MIC2168

Micrel

M9999-111803

6

November 2003

Functional Description

The MIC2168 is a voltage mode, synchronous step-down

switching regulator controller designed for high output power

without the use of an external sense resistor. It includes an

internal soft-start function which reduces the power supply

input surge current at start-up by controlling the output

voltage rise time, a PWM generator, a reference voltage, two

MOSFET drivers, and short-circuit current limiting circuitry to

form a complete 1MHz switching regulator.

Theory of Operation

The MIC2168 is a voltage mode step-down regulator. The

figure above illustrates the block diagram for the voltage

control loop. The output voltage variation due to load or line

changes will be sensed by the inverting input of the

transconductance error amplifier via the feedback resistors

R3, and R2 and compared to a reference voltage at the non-

inverting input. This will cause a small change in the DC

voltage level at the output of the error amplifier which is the

input to the PWM comparator. The other input to the com-

parator is a 0 to 1V triangular waveform. The comparator

generates a rectangular waveform whose width t

to the time from the start of the clock cycle t

the triangle crosses the output waveform of the error ampli-

fier. To illustrate the control loop, let us assume the output

voltage drops due to sudden load turn-on, this would cause

Functional Diagram

Current Limit

Reference

Current Limit

Comparator

Error

Amp

Low-Side

Driver

High-Side

Driver

PWM

Comparator

FB

COMP

GND

LSD

HSD

CS

5V

5V

5V

C2

C1

R1

5V

0.8V

SW

Q2

Q1

L1

Driver

Logic

0.8V

BG Valid

Clamp &

Startup

Current

Enable

Error

Loop

Hys

Comparator

5V LDO

Bandgap

Reference

Soft-Start &

Digital Delay

Counter

MIC2168

Ramp

Clock

BOOST

R2

R3

4

Ω

RSW

RCS

D1

MIC2168 Block Diagram

the inverting input of the error amplifier which is divided down

version of V

causing the output voltage of the error amplifier to go high.

This will cause the PWM comparator to increase t

the top side MOSFET, causing the output voltage to go up

and bringing V

Soft-Start

The COMP/EN pin on the MIC2168 is used for the following

three functions:

1. Disables the part by grounding this pin

2. External compensation to stabilize the voltage

control loop

3. Soft-start

For better understanding of the soft-start feature, let’s as-

sume V

un-grounding the COMP/EN pin. The COMP pin has an

internal 8.5

µA current source that charges the external com-

pensation capacitor. As soon as this voltage rises to 180mV

(t = Cap_COMP

× 0.18V/8.5µA), the MIC2168 allows the

internal V

crosses the undervoltage lockout of 2.6V, the chip’s internal

oscillator starts switching. At this point in time, the COMP pin

current source increases to 40

µA and an internal 11-bit

counter starts counting which takes approximately 2ms to

complete. During counting, the COMP voltage is clamped at