SS6638G datasheet(15/19 Pages) SSC

15 / 19 page

www.SiliconStandard.com

15 of 19

APPLICATION INFORMATION

GENERAL DESCRIPTION

The SS6638G PFM (pulse frequency modulation)

converter IC combines a switch mode converter,

N-channel

power

MOSFET,

precision

voltage

reference,

and

voltage

detector

single

monolithic device. It offers both extreme low

quiescent current, high efficiency, and very low

gate threshold voltage to ensure start-up with low

battery voltage (0.8V typ.). Designed to maximize

battery life in portable products, it minimizes

switching losses by only switching as needed

to service the load.

PFM converters transfer a discrete amount of

energy per cycle and regulate the output voltage

by modulating the switching frequency with a

constant pulse width. Switching frequency

depends on load, input voltage, and inductor

value, and it can range up to 100kHz. The SW

on-resistance is typically 1 to 1.5

Ω to minimize

switching losses.

When

the

output

voltage

drops,

the

error

comparator enables the 100KHz oscillator that turns

the MOSFET on for about 7.5

µs and off for 2.5µs.

Turning on the MOSFET allows inductor current to

ramp up, storing energy in a magnetic field. When

The MOSFET turns off, inductor current is forced

through the diode to the output capacitor and load. As

the stored energy is depleted, the current ramps

down until the diode turns off. At this point,

the inductor may ring due to residual energy and stray

capacitance. The output capacitor stores charge when

the current flowing through the diode is high, and

releases it when current is low, thereby maintaining

a steady voltage across the load.

As the load increases, the output capacitor

discharges faster and the error comparator initiates

cycles sooner, increasing the switching frequency.

The maximum duty cycle ensures adequate time for

energy transfer to the output during the second half

of each cycle. Depending on the circuit, PFM

converters operate in either discontinuous mode

or continuous conduction mode. Continuous

conduction mode means that the inductor current

does not ramp to zero during each cycle.

VIN

VOUT

EXT

IOUT

Isw

Ico

IIN

SS6638G

IPK

IIN

VEXT

ISW

IOUT

TDIS

Charge Co.

VSW

Discharge Co.

Discontinuous Conduction Mode

SS6638G

8/21/2005 Rev.2.3

SS6638G Datasheet(PDF) 15 Page - Silicon Standard Corp.

SS6638G Datasheet(HTML) 15 Page - Silicon Standard Corp.

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL