July 2002

7

MIC2186

MIC2186

Micrel

Functional Diagram

Osc

Error

Amplifier

0.1V

D1

L1

fs/4

Control

Bias

PWM

Comparator

Overcurrent

Comparator

Frequency

Foldback

fs/4

Overcurrent Reset

Corrective

Ramp

On

Gain = 3.7

P

GND

12

CSH

9

OutN

14

V

V

16

1

V

fb

5

S

GND

6

V

DD

10

V

REF

8

COMP

4

Soft Start

3

HiDC

V

DD

13

Sync

11

Freq/2

15

SKIP

2

EN/UVLO

7

gm = 0.0002

Gain = 20

0.3V

100k

R2

R1

Figure 1. MIC2186 PWM Block Diagram

Functional Description

The MIC2186 is a BiCMOS, switched mode multi-topology

controller. It will operate most low side drive topologies

including boost, SEPIC, flyback and forward. The controller

has a low impedance driver capable of switching large N-

channel MOSFETs. It features multiple frequency and duty

cycle settings. 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 shows a block diagram of the MIC2186 configured

as a PWM boost converter.

The switching cycle starts when OutN goes high and turns on

the low side, N-channel MOSFET, Q1. The Vgs of the

MOSFET is equal to V

the inductor. The inductor current flows through the current

sense resistor, Rsense. The voltage across the resistor is

amplified and combined with an internal ramp for stability.

This signal is compared with the error voltage signal from the

error amplifier. When the current signal equals the error

voltage signal, the low side MOSFET is turned off. The

inductor current then flows through the diode, D1, to the

output. The MOSFET remains off until the beginning of the

next switching cycle.