MIC2182 datasheet(13/40 Pages) MICROCHIP

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DS20006644A-page 13

MIC2182

4.0

FUNCTIONAL DESCRIPTION

The MIC2182 is a BiCMOS, switched-mode,

synchronous step-down (buck) converter controller.

Current-mode control is used to achieve superior

transient line and load regulation. An internal corrective

ramp provides slope compensation for stable operation

above 50% duty cycle. The controller is optimized for

high-efficiency, high-performance DC-DC converter

applications.

The MIC2182 block diagrams are shown in the Block

Diagrams section.

The MIC2182 controller is divided into six functions.

• Control Loop

- PWM Operation

- Skip-Mode Operation

• Current Limit

• Reference, Enable, and UVLO

• MOSFET Gate Drive

• Oscillator and Sync

• Soft Start

4.1

Control Loop

4.1.1

PWM AND SKIP MODES OF

OPERATION

The MIC2182 operates in PWM (pulse-width

modulation) mode at heavier output load conditions. At

lighter load conditions, the controller can be configured

to automatically switch to a pulse-skipping mode to

improve efficiency. The potential disadvantage of skip

mode is the variable switching frequency that

accompanies this mode of operation. The occurrence

of switching pulses depends on component values as

well as line and load conditions. There is an external

sync function that is disabled in skip mode. In PWM

mode, the synchronous buck converter forces

continuous current to flow in the inductor. In skip mode,

current through the inductor can settle to zero, causing

voltage ringing across the inductor. Pulling the PWM

pin (Pin 2) low will force the controller to operate in

PWM mode for all load conditions, which will improve

cross regulation of transformer-coupled, multiple

output configurations.

4.1.2

PWM CONTROL LOOP

The MIC2182 uses current-mode control to regulate

the output voltage. This method senses the output

voltage (outer loop) and the inductor current (inner

loop). It uses inductor current and output voltage to

determine the duty cycle of the buck converter.

Sampling the inductor current removes the inductor

from the control loop, which simplifies compensation.

FIGURE 4-1:

PWM Operation.

Current

Sense

Amp

Error

Amp

VIN

VDD

CORRECTIVE

RAMP

PWM

COMPARATOR

RESET

Reference

Oscillator

COMP

100k

MIC2182 [adj.] PWM Mode

VOUT

CSH

VDD

VBST

CIN

VIN

HSD

CBST

COUT

VIN

VOUT

VSW

LSD

PGND

CCOMP

RCOMP

4.7F

1.245V

OUT =+

CONTROL LOGIC AND

PULSE-WIDTH MODULATOR

RCS

gm = 0.2mS

AV = 2

0.024V

PWM Mode

to Skip

Mode

LOW

FORCES

SKIP MODE

(

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