sc1402 Semtech Corporation, sc1402 Datasheet - Page 11

• Manufacturer Part Number: sc1402
• Description: Multi-output, Low-noise Power Supply Controller For Notebook Computers
• Manufacturer: Semtech Corporation


APPLICATION INFORMATION
Introduction
The SC1402 is a versatile dual switching regulator ad-
justable from 2.5V to 5.5V with fixed 5V and 3.3V
modes. In addition, there are two on-chip 12V & 5V lin-
ear regulators capable of supplying 120mA & 50mA of
output current, respectively. The SC1402 is designed
for notebook applications but has applications any-
where high efficiency, small size and low cost are re-
quired.
The Semtech SC1402 EVAL board consists of a 3.3V,
3A switcher, a 5.0V, 3A switcher, an onboard 12V lin-
ear regulator. A 15V flyback supply developed off the
5V SMPS inductor delivers the input voltage to the on-
board 12V linear regulator.
Design Guidelines
The schematic for the EVAL board is shown on page
18. The EVAL board is configured as follows:
Switching Regulator 1
Switching Regulator 2
Linear Regulator 1
Linear Regulator 2
Designing the Output Filter
Before calculating the output filter inductance and out-
put capacitor, an acceptable amount of output ripple
current is to be determined. The ESR of the output ca-
pacitor multiplied by the ripple current sets the maxi-
mum allowable ripple voltage. So once the ripple volt-
age specification is selected, the capacitor ESR is cho-
sen usually based on capacitor cost and size con-
straints. This then sets the maximum output ripple cur-
rent through the capacitor and inductor.
For the EVAL board 3.3V switcher, we selected a max-
imum ripple voltage of 50mV. Choosing two 330uF,
6.3V tantalum capacitors C21 & C22, each having an
ESR of 100m , their combined ESR equaling 50m ,
sets the maximum ripple current as follows:
© 2000 SEMTECH CORP.
December 15, 2000
I
O
ESR

V
O

I
O
0.05
0.05
Vout1 = 3.3V, 3A
Vout2 = 5.0V, 3A
Vout3 = 12V, 120mA
Vout4 = 5V, 50mA
1Amp
Multi-Output, Low-Noise Power Supply
Controller for Notebook Computers
652 MITCHELL ROAD
L1
D
I
Be sure that the two output capacitors can handle this
ripple current. Ripple current specifications are found
in the capacitor data sheet for high quality capacitors
intended for use in switching power supplies.
The inductance can now be found:
These inductance and duty cycle equations exclude
any resistive drops due to winding resistance and
MOSFET on resistance.
Where:
For f = 300KHz, t = 1/f
From this calculation we choose L1 to be 10uH.
For a 200kHz operating frequency, L1 calculates to be
14.5uH.
Choosing Inductor & FET Current Rating
The current carrying capability of the inductor and
MOSFETs should be sized to handle the maximum
current of the supply set by the current sense resistor,
Rsense = R9. Here we use the minimum current
threshold value:
R9
Where Ipeak equals:
Ipeak = 3.5 Amps
R9
Here we chose a sense resistor of 20m . Now we can
determine FET and inductor current carrying capability
PEAK
V
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(V
80mV
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0.080
IN
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INMAX
PEAK
3.5
OUT
;
L1


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0.023
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t
NEWBURY PARK CA 91320
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3.33
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SC1402
11