5

Rev. 08/19/05

SP6128A Low Voltage, Synchronous Step Down PWM Controller

© Copyright 2005 Sipex Corporation

When the overcurrent threshold is exceeded, the

overcurrent comparator sets the fault latch and

terminates the output pulses. The controller

stops switching and goes through a hiccup se-

quence. This prevents excessive power dissipa-

tion in the external power MOSFETs during an

overload condition. An internal delay circuit

prevents that very short and mild overload con-

ditions, that could occur during a load transient,

activate the current limit circuit.

A low power sleep mode can be invoked in the

SP6128A by externally forcing the COMP pin

below 0.3V. Quiescent supply current in sleep

mode is typically less than 30

µA. An internal

5

µA pull-up current at the COMP pin brings the

SP6128A out of shutdown mode.

An internal 0.8V 1.5% reference allows out-

put voltage adjustment for low voltage appli-

cations.

The SP6128A also includes an accurate under-

voltage lockout that shuts down the controller

when the input voltage falls below 2.7V. Output

overvoltage protection is achieved by turning

off the high side switch and turning on the low

side N-channel MOSFET 100% of the time.

Enable

Low quiescent mode or “Sleep Mode” is initi-

ated by pulling the COMP pin below 0.3V with

an external open-drain or open-collector tran-

sistor. Supply current is reduced to 30

µA (typi-

cal) in shutdown. On power-up, assuming that

(2.8V), an internal 5

µA pull-up current at the

COMP pin brings the SP6128A out of shutdown

mode and ensures start-up. During normal oper-

ating conditions and in absence of a fault, an

internal clamp prevents the COMP pin from

swinging below 0.6V. This guarantees that dur-

ing mild transient conditions, due either to line

or load variations, the SP6128A does not enter

shutdown unless it is externally activated.

During Sleep Mode, the high side and low side

MOSFETS are turned off and the internal soft

start voltage is held low.

UVLO

Assuming that there is not shutdown condition

high side and low side MOSFETS off and the

If no faults are present, the SP6128A will ini-

Hysteresis (about 100mV) in the UVLO com-

parator provides noise immunity at start-up.

Soft Start

Soft start is required on step-down controllers to

prevent excess inrush current through the power

train during start-up. Typically this is managed

by sourcing a controlled current into a timing

capacitor and then using the voltage across this

capacitor to slowly ramp up either the error amp

reference or the error amp output (COMP). The

control loop creates narrow width driver pulses

while the output voltage is low and allows these

pulses to increase to their steady-state duty

cycle as the output voltage increases to its regu-

lated value. As a result of controlling the induc-

tor volt*second product during startup, inrush

current is also controlled.

In the SP6128A the duration of the soft-start is

controlled by an internal timing circuit that

provides a 0.3V/mS slew-rate, which is used

during startup and overcurrent to set the hiccup

time. The SP6128A implements soft-start by

ramping up the error amplifier reference voltage

providing a controlled slew-rate of the output

voltage, thereby preventing overshoot and in-

rush current at power up.

The presence of the output capacitor creates extra

dt requires an average sustained current in the

output capacitor and this current must be consid-

ered while calculating peak inrush current and

over current thresholds. An approximate expres-

sion to determine the excess inrush current due to

0.8V

OPERATION: continued