SC4808BIMSTR datasheet(21/31 Pages) SEMTECH

21 / 31 page

 2005 Semtech Corp.

www.semtech.com

SC4808A/B

POWER MANAGEMENT

SOFT START

During start up of the converter, the discharged output ca-

pacitor and the load current have large supply current re-

quirements. To avoid this a soft start scheme is usually

implemented where the duty cycle of the regulator is gradu-

ally increased from 0% until the soft start duration is

elapsed.

SC4808 has an internal soft start circuit that limits the

duty cycle for a duration approximated by the formula be-

low. Also the soft start circuitry is activated if an over cur-

rent condition occurs. After an over current condition, OUTA

and OUTB are disabled and kept low for a duration of about

140µs. After the delay, the OUTA and OUTB are enabled

while the soft start limits the duty cycle. If the over current

condition persists, the soft start cycle repeats indefinitely.

Approximate internal soft start duration can be calculated

as below:

()

























≅

VREF

Ramp

GND

SlopeComp

Internal

SoftStart

If longer soft start durations are required, the simple exter-

nal circuit shown below can be implemented.

R27

15k

200p

C31

VCC

R28

2.2u,16V

C26

REF

0.1u,25V

C33

R33

10k

SYNC

SC4808

REF

LUVLO

SYNC

GND

OUTB

OUTA

VCC

R23

56.2k

Vin

56.2K

Csoft start

REF

MOCD207

C40

R37

Approximate soft start duration can be calculated as be-

low:

SoftStart

≅

Application Information (Cont.)

START UP SEQUENCE

Initially during the power up, the SC4808 is in under volt-

age lock out condition. As the Vcc supply exceeds the UVLO

limit of the SC4808, the internal reference, oscillator, and

logic circuitry are powered up.

The OUTA and OUTB drivers are not enabled until the line

under voltage lock out limit is reached. At that point, once

the FB pin is above 1.5V, soft start circuitry starts the out-

put drivers, and gradually increases the duty cycle from

0%. The soft start duration is internally set (see formula in

Soft Start section).

As the output voltage starts to increase, the error signal

from the error amplifier starts to decrease. If isolation is

required, the error amplifier output can drive the LED of

the opto isolator. The output of the opto is connected in a

common emitter configuration with a pull-up resistor to a

reference voltage connected to the FB pin of the SC4808.

The voltage level at the FB pin provides the duty cycle nec-

essary to achieve regulation.

If an over current condition occurs, the outputs are dis-

abled and after a soft start delay time of about 100µs, the

softstart sequence mentioned above is repeated.

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