October 2003

9

M0334-102203

MIC4682

Micrel

Block Diagram

SW

FB

R1

R2

IN

MIC4682

Internal

Regulator

SHDN

ISET

200kHz

Oscillator

Thermal

Shutdown

Reset

Current

Limit

Com-

parator

Error

Amp

2A

Switch

Driver

1.23V

Bandgap

Reference

GND

VV

R1

R2

1

V

1.23V

R1 R2

V

V

1

R2

R1

V

V

–1

OUT

FB

FB

OUT

FB

OUT

FB

=+









=

=−









=









Figure 1. MIC4682 Block Diagram

Functional Description

The MIC4862 is a constant frequency, voltage mode-switch-

ing regulator. Referring to the block diagram, regulation is

achieved when the feedback voltage is equal to the band gap

reference. The FB pin senses the output voltage and feeds

into the input of the Error Amp. The output of the Error Amp

produces a positive voltage to compare with the 200kHz saw-

tooth waveform. These two signals are fed into the compara-

tor to generate the Pulse Width Modulation (PWM) signal to

turn on and off the internal switch. The duty cycle is defined

as the time the switch turns on divided by the period of the

saw-tooth oscillator. Initially, when power is applied to the IN

pin, the duty cycle is high because the feedback is close to

ground. As the output and feedback voltage start to rise, the

duty cycle decreases.

During the on time, current flows through the switch and into

the inductor until it reaches the peak current limit or the

maximum duty cycle which turns the switch off. The external

resistor at the ISET pin sets the peak current limit. The

maximum duty cycle is controlled by the Reset circuitry. At

this time, energy is stored in the inductor. The current charges

the output capacitor and supplies to the load. The Schottky

diode is reversed bias.

When the internal switch is off, the stored energy in the

inductor starts to collapse. The voltage across the inductor

reverses polarity and the inductor current starts to decrease.

The Schottky diode clamps the switch voltage from going too

negative and provides the path for the inductor current.

During the off time, the inductor and the output capacitor

provide current to the load.

An internal regulator provides power to the control circuitry

and the thermal protection circuitry turns off the internal

switch when the junction temperature exceeds about 160

°C.