SC1402

© 2000 SEMTECH CORP.

652 MITCHELL ROAD

NEWBURY PARK CA 91320

Multi-Output, Low-Noise Power Supply

Controller for Notebook Computers

December 15, 2000

13

If your ESR value varies significantly from the calcu-

lated value and you don’t want to add more capaci-

tance or add a series resistor in the capacitor path as

described above. We recommend that you bench test

the supply over temperature to verify transient re-

sponse and operation of the SMPS.

Input Capacitor Selection

Input capacitor is selected based upon the input ripple

current demand of the converter. First determine the

input ripple current expected and then choose a capac-

itor to meet that demand.

The input RMS ripple current can be calculated as fol-

lows:

The worse case input RMS ripple current occurs at

50% duty cycle (D = 0.5 or Vin = 2 Vout) and therefore

under this condition the I

proximated by:

Therefore, for a maximum load current of 3.0A , the

input capacitors should be able to safely handle 1.5A

of ripple current. For the EVAL board there are two

such regulators that operate simultaneously. Each ca-

pable of 1.5A of ripple current, although it is impossible

for both regulators to be at 50% duty cycle at the same

time since they have different output voltages. For the

EVAL board, we chose four 10uF, 30V OS-CON ca-

pacitors, two for each supply. Each capacitor has a rip-

ple current capability of 1.38A at 100KHz, 45°C. Fol-

lowing the capacitor-derating chart for temperature and

frequency operation at 300KHz, two of these capaci-

tors in parallel will suffice, as calculated below:

The RMS ripple current is under a worst-case condition

at full load, 3A each when both SMPSs are on.

When the 5V output is at maximum ripple of 1.5A (D =

50%), the 3.3V output adds 1.41A of ripple current.

The maximum ripple current is then calculated by:

Conversely:

When the 3V output is at maximum ripple 1.5A (D =

50%), the 5V output adds 1.29A of ripple current.

The worse case ripple current is then calculated by:

Clearly, the combined input capacitor bank must be

chosen to handle 2A of ripple current under worst-

case conditions.

MOSFET Switches

After selecting the voltage and current requirements

of each MOSFET device for the upper and lower

switches, the next step is to determine their power

handling capability. For the EVAL board the IRF7413

met the voltage and current requirements. These are

30V, 9A FET’s. Based on 85

150

their power handling is calculated as follows:

Power Limit for Upper & Lower FET:

T

Each FET must not exceed 1.3W of power dissipa-

tion. The conduction losses for the upper & lower

FET can be determined. For the calculations below,

a nominal input voltage of 12V, for Vout = 3.3V, Iout

= 3A and f = 300KHz. The Rdson value for the upper

& lower FET is 11m

Ω. We will calculate the conduc-

tion losses and switching losses for each FET. From

the calculations below we are well within the 1.3W

dissipation limit as calculated above.

Conduction Losses Upper FET:

Conduction Losses Lower FET:

IN

OUT

OUT

IN

OUT

RMS

V

I

)

V

(V

V

I

•

−

=

•

2

I

I

LOAD

2.06A

1.41

1.5

I

2

2

RMS(MAX)

=

+

=

1.98A

1.29

1.5

I

2

2

RMS(MAX)

=

+

=

1.3W

50

85

150

T

T

P

JA

A

J

T

=

−

=

−

=

θ

2

DS

CU

I

D

R

P

•

•

=

0.027W

3

12

3.3

0.011

P

2

CU

=

•

•

=

2

DS

CL

I

D)

(1

R

P

•

−

•

=

0.072W

3

12

3.3

1

0.011

P

2

CL

=

•

ö

ç

è

æ −

•

=