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SC1402

Multi-Output, Low-Noise Power Supply Controller for Notebook Computers

Semtech Corporation 

Multi-Output, Low-Noise Power Supply

Controller for Notebook Computers

SC1402

December 15, 2000

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

Vout1 = 3.3V, 3A

Switching Regulator 2

Vout2 = 5.0V, 3A

Linear Regulator 1

Linear Regulator 2

Vout3 = 12V, 120mA

Vout4 = 5V, 50mA

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 100mΩ, their combined ESR equaling 50mΩ,

sets the maximum ripple current as follows:

∆ IO

=

∆VO

ESR

∆IO

=

0.05

0.05

= 1Amp

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:

L1 =

(VINMAX

− VO ) • D • t

∆IO

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

D=

VO

VIN

(28

;L1 =

−

3.3)

•

3.3

28

1

• 3.33

• 10 −6

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

=

80mV

IPEAK

Where Ipeak equals:

IPEAK

= IOUT

+

∆IO

2

Ipeak = 3.5 Amps

R9

=

0.080

3.5

= 0.023 Ω

Here we chose a sense resistor of 20mΩ. Now we can

determine FET and inductor current carrying capability

© 2000 SEMTECH CORP.

11

652 MITCHELL ROAD NEWBURY PARK CA 91320

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