MIC2172YN datasheet(10/20 Pages) MICREL

10 / 20 page

Micrel

MIC2172/3172

April 2006

M9999-041806

(408) 955-1690

Functional Description

Refer to “Block Diagram MIC2172” and “Block Diagram

MIC3172.”

Internal Power

The MIC2172/3172 operates when VIN is

≥ 2.6V (and

VEN ≥ 2.0V for the MIC3172). An internal 2.3V regulator

supplies biasing to all internal circuitry including a

precision 1.24V band gap reference.

The enable control (MIC3172 only) enables or disables

the internal regulator which supplies power to all other

internal circuitry.

PWM Operation

The 100kHz oscillator generates a signal with a duty

cycle

approximately

90%.

The

current-mode

comparator output is used to reduce the duty cycle when

the current amplifier output voltage exceeds the error

amplifier output voltage. The resulting PWM signal

controls a driver which supplies base current to output

transistor Q1.

Current Mode Advantages

The MIC2172/3172 operates in current mode rather than

voltage mode. There are three distinct advantages to

this technique. Feedback loop compensation is greatly

simplified because inductor current sensing removes a

pole from the closed loop response. Inherent cycle-by-

cycle current limiting greatly improves the power switch

reliability and provides automatic output current limiting.

Finally, current-mode operation provides automatic input

voltage feed forward which prevents instantaneous input

voltage changes from disturbing the output voltage

setting.

Anti-Saturation

The anti-saturation diode (D1) increases the usable duty

cycle range of the MIC2172/3172 by eliminating the

base to collector stored charge which would delay Q1’s

turnoff.

Compensation

Loop stability compensation of the MIC2172/3172 can

be accomplished by connecting an appropriate network

from

either

COMP

circuit

ground

(Typical

Applications) or COMP to FB.

The error amplifier output (COMP) is also useful for soft

start and current limiting. Because the error amplifier

output is a transconductance type, the output impedance

is relatively high which means the output voltage can be

easily clamped or adjusted externally.

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