US3012/3012A
5 BIT PROGRAMMABLE SYNCHRONOUS BUCK
CONTROLLER IC
PRELIMINARY DATASHEET
FEATURES
DESCRIPTION
Dual Layout Compatible with RC5051
Designed to meet Intel specification of VRM8.2
& VRM8.3 for Pentium II
On board DAC programs the output voltage
from 1.3V to 3.5V (US3010) & 1.8V to 3.5V for
US3010A
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
Soft Start
High current totem pole driver for direct
driving of the external Power MOSFET
Power Good function
APPLICATIONS
Pentium II & Pentium Pro processor DC to DC
converter application
Low cost Pentium with AGP
The US3012 family of controller ICs are specifically designed to meet Intel specification for Pentium II and
Pentium Pro microprocessor applications as well as
the next generation P6 family processors. These products feature a patented topology that in combination
with a few external components as shown in the typical
application circuit below ,will provide in excess of 16A
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 feature, 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 .
Other features of the device are ; Undervoltage lockout
for both 5V and 12V supplies as well as an external
programmable soft start function as well as programming the oscillator frequency by using an external capacitor.
TYPICAL APPLICATION
L1
L2
R8
Q1
5V
C5
R2
R4
R7
R9
C7
C1
C3
Q2
C11
R6
R3
C4
R5
C6
R10
R1
R12
D1
12V
R13
R11
13
V12
6
V5
4
CS+
12
HDrv
5
CS-
9
LDrv
10
Gnd
7 NC/Vccp
14
Vfb/
GndD
En 2
OutEn
US3012
16 SS/Vref
C2
D4
8
D3
17
D2
18
D1
19
D0
20
VID4
NC/
GndP
11
NC/
GndA
15
Power Good
PGd 3
Ct
1
C9
C10
C8
VID3
VID2
VID1
3012app1-1.1
VID0
Notes: Pentium II and Pentium Pro are
trade marks of Intel Corp.
PACKAGE ORDER INFORMATION
Ta (°C)
0 TO 70
0 TO 70
Rev. 1.0
5/6/98
Device
US3012CW
US3012ACW
Package
20 pin Plastic SOIC WB
20 pin Plastic SOIC WB
VID Voltage Range
1.3V to 3.5V
1.8V to 3.5V
4-1
US3012/3012A
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
20 PIN WIDE BODY PLASTIC SOIC (W)
TOP VIEW
Ct 1
20 D0
En 2
19 D1
PGd 3
18 D2
CS+ 4
17 D3
CS- 5
16 SS
V5 6
15 Gnd
NC 7
14 Vfb
D4 8
13 V12
LDrv 9
PGnd 10
12 HDrv
11 NC
θJA =85°C/W
ELECTRICAL SPECIFICATIONS
Unless otherwise specified ,these specifications apply over ,V12 = 12V, V5 = 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.
PARAMETER
SYM TEST CONDITION
VID Section
DAC output voltage
(note 1)
DAC Output Line Regulation
DAC Output Temp Variation
VID Input LO
VID Input HI
VID input internal pull-up
resistor to V5
Power Good Section
Under voltage lower trip point
Vout ramping down
Under voltage upper trip point
Vout ramping up
UV Hysterises
Over voltage upper trip point
Vout ramping up
Over voltage lower trip point
Vout ramping down
OV Hysterises
Power Good Output LO
RL=3mA
Power Good Output HI
RL=5K pull up to 5V
Soft Start Section
Soft Start Current
CS+ =0V , CS- =5V
4-2
MIN
TYP
MAX
UNITS
0.99Vs
Vs
1.01Vs
V
0.1
0.5
0.4
%
%
V
V
kΩ
0.91Vs
V
V
V
V
V
V
V
V
2
27
0.89Vs
.015Vs
1.09Vs
.015Vs
0.90Vs
0.92Vs
.02Vs
1.10Vs
1.08Vs
.02Vs
4.8
10
.025Vs
1.11Vs
.025Vs
0.4
uA
Rev. 1.0
5/6/98
US3012/3012A
UVLO Section
UVLO Threshold-12V
UVLO Hysterises-12V
UVLO Threshold-5V
UVLO Hysterises-5V
Error Comparator Section
Input bias current
Input Offset Voltage
Delay to Output
Current Limit Section
C.S Threshold Set Current
C.S Comp Offset Voltage
Hiccup Duty Cycle
Supply Current
Operating Supply Current
Output Drivers Section
Rise Time
Fall Time
Dead band Time
Oscillator Section
Osc Frequency
Osc Valley
Osc Peak
Output Enable Section
Pull up Resistor to V5
HI Threshold Voltage
LO Threshold Voltage
Supply ramping up
Supply ramping up
9.2
0.3
4.1
0.2
10
0.4
4.3
0.3
-2
Vdiff=10mV
160
-5
200
Css=0.1 uF
10.8
0.5
4.5
0.4
V
V
V
V
2
+2
100
uA
mV
nS
240
+5
2
uA
mV
%
CL=3000pF
V5
V12
20
14
CL=3000pF
CL=3000pF
CL=3000pF
100
70
70
200
100
130
300
nS
nS
nS
Ct=150pF
190
220
250
0.2
Khz
V
V
mA
V5
35
2
0.8
kΩ
V
V
Note 1: Vs refers to the set point voltage given in Table 1.
