5

 2007 Semtech Corp.

www.semtech.com

SC2603A

POWER MANAGEMENT

Applications Information

Theory of Operation

The SC2603A is a versatile, low-cost, voltage-mode PWM

controller designed for DC/DC power supply applications.

In normal condition, the SC2603A operates at a fixed

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

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

fourth of 200kHz.

SC2603A 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.

Setting the Output Voltage

As shown in Fig.1, an external resistive divider R2 and R4

with its center tap tied to the FB pin sets the output volt-

age.

Soft-Start

Soft-start prevents a DC-DC converter from drawing exces-

sive 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 00mV

is reached at the EN/SS pin, the internal comp node is at

0.4V. The SC2603A 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 reference voltage.

Under Voltage Lockout

The under voltage lockout circuit of the SC2603A assures

that the Gate output remains in the off state whenever the

supply voltage drops below set parameters. Lockout occurs

rises above 4.2V.

MAX

IN

OUT

D





V

V

−

=

Maximum Duty Cycle

The maximum duty cycle, D

power conversion ratio

the supply voltage is higher than 6V, a Zener diode D2

is required to limit the SC2603A supply voltage, as shown

in Fig.1. The clamping voltage of D2 has to be not higher

than 5V.

Over Current Protection (OCP)

The over-current protection is implemented by a sensing

resistor and an internal current-limit comparator with a

threshold of 70mV. In each switching cycle, if the switching

current through the sensing resistor causing the voltage at

the CS pin reaches 70mV, the SC2603A will immediately

turn the gate pulse off to limit the current through the

power switch.

The figure below shows the current waveforms run into

over-current protection. At t1, the input current Iin begins

to increase as the load Iout builds up until it hits the OCP

threshold at t2. At t2, Vout begins to drop as Iout starts

increasing toward t3 while Vin and Iin remain the same.

At t3, there two events are happening. ) Vout is at half of

its regulating voltage. At this point, the internal oscillator

runs into frequency shift mode and shifts the frequency to

one fourth of the operating frequency. 2) The diode starts

conducting current without limit causing Vin to fall due to

supply current limit. Finally, Vin and Vout both drops to zero

and Iin becomes the input supply current limit.

A RC filter at the CS pin is necessary to filter out noise to

ensure accurate sensing.

The value of the sensing resistor can be calculated by the

following equation:

Rs = 70mV/Ipeak













−

=



V

25

.



V

R

R

OUT

4

2