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Date: 7/12/04

SP6655 High Efficiency 400mA Synchronous Buck Regulator

© Copyright 2004 Sipex Corporation

current demands. The converter goes in to a

standard pulse frequency modulation (PFM)

mode where the switching frequency is propor-

tional to the load current.

Low Dropout and Load Transient Operation

AND’ing the loop comparator also increases the

including 100%. 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 regu-

lation.

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, causing the loop compara-

tor to hold the converter 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 SP6655 is equal

the DC resistance of the inductor.

The SP6655 has been designed to operate in

dropout with a light load Iq of only 80

µA. The

on-time control circuit seamlessly operates the

converter between CCM, DCM, and low drop-

out modes without the need for compensation.

The converter’s transient response is quick since

there is no compensated error amplifier in the loop.

Inductor Over-Current Protection

To reduce the light load dropout Iq, the SP6655

400mA. The inductor over-current protection

circuitry is programmed to limit the peak induc-

tor current to 0.625A. This is done during the

on-time by comparing the source to drain volt-

age drop of the PMOS passing the inductor

current with a second voltage drop representing

the maximum 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

causing the inductor current to decrease. At the

end of the off-time, loop control is handed back

to the AND’d 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 over-

load conditions, the converter behaves like a

current source at the programmed limit minus

controlled by the equation:

Under-Voltage Lockout

The SP6655 is equipped with a programmable

under-voltage lockout to protect the input bat-

tery source from excessive currents when sub-

stantially discharged. When the input supply is

below the UVLO threshold both power switches

are open to prevent inductor current from flow-

ing. The three levels of falling input voltage

UVLO threshold are shown in Table 1, with a

typical hysteresis of 120mV to prevent chatter-

ing due to the impedance of the input source.

During UVLO, BLON is forced low.

Under-Current Detection

The synchronous rectifier is comprised of an

inductor discharge switch, a voltage compara-

tor, 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 inductor current approaches zero,

the channel voltage sign goes from negative to

positive, causing the comparator to trigger the

THEORY OF OPERATION