MIC4685

Micrel

September 2002

9

MIC4685

Block Diagrams

SW

FB

R1

R2

IN

MIC4685

Internal

Regulator

Bootstrap

Charger

Enable

200kHz

Oscillator

Thermal

Shutdown

Reset

Current

Limit

Com-

parator

Error

Amp

Driver

1.235V

Bandgap

Reference

VV

R1

R2

1

R1 R2

V

V

1

V

1.235V

OUT

REF

OUT

REF

REF

=+









=−









=

FIgure 1. Adjustable Regulator

Functional Description

The MIC4685 is a variable duty cycle switch-mode regulator

with an internal power switch. Refer to the above block

diagram.

Supply Voltage

The MIC4685 operates from a +4V to +30V (34V transient)

unregulated input. Highest efficiency operation is from a

supply voltage around +12V. See the efficiency curves in the

Enable/Shutdown

The enable (EN) input is TTL compatible. Tie the input high

if unused. A logic-high enables the regulator. A logic-low

shuts down the internal regulator which reduces the current

to typically 150

µA when V

Feedback

An external resistive voltage divider is required from the

Table 1 and Table 2 for recommended resistor values.

Duty Cycle Control

A fixed-gain error amplifier compares the feedback signal

with a 1.235V bandgap voltage reference. The resulting error

amplifier output voltage is compared to a 200kHz sawtooth

waveform to produce a voltage controlled variable duty cycle

output.

A higher feedback voltage increases the error amplifier

output voltage. A higher error amplifier voltage (comparator

inverting input) causes the comparator to detect only the

peaks of the sawtooth, reducing the duty cycle of the com-

parator output. A lower feedback voltage increases the duty

cycle. The MIC4685 uses a voltage-mode control architec-

ture.

Output Switching

When the internal switch is ON, an increasing current flows

from the supply V

output capacitor C

inductor as the current increases with time.

When the internal switch is turned OFF, the collapse of the

magnetic field in L1 forces current to flow through fast

recovery diode D1, charging C

Output Capacitor

External output capacitor C

reduces ripple.

Return Paths

During the ON portion of the cycle, the output capacitor and

load currents return to the supply ground. During the OFF

portion of the cycle, current is being supplied to the output

capacitor and load by storage inductor L1, which means that

D1 is part of the high-current return path.