL" series.
Reducing the Output Capacitors Using Voltage Level
Shifting Technique
The trace resistance or an external resistor from the output
of the switching regulator to the Slot 1 can be used to
the circuit advantage and possibly reduce the number
of output capacitors, by level shifting the DC regu-
lation point when transitioninig from light load to
full load and vice versa
. To accomplish this, the out-
put of the regulator is typically set about half the DC
drop that results from light load to full load. For example,
if the total resistance from the output capacitors to the
Slot 1 and back to the GND pin of the device is 5m
and
if the total
I, the change from light load to full load is
14A, then the output voltage measured at the top of the
resistor divider which is also connected to the output
capacitors in this case, must be set at half of the 70 mV
or 35mV higher than the DAC voltage setting.
This intentional voltage level shifting during the load tran-
sient eases the requirement for the output capacitor ESR
at the cost of load regulation. One can show that the
new ESR requirement eases up by half the total trace
resistance.
For example, if the ESR requirement of the
output capacitors without voltage level shifting must be
7m
then after level shifting the new ESR will only need
to be 8.5m
if the trace resistance is 5m
(7+5/2=9.5).
However, one must be careful that the combined "volt-
age level shifting" and the transient response is still within
the maximum tolerance of the Intel specification. To in-
sure this, the maximum trace resistance must be less
than:
Rs
2(Vspec - 0.02*Vo -
Vo)/
I
Where :
Rs=Total maximum trace resistance allowed
Vspec=Intel total voltage spec
Vo=Output voltage
Vo=Output ripple voltage
I=load current step
For example, assuming:
Vspec=
140 mV=
0.1V for 2V output
Vo=2V
Vo=assume 10mV=0.01V
I=14.2A
Then the Rs is calculated to be:
Rs
2(0.140 - 0.02*2 - 0.01)/14.2=12.6m
However, if a resistor of this value is used, the maximum
power dissipated in the trace (or if an external resistor is
being used) must also be considered. For example if
Rs=12.6 m
, the power dissipated is
(Io^2)*Rs=(14.2^2)*12.6=2.54W. This is a lot of power to
be dissipated in a system. So, if the Rs=5m
, then the
power dissipated is about 1W which is much more ac-
ceptable. If level shifting is not implemented, then the
maximum output capacitor ESR was shown previously
to be 7m
which translated to
6 of the 1500uF,
6MV1500GX type Sanyo capacitors. With Rs=5m
, the
maximum ESR becomes 9.5m
which is equivalent to
4 caps. Another important consideration is that if a
trace is being used to implement the resistor, the
power dissipated by the trace increases the case
temperature of the output capacitors which could
seriously effect the life time of the output capaci-
tors.
Output Inductor Selection
The output inductance must be selected such that un-
der low line and the maximum output voltage condition,
the inductor current slope times the output capacitor
ESR is ramping up faster than the capacitor voltage is
US3011
4-9
Rev. 1.3
12/8/00
drooping during a load current step. However if the in-
ductor is too small , the output ripple current and ripple
voltage become too large. One solution to bring the ripple
current down is to increase the switching frequency ,
however that will be at the cost of reduced efficiency and
higher system cost. The following set of formulas are
derived to achieve the optimum performance without
many design iterations.
The maximum output inductance is calculated using the
following equation :
L = ESR * C * ( Vinmin - Vomax ) / ( 2*
I )
Where :
Vinmin = Minimum input voltage
For Vo = 2.8 V ,
I = 14.2 A
L =0.006 * 9000 * ( 4.75 - 2.8) / (2 * 14.2) = 3.7 uH
Assuming that the programmed switching frequency is
set at 200 KHZ , an inductor is designed using the
Micrometals' powder iron core material. The summary
of the design is outlined below :
The selected core material is Powder Iron , the
selected core is T50-52D from Micro Metal wounded
with 8 Turns of # 16 AWG wire, resulting in 3 uH
inductance with
3 m
of DC resistance.
