SC1402 datasheet(14/18 Pages) SEMTECH

14 / 18 page

SC1402

652 MITCHELL ROAD

NEWBURY PARK CA 91320

Multi-Output, Low-Noise Power Supply

Controller for Notebook Computers

December 15, 2000

Switching Losses Upper FET:

Note: switching losses exist on the upper FET only

because the clamp diode across the lower FET will

turn on prior to the lower FET turning on.

Where Ig is the gate driver current. This is equal to

1A for the SC1402.

Crss is the reverse transfer capacitance of the FET;

in this case it equals 240pF for the IRF7413.

So the total FET losses equate to:

FETS = 0.027+0.072+0.031 = 0.130W

Note that as Vin increases the power dissipation from

switching losses will also increase. This is especially

important if the input to the supply is from an AC

adapter. Therefore, it is necessary to check the cal-

culations with your maximum input voltage specifica-

tion. In addition, the distribution of power in the upper

and lower FET will change as input voltage in-

creases.

Other losses to consider are gate charge losses, in-

ductor switching and copper losses, and losses in the

input and output capacitors. All these items will de-

crease efficiency and need to be carefully analyzed to

obtain the highest efficiency possible, especially if

running off battery power.

Basic Application Circuit

The basic dual-output 3.3V / 5V synchronous buck

converter is shown in Figure 2. This circuit shows the

minimum requirements for successful operation. For

varying current levels, Table 3 provides a useful se-

lection of components for varying supply require-

ments and is based on the calculations described

previously. Input voltage ranges for all designs in the

table are 6.5V to 28V. Frequency used in calculations

is 300KHz.

High & Low

Side FET’s

IR IRF7901D1

Siliconix Si4412

IR IRF7413

Siliconix Si4412

IR IRF7805

Siliconix Si4410

Input

Capacitor

10uF, 30V Sanyo

OS-CON

2 X 10uF, 30V Sanyo

OS-CON

3 X 10uF, 30V Sanyo

OS-CON

Output

Capacitor

330uF, 6.3V/10V*

AVX TPS

2X 330uF, 6.3V/10V*

AVX TPS

4X 330uF, 6.3V/10V*

AVX TPS

Resistor

0.033

Ω, Dale

WSL2010-R033-F

0.02

Ω, Dale

WSL2010-R020-F

0.012

Ω, Dale

WSL2512-R012-F

Inductor

15uF, Coilcraft

DO-3316P-153

10uF, Coilcraft

DO3316P-103

4.7uF, Coilcraft

DO3316P-472

*10V for 5V SMPS, 6.3V for 3V SMPS

Table 3

0.031W

300000

240

OUT

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