MIC2171 datasheet(8/24 Pages) MICROCHIP

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MIC2171

DS20006355A-page 8

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2022 Microchip Technology Inc. and its subsidiaries

4.0

FUNCTIONAL DESCRIPTION

Refer to Functional Block Diagram section.

4.1

Internal Power

The MIC2171 operates when VIN is ≥ 2.6V. An internal

2.3V regulator supplies biasing to all internal circuitry,

including a precision 1.24V band gap reference.

4.2

PWM Operation

The 100 kHz 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

the output transistor Q1.

4.3

Current Mode Advantages

The MIC2171 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.

4.4

Anti-Saturation

The anti-saturation diode (D1) increases the usable

duty cycle range of the MIC2171 by eliminating the

base to collector stored charge which would delay Q1’s

turnoff.

4.5

Compensation

Loop stability compensation of the MIC21712 can be

accomplished by connecting an appropriate RC

network from either COMP to circuit ground (Typical

Application Circuits) or from 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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