June 2001

7

MIC4690

MIC4690

Micrel

Block Diagrams

SW

FB

R1

R2

IN

MIC4690 [adj.]

Internal

Regulator

SHDN

500kHz

Oscillator

Thermal

Shutdown

Reset

Current

Limit

Com-

parator

Error

Amp

Driver

1.23V

Bandgap

Reference

VV

R1

R2

1

R1 R2

V

V

1

V

1.23V

OUT

REF

OUT

REF

REF

=+









=−









=

Adjustable Regulator

Functional Description

The MIC4690 is a variable duty cycle switch-mode regulator

with an internal power switch. Refer to the block diagrams.

Supply Voltage

The MIC4690 operates from a +4V to +30V (transients to

34V) unregulated input. Highest efficiency operation is from

a supply voltage around +12V. See the efficiency curve on

page 5.

Enable/Shutdown

enables the regulator. A logic-high shuts down the internal

regulator which reduces the current to typically 1.5

µA when

V

“Shutdown Input Behavior: Shutdown Hysteresis.”

Feedback

Fixed-voltage versions of the regulator have an internal

directly to the output voltage.

Adjustable versions require an external resistive voltage

divider from the output voltage to ground, center tapped to the

FB

pin. See Figure 1b for recommended resistor values.

Duty Cycle Control

A fixed-gain error amplifier compares the feedback signal

with a 1.23V bandgap voltage reference. The resulting error

amplifier output voltage is compared to a 500kHz 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 MIC4690 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.