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Date: 3/6/06

SP6656, 400mA Synchronous Buck Regulator with Dynamically Adjustable Voltage Output

© Copyright 2006 Sipex Corporation

Low Dropout and Load Transient

Operation

Using AND logic function the loop compara-

tor also increases the duty ratio past the

Under a light to heavy load transient, the

loop comparator will hold the main switch

on longer than the minimum on timer until

the output is brought back into regulation.

Also, as the input voltage supply drops

down close to the output voltage, the main

MOSFET resistance loss will dictate a much

higher duty ratio to regulate the output.

Eventually as the input voltage drops low

enough, the output voltage will follow, caus-

ing the loop comparator to hold the con-

verter at 100% duty cycle.

This mode is critical in extending battery life

when the output voltage is at or above the

minimum usable input voltage. The dropout

which the output regulation cannot be main-

tained. The dropout voltage of SP6656 is

The SP6656 has been designed to operate

in dropout with a light load Iq of only 80µA.

The on-time control circuit seamlessly op-

erates the converter between CCM, DCM,

and low dropout modes without the need for

compensation. The converter’s transient

response is quick since there is no compen-

sated error amplifier in the loop.

Inductor Over-Current Protection

To reduce the light load dropout Iq, the

SP6656 over-current system is only en-

current protection circuitry is programmed

to limit the peak inductor current to 0.625A.

This is done during the on-time by compar-

ing the source to drain voltage drop of the

PMOS passing the inductor current with a

second voltage drop representing the maxi-

mum allowable inductor current. As the two

voltages become equal, the over-current

comparator triggers a minimum off-time one

shot. The off-time one shot forces the loop

into the discharge phase for a minimum

decrease. At the end of the off-time, loop

control is handed back to the AND logic

function on-time signal. If the output voltage

is still low, charging begins until the output

is in regulation or the current limit has been

reached again. During startup and overload

conditions, the converter behaves like a

current source at the programmed limit mi-

nus half the current ripple. The minimum

Under-Voltage Lockout

The SP6656 is equipped with a program-

mable under-voltage lockout to protect the

input battery source from excessive cur-

rents when substantially discharged. When

the input supply is below the UVLO thresh-

old both power switches are open to pre-

vent inductor current from flowing. The level

of falling input voltage has a typical hyster-

esis of 120mV to prevent chattering due to

the impedance of the input source.

Under-Current Detection

The synchronous rectifier is comprised of

an inductor discharge switch, a voltage com-

parator, and a driver latch. During the off-

time, positive inductor current flows into the

PGND pin 9 through the low side NMOS

switch to LX pin 10, through the inductor

and the output capacitor, and back to pin 9.

The comparator monitors the voltage drop

across the discharge NMOS. As the induc-

tor current approaches zero, the channel

voltage sign goes from negative to positive,

causing the comparator to trigger the driver

latch and open the switch to prevent induc-

tor current reversal. This circuit along with

the on-timer puts the converter into PFM

mode and improves light load efficiency

THEORY OF OPERATION