SC4810EITSTRT datasheet(13/24 Pages) SEMTECH

13 / 24 page

 2004 Semtech Corp.

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SC4810A/B/C/D/E

POWER MANAGEMENT

Application Information (Cont.)

VDD and LUVLO

SC4810 features three different input turn-on voltage

thresholds, as specified in the Electrical Characteristics

on page 2. V

REF starts to regulate when the supply voltage

on the VDD pin is above the turn-on voltage threshold.

REF drops to ground when VDD is lower than the turn-on

threshold minus the hysteresis value. The soft start cap

remains grounded as long as LUVLO is below the thresh-

old voltage 3V. The soft start cap will be charged up

through an internal 5uA current source when LUVLO is

above the threshold voltage.

Soft Start

The soft-start function is implemented by charging the

soft-start cap through an internal 5uA current source.

Under normal soft-start, the SS pin is discharged below

0.65V and ramps up to 1V, during which time the output

driving signals OUT1 and OUT2 are held low. During the

time when the SS pin is charged from 1V to 2.5V, soft-

start is implemented by an increasing output duty ratio.

The duty ratio is completely under the control of the

feedback after the SS pin is above 2.5V.

When the SS pin is pulled down below 0.5V, OUT1 and

OUT2 will be held low and the VREF pin will be grounded

via an internal FET.

Complementary Driving with Programmable Delays

The SC4810 features dual driving signals to drive two

power switches complementarily. This feature makes the

SC4810 suitable for a variety of applications in which

dual complimentary driving signals are needed. The

SC4810 even provides programmable driving delay as

an extra feature for applications such as active-clamp

forward topology. The users can program the driving delay

by adjusting the resistors tied to pin DELAY1 and pin

DELAY2 respectively to achieve the optimum delay for

each output. The delay of OUT1 is controlled by the

resistor tied to pin DELAY1 and the delay of OUT2 is

controlled by the resistor tied to pin DELAY2. For

illustration, see Fig. 8.

Over Current Protection

The SC4810 provides Hiccup mode over-current

protection when the sensed current signals are beyond

0.6V. When the hiccup mode over-current protection is

triggered, the soft-start cap will be discharged

immediately by an internal grounded FET. When the soft-

start pin SS is pulled down below 1V, OUT1 and OUT2 will

be disabled, and a soft re-start sequence will follow.

SC4810 can also be configured to implement cycle-by-

cycle over-current limit. As illustrated in Fig. 9, cycle-by-

cycle over-current limitation can be achieved by adjusting

the values of R1 and R2 to limit the voltage of FB pin to

less than the threshold voltage (0.6Volt) of the hiccup

over-current protection, using equations (3) and (4).

SC4810EITSTRT Datasheet(PDF) 13 Page - Semtech Corporation

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