MIC4682 datasheet(9/15 Pages) MICREL

9 / 15 page

Micrel, Inc.

MIC4682

June 2007

M9999-061507

Functional Diagram

OUT

MIC4682

Internal

Regulator

SHDN

ISET

200kHz

Oscillator

Thermal

Shutdown

Reset

Current

Limit

Com-

parator

Error

Amp

Switch

Driver

1.23V

Bandgap

Reference

GND

1.23V

R1 R2

–1

OUT

⎛

⎝⎜

⎞

⎠⎟

=−

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

Figure 1. MIC4682 Block Diagram

Functional Description

The MIC4862 is a constant frequency, voltage mode-

switching 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 comparator 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.

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