Assuming L = 3 uH and the switching frequency ; Fsw =
200 KHZ , the inductor ripple current and the output
ripple voltage is calculated using the following set of
equations :
T = 1/Fsw
T
Switching Period
D
( Vo + Vsync ) / ( Vin - Vsw + Vsync )
D
Duty Cycle
Ton = D * T
Vsw
High side Mosfet ON Voltage = Io * Rds
Rds
Mosfet On Resistance
Toff = T - Ton
Vsync
Synchronous MOSFET ON Voltage=Io * Rds
Ir = ( Vo + Vsync ) * Toff /L
Ir
Inductor Ripple Current
Vo =
Ir * ESR
Vo
Output Ripple Voltage
In our example for Vo = 2.8V and 14.2 A load , Assum-
ing IRL3103 MOSFET for both switches with maximum
on resistance of 19 m
, we have :
T = 1 / 200000 = 5 uSec
Vsw =Vsync= 14.2*0.019=0.27 V
D
( 2.8 + 0.27 ) / ( 5 - 0.27 + 0.27 ) = 0.61
Ton = 0.61 * 5 = 3.1 uSec
Toff = 5 - 3.1 = 1.9 uSec
Ir = ( 2.8 + 0.27 ) * 1.9 / 3 = 1.94 A
Vo = 1.94 * .006 = .011 V = 11 mV
Power Component Selection
Assuming IRL3103 MOSFETs as power components,
we will calculate the maximum power dissipation as fol-
lows:
For high side switch the maximum power dissipation
happens at maximum Vo and maximum duty cycle.
Dmax
( 2.8 + 0.27 ) / ( 4.75 - 0.27 + 0.27 ) = 0.65
Pdh = Dmax * Io^2*Rds(max)
Pdh= 0.65*14.2^2*0.029=3.8 W
Rds(max)=Maximum Rds-on of the MOSFET at 125
C
For synch MOSFET, maximum power dissipation hap-
pens at minimum Vo and minimum duty cycle.
Dmin
( 2 + 0.27 ) / ( 5.25 - 0.27 + 0.27 ) = 0.43
Pds = (1-Dmin)*Io^2*Rds(max)
Pds=(1 - 0.43) * 14.2^2 * 0.029 = 3.33 W
Heatsink Selection
Selection of the heat sink is based on the maximum
allowable junction temperature of the MOSFETS. Since
we previously selected the maximum Rds-on at 125
C,
then we must keep the junction below this temperature.
Selecting TO220 package gives
jc=1.8
C/W ( From the
venders' datasheet ) and assuming that the selected
heatsink is Black Anodized , the Heat sink to Case ther-
mal resistance is ;
cs=0.05
C/W , the maximum heat
sink temperature is then calculated as :
Ts = Tj - Pd * (
jc +
cs)
Ts = 125 - 3.82 * (1.8 + 0.05) = 118
C
With the maximum heat sink temperature calculated in
the previous step, the Heat Sink to Air thermal resis-
tance (
sa) is calculated as follows :
Assuming Ta=35
C
T = Ts - Ta = 118 - 35 = 83
C Temperature Rise
Above Ambient
sa =
T/Pd
sa = 83 / 3.82 = 22
C/W
Next , a heat sink with lower
sa than the one calcu-
lated in the previous step must be selected. One way to
do this is to simply look at the graphs of the "Heat Sink
Temp Rise Above the Ambient" vs. the "Power Dissipa-
tion" given in the heatsink manufacturers' catalog and
select a heat sink that results in lower temperature rise
than the one calculated in previous step. The following
heat sinks from AAVID and Thermaloy meet this crite-
ria.
Co.
Part #
Thermalloy
6078B
AAVID
577002
4-10
Rev. 1.3
12/8/00
US3011
Following the same procedure for the Schottcky diode
results in a heatsink with
sa = 25
C/W. Although it is
possible to select a slightly smaller heatsink, for sim-
plicity the same heatsink as the one for the high side
MOSFET is also selected for the synchronous MOSFET.
Switcher Current Limit Protection
The PWM controller uses the MOSFET Rds-on as the
sensing resistor to sense the MOSFET current and com-
pares to a programmed voltage which is set externally
via a resistor (Rcs) placed between the drain of the
MOSFET and the "CS+" terminal of the IC as shown in
the application circuit. For example, if the desired cur-
rent limit point is set to be 22A and from our previous
selection, the maximum MOSFET Rds-on=19m
, then
the current sense resistor, Rcs is calculated as :
Vcs=IcL*Rds=22*0.019=0.418V
Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1k
Where: Ib=200uA is the internal current setting of the
device
Switcher Timing Capacitor Selection
The switching frequency can be programmed using an
external timing capacitor. The value of Ct can be ap-
proximated using the equation below:
Switcher Output Voltage Adjust
As it was discussed earlier,the trace resistance from
the output of the switching regulator to the Slot 1 can be
used to the circuit advantage and possibly reduce the
number of output capacitors, by level shifting the DC
regulation point when transitioninig from light load to full
load and vice versa. To account for the DC drop, the
output of the regulator is typically set about half the DC
drop that results from light load to full load. For example,
if the total resistance from the output capacitors to the
F
C
Where
C =Ti
g Capacitor
F
Switching Frequency
SW
T
T
SW
=
-
3 5 10
5
.
