MIC2183

Micrel

MIC2183

8

December 10, 2001

Functional Characteristics

Controller Overview and Functional Description

The MIC2183 is a BiCMOS, switched mode, synchronous,

step down (buck) converter controller. It uses both N and P-

Channel MOSFETs, which allows the controller to operate at

100% duty cycle and eliminates the need for a high side drive

bootstrap circuit. Current mode control is used to achieve

superior transient line and load regulation. An internal correc-

tive ramp provides slope compensation for stable operation

above a 50% duty cycle. The controller is optimized for high

efficiency, high performance DC-DC converter applications.

Figure 1 is a block diagram of the MIC2183 configured as a

synchronous buck converter. At the beginning of the switch-

ing cycle, the OUTP pin pulls low and turns on the high-side

Functional Diagram

FREQUENCY

FOLDBACK

ERROR

AMP

PWM

COMPARATOR

OVERCURRENT

COMPARATOR

L1

CURRENT

SENSE

AMP

0.3V

100k

gm = 0.0002

gain = 20

fs/4

COMP

SS

FreqOut

FREQ/2

SYNC

ON

GAIN

3.7

PGND

RESET

VREF

1.245V

4

3

2

15

11

VDD

SLOPE

COMPENSATION

∑

OSC

fs/4

CONTROL

BIAS

÷2

CSH

9

CSL

8

VINP

16

OUTP

14

OUTN

13

PGND

Q2

Q1

D1

12

FB

6

VINA

0.1V

1

SGND

5

Figure 1. MIC2183 Block Diagram

P-Channel MOSFET, Q1. Current flows from the input to the

output through the current sense resistor, MOSFET and

inductor. The current amplitude increases, controlled by the

inductor. The voltage developed across the current sense

resistor, R

bined with an internal ramp for stability. This signal is com-

pared to the output of the error amplifier. When the current

signal equals the error voltage signal, the P-channel MOSFET

is turned off. The inductor current flows through the diode, D1,

until the synchronous, N-Channel MOSFET turns on. The

voltage drop across the MOSFET is less than the forward

voltage drop of the diode, which improves the converter

efficiency. At the end of the switching period, the synchro-

nous MOSFET is turned off and the switching cycle repeats.