November 2004

7

MIC2199

MIC2199

Micrel

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

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.

Current

Sense

Amp

Error

Amp

CORRECTIVE

RAMP

PWM

COMPARATOR

RESET

Oscillator

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

S

R

Q

CONTROL LOGIC AND

PULSE-WIDTH MODULATOR

-3

V

0.8V

R1

R2

1

Reference

⎞

⎠

⎛

⎝

Figure 2. PWM Operation

0V

0V

0V

0V

0A

Reset

Pulse

Figure 3. PWM-Mode Timing