MIC2204

Micrel

M0214-012904

10

January 2004

PWM Operation

The MIC2204 is a pulsewidth modulation (PWM) controller.

By controlling the ratio of on-to-off time, or duty cycle, a

regulated DC output voltage is achieved. As load or supply

voltage changes, so does the duty cycle to maintain a

constant output voltage. In cases where the input supply runs

into a dropout condition, the MIC2204 will run at 100% duty

cycle.

The MIC2204 provides constant switching at 2MHz with

synchronous internal MOSFETs. The internal MOSFETs

include a high-side P-Channel MOSFET from the input

supply to the switch pin and an N-Channel MOSFET from the

switch pin to ground. Since the low-side N-Channel MOSFET

provides the current during the off cycle, a free wheeling

Schottky diode from the switch node to ground is not required.

PWM control provides fixed frequency operation. By main-

taining a constant switching frequency, predictable funda-

mental and harmonic frequencies are achieved. Other meth-

ods of regulation, such as burst and skip modes, have

frequency spectrums that change with load and can interfere

with sensitive communication equipment.

Synchronization

SYNC_IN allows the user to change the frequency from

2MHz up to 2.5MHz or down to 1.8MHz. This controls the

fundamental frequency and all the resultant harmonics. Main-

taining a predictable frequency creates the ability to either

shift the harmonics away from sensitive carrier and IF fre-

quency bands, or to accurately filter out specific harmonic

frequencies.

Connecting the SYNC_OUT function pin to the SYNC_IN of

other MIC2204s will synchronize multiple MIC2204s in a

daisy-chain. Synchronizing multiple MIC2204s means that

regulators will run at the same fundamental frequency, result-

ing in matched harmonic frequencies and simplifying design

for sensitive communication equipment.