MIC2171 Datasheet, PDF(5/10 Page) Micrel Semiconductor – 100kHz 2.5A Switching Regulator Preliminary Information

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MIC2171 Datasheet, PDF (5/10 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator Preliminary Information

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MIC2171

Block Diagram MIC2171

2.3V

Reg.

100kHz

Osc.

Logic

Error

1.24V Amp.

Ref.

Com-

parator

Anti-Sat.

Driver

Current

Amp.

COMP

GND

Micrel

Functional Description

Refer to âBlock Diagram MIC2171â.

Internal Power

The MIC2171 operates when VIN is â¥ 2.6V. An internal 2.3V

regulator supplies biasing to all internal circuitry including a

precision 1.24V band gap reference.

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 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 pro-

vides 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 MIC2171 by eliminating the base to

collector stored charge which would delay Q1âs turnoff.

Compensation

Loop stability compensation of the MIC2171 can be accom-

plished by connecting an appropriate network from either

COMP to circuit ground (see 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.

1997

4-7

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