:
min
If, F =
kHz :
C
pF
SW
T
200
3 5 10
200 10
175
5
3
=
-
.
Slot 1 and back to the GND pin of the device is 5m
and
if the total
I, the change from light load to full load is
14A, then the output voltage measured at the top of the
resistor divider which is also connected to the output
capacitors in this case, must be set at half of the 70 mV
or 35mV higher than the DAC voltage setting. To do this,
the top resistor of the resistor divider, Rtop is set at 100
,
and the bottom resistor, Rb is calculated. For example,
if DAC voltage setting is for 2.8V and the desired output
under light load is 2.835V, then Rb is calculated using
the following formula :
Rb= 100*
{
Vdac /(Vo - 1.004*Vdac)
}
[]
Rb= 100*
{
2.8 /(2.835 - 1.004*2.800)
}
= 11.76 k
Select 11.8 k
, 1%
Note: The value of the top resistor must not exceed
100
. The bottom resistor can then be adjusted to raise
the output voltage.
Soft Start Capacitor Selection
The soft start capacitor must be selected such that dur-
ing the start up when the output capacitors are charging
up, the peak inductor current does not reach the current
limit treshold. A minimum of 1uF capacitor insures this
for most applications. An internal 10uA current source
charges the soft start capacitor which slowly ramps up
the inverting input of the PWM comparator Vfb3. This
insures the output voltage to ramp at the same rate as
the soft start cap thereby limiting the input current. For
example, with 1uF and the 10uA internal current source
the ramp up rate is (
V/
t)=I/C = 1V/100mS. Assum-
ing that the output capacitance is 9000uF, the maxi-
mum start up current will be:
I=9000uF*(1V/100mS)=0.09A
Input Filter
It is highly recommended to place an inductor between
the system 5V supply and the input capacitors of the
switching regulator to isolate the 5V supply from the
switching noise that occurs during the turn on and off of
the switching components. Typically an inductor in the
range of 1 to 3 uH will be sufficient in this type of appli-
cation.
Switcher External Shutdown
The best way to shutdown the part is to pull down on the
soft start pin using an external small signal transistor
such as 2N3904 or 2N7002 small signal MOSFET. This
allows slow ramp up of the output, the same as the power
up.
US3011
4-11
Rev. 1.3
12/8/00
Layout Considerations
Switching regulators require careful attention to the lay-
out of the components, specifically power components
since they switch large currents. These switching com-
ponents can create large amount of voltage spikes and
high frequency harmonics if some of the critical compo-
nents are far away from each other and are connected
with inductive traces. The following is a guideline of how
to place the critical components and the connections
between them in order to minimize the above issues.
Start the layout by first placing the power components:
1) Place the input capacitors C3 and the high side
mosfet ,Q1 as close to each other as possible
2) Place the synchronous mosfet,Q2 and the Q1 as
close to each other as possible with the intention that
the source of Q1 and drain of the Q2 has the shortest
length.
3) Place the snubber R4 & C7 between Q1 & Q2.
4) Place the output inductor ,L2 and the output capaci-
tors ,C10 between the mosfet and the load with output
capacitors distributed along the slot 1 and close to it.
5) Place the bypass capacitors, C6 and C9 right next to
12V and 5V pins. C6 next to the 12V, pin 18 and C9
next to the 5V, pin 9.
6) Place the IC such that the pwm output drives, pins
14 and 17 are relatively short distance from gates of Q1
and Q2.
7) If the ouput voltage is to be adjusted, place resistor
dividers close to the feedback pin.
Note 1: Although, the device does not require resistor
dividers and the feedback pin can be directly connected
to the output, they can be used to set the outputs slightly
higher to account for any output drop at the load due to
the trace resistance. See the application note.
8) Place timing capacitor C7 close to pin20 and soft
start capacitor C2 close to pin 3.
Component connections:
Note : It is extremely important that no data bus
should be passing through the switching regulator
section specifically close to the fast transition nodes
such as PWM drives or the inductor voltage.
Using 4 layer board, dedicate on layer to GND, another
layer as the power layer for the 5V, 3.3V and Vcore.
Connect all grounds to the ground plane using di-
rect vias to the ground plane.
Use large low inductance/low impedance plane to con-
nect the following connections either using component
side or the solder side.
a) C3 to Q1 Drain
b) Q1 Source to Q2 Drain
c) Q2 drain to L2
d) L2 to the output capacitors, C10
e) C10 to the slot 1
f) Input filter L1 to the C3
Connect the rest of the components using the shortest
connection possible