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POWER MANAGEMENT

SC2602L

Soft Start

Initially, SS/SHDN 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/SHDN. 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 ex-

ternal resistor from the V

age drop across this resistor is due to the 200µA inter-

nal sink sets the voltage at the pin. This voltage is com-

pared to the voltage at the PHASE node. This compari-

son is made only when the high-side drive is high to

avoid false current limit triggering due to uncontributing

measurements from the MOSFETs 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 synchronous switch turns off and SS/

SHDN starts to sink 1.5µA. When SS/SHDN reaches

0.8V, it then starts to source 10µA and a new cycle be-

gins.

Hiccup Mode

During power up, the SS/SHDN 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 200kHz, switching oc-

curs. As the error signal crosses over the oscillator sig-

nal, the duty cycle of the PWM signal continues to in-

crease until the output comes into regulation. If an over-

current condition has not occurred the soft-start voltage

will continue to rise and level off at about 2.2V.

Applications Information (Cont.)

An over-current condition occurs when the high-side drive

is turned on, but the PHASE node does not reach the

voltage level set at the OCSET pin. The PHASE node is

sampled only once per cycle during the valley of the tri-

angular oscillator. Once an over-current occurs, the high-

side drive is turned off and the low-side drive turns on

and the SS/SHDN pin begins to sink 1.5µA. The soft-

start voltage will begin to decrease as the 1.5µA of cur-

rent discharges the external capacitor. When the soft-

start voltage reaches 0.8V, the SS/SHDN pin will begin

to source 10µA and begin to charge the external capaci-

tor causing the soft-start voltage to rise again. Again,

when the soft-start voltage reaches the level of the in-

ternal oscillator, switching will occur.

If the over-current condition is no longer present, nor-

mal operation will continue. If the over-current condition

is still present, the SS/SHDN pin will again begin to sink

1.5µA. This cycle will continue indefinitely until the over-

current condition is removed.

In conclusion, below is shown a typical “12V Application

Circuit” which has a BSTH voltage derived by

bootstrapping input voltage to the PHASE node through

diode D1. This circuit is very useful in cases where only

input power of 12V is available.

In order to prevent substrate glitching, a small-signal di-

ode should be placed in close proximity to the chip with

cathode connected to PHASE and anode connected to

PGND.