D4
D3
D2
D1
D0
Vs
D4
D3
D2
0
1
1
1
1
1.30*
1
1
1
0
1
1
1
0
1.35*
1
1
1
0
1
1
0
1
1.40*
1
1
1
0
1
1
0
0
1.45*
1
1
1
0
1
0
1
1
1.50*
1
1
0
0
1
0
1
0
1.55*
1
1
0
0
1
0
0
1
1.60*
1
1
0
0
1
0
0
0
1.65*
1
1
0
0
0
1
1
1
1.70*
1
0
1
0
0
1
1
0
1.75*
1
0
1
0
0
1
0
1
1.80
1
0
1
0
0
1
0
0
1.85
1
0
1
0
0
0
1
1
1.90
1
0
0
0
0
0
1
0
1.95
1
0
0
0
0
0
0
1
2.00
1
0
0
0
0
0
0
0
2.05
1
0
0
* Output voltage is disabled for US3012A.
** Output voltage is disabled for all versions.
Table 1 - Set point voltage vs. VID codes
Rev. 1.0
5/6/98
D1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
D0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
Vs
**
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
4-3
US3012/3012A
PIN DESCRIPTIONS
PIN# PIN SYMBOL
20
D0
19
D1
18
D2
17
D3
8
D4
3
PGd
14
Vfb
4
CS+
5
CS-
16
SS
1
Ct
10
Gnd
9
12
13
LDrv
HDrv
V12
6
2
V5
OUTEN
7,11
15
N.C
4-4
Pin Description
LSB 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 10kΩ 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 externally by a 10k resistor to either 3.3V or 5V supply.
This pin selects a range of output voltages for the DAC. The voltage range for both the "A"
and the none "A" versions of the device is given in table 1.
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 resistor 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 transitions.
5V supply voltage.
This is the output enable pin.This pin is internally pulled high through a resistor to 5V
supply. A low signal on this pin disables the output.
No connect.
Rev. 1.0
5/6/98
US3012/3012A
BLOCK DIAGRAM
Vfb
En
Enable
V12
Vset
V12
HDrv
Enable
UVLO
PWM
Control
V5
+
Vset
D0
D1
D2
D3
D4
Enable
5Bit
DAC,
Ctrl
Logic
V12
Slope
Comp
LDrv
Osc
CS-
Soft
Start &
Fault
Logic
CS+
Over
Current
200uA
Enable
Ct
SS
1.1Vset
PGd
Gnd
PGnd
0.9Vset
3012Ablk1-1.1
Figure 1 - Simplified block diagram of the US3012/3012A.
