SC1409

© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

1 A Synchronous Switching Regulator

June 28, 2000

7

COMPONENT SELECTION

Input Capacitor

The Input capacitor(s) must be capable of 0.5A RMS

ripple current. Total capacitance values of 22uF to

47uF are suitable with Esr of less than 350m

Ω. Typical

capacitors would be AVX TPS Tantalum surface

mount type TPSC476M010R350 or Panasonic Spe-

cialty Polymer Aluminum Electrolytic surface mount

type ECGC0KB470RA. Other types of capacitor are

also suitable, such as high value ceramic or Aluminum

electrolytics.

Inductor

Suitable Inductor values are in the range of 47uH to

330uH, higher inductor values give lower output volt-

age ripple and slower transient response time. Inductor

ripple current acts with the output capacitor ESR to

produce output voltage ripple. For a given output volt-

age ripple requirement, L can be calculated from

Output Capacitor

The output capacitor requirements are similar to the

input capacitor except ripple current is lower. Similar

capacitors will be suitable.

Soft Start Capacitor

The soft start capacitor value determines the soft start

time according to the following equation:-

C = 0.015t

where C is the soft start capacitor value in uF

and t is the soft start time in mS

Startup from a shutdown condition will take longer than

this since the soft start capacitor must charge from its

shutdown value to approximately 1V at a charge cur-

rent of 1uA before the shutdown is released and true

soft start begins.

Divider Resistors

The current through the divider chain should be at least

1000 times higher than the FB input bias current to

eliminate ratio errors. This gives a maximum value for

the bottom divider resistor (R4) of 24k

Ω

The top resistor (R2) can then be determined from

R2 = (Vo-1.203)·R4/1.203

Compensation

Normally, external compensation is not recommended,

however, if required, loop response can be tailored

with suitable values for R3, R5, C4 and C5. Consult the

factory for details.

C5

OPEN

R3

OPEN

Place C6 across pins 7 and 5

of SC1409, as close as

possible to package

U1

SC1409IS

8

1

2

3

4

5

6

7

ISET

SYNC/FSET

EN/SS

AGND

FB

PGND

PHASE

VIN

R2

1.74k*

GND

5V in

R4

1.0k

+

C1

47uF

C6

0.1uF

C3

0.033uF

C4

OPEN

+

C2

47uF

R5

OPEN

R1

OPEN

3.3V Out

L1

100uH

Coilcraft

DO3316P-104

Typical Application Circuit

()

IN

RIPPLE

OUT

OUT

IN

V

f

V

V

ESR

V

V

L

⋅

⋅

⋅

⋅

−

≥