MIC2198

Micrel, Inc.

MIC2198

6

October 2005

A ramp is added to the amplified current-sense signal to

provide slope compensation, which is required to prevent

unstable operation at duty cycles greater than 50%.

A transconductance amplifier is used for the error amplifier,

which compares an attenuated sample of the output voltage

with a reference voltage. The output of the error amplifier is

Current

Error

CORRECTIVE

PWM

RESET

Oscillator

7

10

6

12

11

8

9

4

5

3

1

COMP

100k

MIC2198

FB

R2

R1

VOUT

CSH

VDD

BST

VIN

HSD

Q2

Q1

D1

D2

L1

VSW

LSD

PGND

4.7µF

S

R

Q

CONTROL LOGIC AND

V

0.8V

R1

R2

+

1

Reference









PULSE-WIDTH MODULATOR

COMPARATOR

RAMP

Amp

Sense

Amp

Figure 2. PWM Operation

0V

0V

0V

0V

0A

Reset

Pulse

Figure 3. PWM-Mode Timing

the COMP (compensation) pin, which is compared to the

current-sense waveform in the PWM block. When the current

signal becomes greater than the error signal, the comparator

turns off the high-side drive. The COMP pin (pin 1) provides

access to the output of the error amplifier and allows the use

of external components to stabilize the voltage loop.