Rev. 1.0
5/6/98
4-5
US3012/3012A
TYPICAL APPLICATION
SYNCHRONOUS OPERATION
(Dual Layout with RC5051)
L1
L2
R8
Q1
5V
C5
R2
R4
R7
R9
C7
C1
C3
Q2
C11
R6
R3
C4
R5
C6
R10
R1
R12
D1
12V
13
V12
6
V5
4
CS+
12
HDrv
5
CS-
9
LDrv
10
Gnd
7 NC/Vccp
R13
R11
14
Vfb/
GndD
En 2
OutEn
US3012
16 SS/Vref
C2
D4
8
D3
17
D2
18
D1
19
D0
20
NC/
GndP
11
NC/
GndA
15
VID4
Power Good
PGd 3
Ct
1
C9
C10
C8
VID3
VID2
VID1
3012app1-1.1
VID0
Typical application of US3012/3012A in an on board DC-DC converter providing the Core supply for the Pentium II
microprocessor.
Table of components that need to be modified to make the dual layout work for US3012A* and RC5051.
Part #
R1
RC5051
V
US3012A* S
R2
O
V
R4
O
S
R7
V
O
R8
V
S
R9
V
O
R10
O
V
R11
S
O
C2
V
V
C5
O
V
C8
V
V
D1
V
O
S - Short
O - Open
V - See Unisem or Raytheon parts list for the value.
* Table also applies to the none "A" version of the part.
Note 1 : R8 can be replaced with shorting wire of #20 AWG or lower. This elliminates the expensive current
sense resistor that otherwise is needed with RC5051.
4-6
Rev. 1.0
5/6/98
US3012/3012A
US3012/3012A and RC5051 Dual Layout Parts List
Ref Desig
Q1
Description
MOSFET
Qty
1
Q2
MOSFET
1
L1
L2
Inductor
Inductor
1
1
D1
C11
C3
C1
C2
C4
C5
C6
C7
C8
C9
C10
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
HS1
HS2
Diode
Capacitor ,Electrolytic
Capacitor ,Electrolytic
Capacitor ,Electrolytic
Capacitor , Ceramic
Capacitor , Ceramic
Capacitor , Ceramic
Capacitor , Ceramic
Capacitor , Ceramic
Capacitor , Ceramic
Capacitor , Ceramic
Capacitor , Ceramic
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Q1 Heatsink
Q2 Heatsink
1
5
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Part #
IRL3103
IRL3103S (Note 1)
IRL3103
IRL3103S (Note 1)
L=1 uH
Core:T50-18
Turns:14 AWG
L=2.5 uH R=2 mohm
open
6MV1500GX ,1500uF,6.3V,
6MV1500GX ,1500uF,6.3V,
680uF,10V, EEUFA1A681L
1 uF , SMT
1 uF , SMT
220 pF , SMT
1 uF , SMT
470 pF , SMT
150pF , X7R , SMT
0.01 uF , SMT
0.01 uF , SMT
Short, SMT
2.21 kΩ , 1% ,SMT
10Ω , 5%, SMT , 1206 size
Short, SMT
10Ω , 5%, SMT , 1206 size
10Ω , 5%, SMT , 1206 size
open , SMT
short , #20 AWG wire
open , SMT
100Ω, 1%, SMT
10 kΩ , 1% ,SMT
100 kΩ , 5%, SMT
See Raytheon Parts List , SMT
6270
6270
Manuf
IR
IR
Micro Metal
Sanyo
Sanyo
Panasonic
Thermalloy
Thermalloy
Note 1 : For the applications where it is desirable not to use the Heatsink, the IRL3103S MOSFET in the TO263
SMT package with 1″ square of pad area using top and bottom layers of the board as a minimum is required.
Rev. 1.0
5/6/98
4-7
US3012/3012A
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 regulator 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 voltage, then the maximum ESR of the output capacitor is
calculated as :
ESR ≤
100
= 7 mΩ
14.2
The Sanyo M V G X series is a good choice to achieve
both the price and perform ance goals.The 6M V1500G X
, 1500uF, 6.3V has an E S R of less than 36 m Ω typ .
Selecting 6 of these capacitors in parallel has an ESR
of ≈6 mΩ which achieves our low ESR goal.
Other type of Electrolytic capacitors from other manufacturers 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 regulation point when transitioninig from light load to
full load and vice versa. To accomplish this, 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
Slot 1 and back to the GND pin of the device is 5mΩ 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.
4-8
This intentional voltage level shifting during the load transient 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
7mΩ then after level shifting the new ESR will only need
to be 8.5mΩ if the trace resistance is 5mΩ (7+5/2=9.5).
