5

 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2612E

Theory of Operation

The SC2612E is a step down DC/DC controller designed

for minimum cost and size without sacrificing accuracy

and protection. Overcurrent protection is implemented

by a simple undervoltage detection scheme and is dis-

abled until soft start has been completed to eliminate

false trips due to output capacitor charging. The SS/EN

pin is held low, as are the DH and DL pins, until the

undervoltage lockout points are exceeded. Once the VCC

and BST pins both rise above their undervoltage lockout

points, the SS capacitor begins to charge, controlling the

duty cycle of the switcher, and therefore slowly ramping

up the switcher output voltage. Once the SS capacitor is

charged, the current limit circuitry is enabled. If a short

circuit is applied , the output will be pulled down below

it’s trip point and shut down. The device may be restarted

by either cycling power, or momentarily pulling SS/EN low.

Component Selection

OUTPUT INDUCTOR

OUTPUT INDUCTOR

OUTPUT INDUCTOR

OUTPUT INDUCTOR

OUTPUT INDUCTOR - A good starting point for output

filter component selection is to choose an inductor value

that will give an inductor ripple current of approximately

20% of max. output current.

Inductor ripple current is given by:-

OSC

IN

O

O

RIPPLE

L

f

L

V

V

V

I

×

÷÷ø

ö

ççè

æ

-

×

=

1

So choose inductor value from:-

OSC

O

IN

O

O

f

I

V

V

V

L

×

÷÷ø

ö

ççè

æ

-

×

×

=

1

5

OUTPUT CAP

OUTPUT CAP

OUTPUT CAP

OUTPUT CAP

OUTPUT CAPAAAAACIT

CIT

CIT

CIT

CITOR(S)

OR(S)

OR(S)

OR(S)

OR(S) - The output capacitors should

be selected to meet output ripple and transient response

criteria. Output ripple voltage is caused by the inductor

ripple current flowing in the output capacitor’s ESR (There

is also a component due to the inductor ripple current

charging and discharging the output capacitor itself, but

this component is usually small and can often be ignored).

Given a maximum output voltage ripple requirement, ESR

is given by:-

OSC

IN

O

RIPPLE

O

ESR

f

L

V

V

V

V

R

×

÷÷ø

ö

ççè

æ

-

×

×

<

1

Output voltage transient excursions are a function of load

current transient levels, input and output voltages and

inductor and capacitor values.

Capacitance and R

sient condition can be calculated from:-

release)

(load

transients

positive

for

V

V

and

n)

applicatio

(load

transients

negative

for

V

V

V

where

V

V

I

L

C

I

V

R

O

A

O

IN

A

A

T

T

T

T

ESR

=

-

=

×

×

×

>

<

2

2

values for positive and negative transients must be cal-

culated seperately and the worst case value chosen. For

Capacitor values, the calculated value should be doubled

to allow for duty cycle limitation and voltage drop issues.