MIC2199

Micrel

MIC2199

6

November 2004

Block Diagram

-3

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

V6 V

.0

Current

Sense

Amp

Control

Logic

⎞

⎠

⎛

⎝

PWM

Figure 1. Internal Block Diagram

Functional Description

The MIC2199 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 a 50% duty cycle. The controller is

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

verter applications.

The MIC2199 block diagram is shown above.

The MIC2199 controller is divided into 5 functions.

• Control loop

• Current limit

• Reference, enable and UVLO

• MOSFET gate drive

• Oscillator

Control Loop

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

mode. In PWM mode, the synchronous buck converter forces

continuous current to flow in the inductor which also improves

cross regulation of transformer coupled, multiple output con-

figurations.

PWM Control Loop

The MIC2199 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 compensa-

tion.

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

is sensed by measuring the voltage across the resistor, R