7

 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1110

Soft Start

Initially, SS/ENABLE sources 10µA of current to charge

an external capacitor. The outputs of the error amplifiers

are clamped to a voltage proportional to the voltage on

SS/ENABLE. This limits the on-time of the high-side

MOSFETs, thus leading to a controlled ramp-up of the

output voltages.

R

The current limit threshold is set by connecting an exter-

nal resistor from the V

drop across this resistor is due to the 200µA internal

sink sets the voltage at the pin. This voltage is compared

to the voltage at the PHASE node. This comparison is

made only when the high-side drive is high to avoid false

current limit triggering due to uncontributing measure-

ments from the MOSFET’s off-voltage. When the voltage

at PHASE is less than the voltage at OCSET, an overcurrent

condition occurs and the soft start cycle is initiated. The

Applications Information

synchronous switcher turns off and SS/ENABLE starts

to sink 2µA. When SS/ENABLE reaches 0.8V, it then

starts to source 10µA and a new cycle begins.

Hiccup Mode

During power up, the SS/ENABLE pin is internally pulled

low until VCC reaches the undervoltage lockout level of

4.2V. Once V

released and begins to source 10µA of current to the

external soft-start capacitor. As the soft-start voltage

rises, the output of the internal error amplifier is clamped

to this voltage. When the error signal reaches the level

of the internal triangular oscillator, which swings from 1V

to 2V at a fixed frequency of 250 kHz, switching occurs.

As the error signal crosses over the oscillator signal, the

duty cycle of the PWM signal continues to increase until

the output comes into regulation. If an over-current con-

dition has not occurred the soft-start voltage will con-

tinue to rise and level off at about 2.2V.