www.SiliconStandard.com

8 of 12

SS6639

Pin 1:

GND:

Ground. Must be low impedance;

solder directly to ground plane.

Pin 2:

VOUT:

IC supply pin. Connect Vout to the

regular output.

Pin 3:

EXT:

Push-pull driver output for external

power. Switch.

General Description

The SS6639 PFM (pulse frequency modulation)

controller IC combines a switch mode regulator, a

push-pull driver, a precision voltage reference, and a

voltage detector in a single monolithic device. It

offers

extremely

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 controllers transfer a discrete amount of energy

per cycle and regulate the output voltage by

modulating the switching frequency with a constant

turn-on time. Switching frequency depends on load,

input voltage, and inductor value and can range up

to 100 KHz.

When the output voltage drops, the error comparator

enables the 100 kHz oscillator which turns the

MOSFET on for around 7.5us and off for 2.5

µs.

Turning on the MOSFET allows inductor current to

ramp up, storing energy in the magnetic field.

When the MOSFET turns off, the inductor forces

current through the diode to the output capacitor and

the 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 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

controllers can operate in either discontinuous mode

or continuous conduction mode. The continuous

conduction mode means that the inductor current

does not ramp to zero during each cycle.

+

V

IN

SW

V

OUT

EXT

I

D

I

OUT

Isw

Ico

I

IN

Discontinuous Conduction Mode

Rev.2.01 6/26/2003