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© 2005 Semtech Corp.

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SC2603

POWER MANAGEMENT

Application Information

Soft-Start

Soft-start prevents a DC-DC converter from drawing

excessive current from the power source during start up.

If the soft-start time is made sufficiently long, then the

output will enter regulation without overshoot. An external

capacitor is connected from the EN/SS pin to the ground.

After the part’s input voltage surpasses the UVLO, the

EN/SS pin is allowed to charge its capacitor with 60µA.

When 100mV is reached at the EN/SS pin, the internal

comp node is at 0.4V. The SC2603 will start switching,

and the converter output comes into regulation. Because

the FB pin voltage follows EN/SS pin voltage, finally, the

EN/SS pin voltage will be clamped at 1.25V same as the

refernece voltage.

Setting the Output Voltage

An external resistive divider R

tap tied to the FB pin sets the output voltage.













−

=

1

25

.

1

2

1

OUT

V

R

R

Under Voltage Lockout

The under voltage lockout circuit of the SC2603 assures

that the gate driver output remains in the off state when-

ever the supply voltage drops below set parameters. Lock-

out occurs if V

sumes once V

Theory of Operation

The SC2603 is a versatile, low-cost, voltage-mode PWM

controller designed for DC/DC power supply applications.

In normal condition, the SC2603 operats at a fixed

200KHz. One exception is that when the FB pin voltage

drops to one half, the frequency will be shifted to one

fouth of 200KHz.

SC2603 features include a temperature compensated

voltage reference, triangle wave oscillator, current limit

comparator, frequency shift over-current protection, and

an internally compensated error amplifier. Pulse by pulse

current limiting is implemented by sensing the

differential voltage across an external resistor, or an

appropriately sized PC board trace.

VOUT

R2

R1

FB

1

SC2603