MIC2172YN datasheet(8/17 Pages) MICREL

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MIC2172/3172

Micrel, Inc.

M9999-042205

April 2005

Functional Description

Refer to “Block Diagram MIC2172” and “Block Diagram

MIC3172.”

Internal Power

The MIC2172/3172 operates when V

IN 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 of

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 limit-

ing greatly improves the power switch reliability and provides

automatic output current limiting. Finally, current-mode op-

eration provides automatic input voltage feed forward which

prevents instantaneous input voltage changes from disturb-

ing 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 to 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.

Block Diagram MIC3172

Current

Amp.

Error

Amp.

1.24V

Ref.

Logic

Driver

Anti-Sat.

Pin 7

100kHz

Osc.

Pin 4

Reg.

Pin 3

COMP

Pin 2

GND

Pin 6

GND

Pin 8

VIN

Pin 5

GND

Pin 1

2.3V

Com-

parator

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