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SC1102A

Synchronous Voltage Mode Controller for Distributed Power Supply Applications

Semtech Corporation 

SC1102/SC1102A

POWER MANAGEMENT

Applications Information (Cont.)

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.

RDS(ON) Current Limiting

The current limit threshold is set by connecting an exter-

nal resistor from the VCC supply to OCSET. The voltage

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 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 2µA. When SS/SHDN reaches 0.8V, it then starts

to source 10µA and a new cycle begins.

Hiccup Mode

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

low until VCC reaches the undervoltage lockout level of

4.2V. Once VCC has reached 4.2V, the SS/SHDN pin is

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/500kHz, switching

occurs. As the error signal crosses over the oscillator

signal, 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.

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 2uA. The soft-start

voltage will begin to decrease as the 2uA of current dis-

charges the external capacitor. When the soft-start volt-

age reaches 0.8V, the SS/SHDN pin will begin to source

10uA and begin to charge the external capacitor causing

the soft-start voltage to rise again. Again, when the soft-

start voltage reaches the level of the internal oscillator,

switching will occur.

If the over-current condition is no longer present, normal

operation will continue. If the over-current condition is

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

2uA. 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.

 2006 Semtech Corp.

6

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