However, one must be careful that the combined “voltage level shifting” and the transient response is still within
the maximum tolerance of the Intel specification. To insure 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=12.6mΩ
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 mΩ , 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=5mΩ, then the
power dissipated is about 1W which is much more acceptable. If level shifting is not implemented, then the
maximum output capacitor ESR was shown previously
to be 7mΩ which translated to ≈ 6 of the 1500uF,
6MV1500GX type Sanyo capacitors. With Rs=5mΩ, the
maximum ESR becomes 9.5mΩ 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 capacitors.
Output Inductor Selection
The output inductance must be selected such that under 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
Rev. 1.0
5/6/98
US3012/3012A
drooping during a load current step. However if the inductor 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 * ( Vinm in - Vom ax )/(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 mΩ 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 , Assuming IRL3103 MOSFET for both switches with maximum
on resistance of 19 mΩ, 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
Rev. 1.0
5/6/98
Power Component Selection
Assuming IRL3103 MOSFETs as power components,
we will calculate the maximum power dissipation as follows:
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 happens 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 θjc=1.8°C/W ( From the
venders’ datasheet ) and assuming that the selected
heatsink is Black Anodized , the Heat sink to Case thermal resistance is ; θcs=0.05°C/W , the maximum heat
sink temperature is then calculated as :
Ts = Tj - Pd * (θjc + θcs)
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 resistance (θsa) is calculated as follows :
Assuming Ta=35 °C
∆T = Ts - Ta = 118 - 35 = 83 °C Temperature Rise
Above Ambient
θsa = ∆T/Pd
θsa = 83 / 3.82 = 22 °C/W
Next , a heat sink with lower θsa than the one calculated 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 Dissipation” 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 criteria.
Co.
Part #
Thermalloy
6078B
AAVID
577002
4-9
US3012/3012A
Following the same procedure for the Schottcky diode
results in a heatsink with θsa = 25 °C/W. Although it is
possible to select a slightly smaller heatsink, for simplicity 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 compares 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 current limit point is set to be 22A and from our previous
selection,the m axim um M O SFET Rds-on=19m Ω, then
the current sense resistor, Rcs is calculated as :
Vcs=IcL*Rds=22*0.019=0.418V
Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1kΩ
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 Ct value can be approximated using the equation below:
3.5 × 10 −5
CT
Where :
CT=Ti min g Capacitor
FSW = Switching Frequency
FSW ≈
If, FSW=200 kHz :
CT ≈
3.5 × 10 −5
= 175 pF
200 × 10 3
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
4-10
Slot 1 and back to the GND pin of the device is 5mΩ 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(R10 in the application circuit) is set at 100Ω, and the R11 is calculated.
For example, if DAC voltage setting is for 2.8V and the
desired output under light load is 2.835V, then R11 is
calculated using the following formula :
R11= 100*{Vdac /(Vo - 1.004*Vdac)} [Ω]
R11= 100*{2.8 /(2.835 - 1.004*2.800)} = 11.76 kΩ
Select 11.8 kΩ , 1%
Note: The value of the top resistor must not exceed
100Ω
Ω . 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 during 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. Assuming that the output capacitance is 9000uF, the maximum 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 application.
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.
Rev. 1.0
5/6/98
US3012/3012A
Layout Considerations
Switching regulators require careful attention to the layout of the components, specifically power components
since they switch large currents. These switching components can create large amount of voltage spikes and
high frequency harmonics if some of the critical components 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:
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
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 capacitors ,C10 between the mosfet and the load with output
capacitors distributed along the slot 1 and close to it.
5) Place the bypass capacitors, C4 and C6 right next to
12V and 5V pins. C4 next to the 12V, pin 13 and C6
next to the 5V, pin 6.
6) Place the IC such that the pwm output drives, pins
12 and 9 are relatively short distance from gates of Q1
and Q2.
7) If the ouput voltage is to be adjusted, place resistor
dividers, R10 & R11 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 C8 close to pin1 and soft start
capacitor C2 close to pin 16.
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 direct vias to the ground plane.
Use large low inductance/low impedance plane to connect the following connections either using component
side or the solder side.
a) C3 to Q1 Drain
b) Q1 Source to Q2 Drain
Rev. 1.0
5/6/98
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