Micrel, Inc.

MIC2199.

January 2010

5

M9999-011310

Block Diagram

Current

Limit

Error

Amp

CORRECTIVE

RAMP

RESET

Reference

Oscillator

EN/UVLO

2

7

10

6

12

11

8

9

4

5

3

1

COMP

100k

MIC2199

FB

R2

R1

VOUT

CSH

VDD

BST

VIN

HSD

Q2

Q1

D1

D2

L1

VSW

LSD

PGND

4.7µF

V

0.8V

R1

R2

+

1

V

6 V

Current

Sense

Amp

Control

Logic









PWM

Figure 1. Internal Block Diagram

Functional Description

TheMIC2199isaBiCMOS,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

forstableoperationabovea50%dutycycle.Thecontroller

is optimized for high-efficiency, high-performance DC-DC

converter applications.

The MIC2199 block diagram is shown above.

The MIC2199 controller is divided into 5 functions.

• Controlloop

• Currentlimit

• Reference,enableandUVLO

• MOSFETgatedrive

• Oscillator

Control Loop

The MIC2199 operates in PWM (pulse-width-modulation)

mode. In PWM mode, the synchronous buck converter

forcescontinuouscurrenttoflowintheinductorwhichalso

improves cross regulation of transformer coupled, multiple

outputconfigurations.

PWM Control Loop

TheMIC2199usescurrent-modecontroltoregulatetheoutput

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.Samplingtheinductorcurrentremovestheinductor

fromthecontrolloop,whichsimplifiescompensation.

A block diagram of the MIC2199 PWM current-mode control

loop is shown in Figure 2 and the PWM mode voltage and

current waveform is shown in Figure 3. The inductor current