6

FN9011.3

General Circuit Description

The ISL6560 is a two-power channel, current mode PWM

controller with input current sensing. A transconductance

error amplifier helps establish the desired droop voltage for

microprocessor power supplies and will be explained later.

Figure 1 is a functional system block diagram of the IC in a

power supply application. A single current sampling resistor,

for both channels via a comparator within the ISL6560. A

single comparator insures that both channels are monitored

by the same circuitry, helping to balance the operating

current of each channel. During normal operation the

comparator is tripped by the peak inductor current,

terminating the conduction cycle. As more current is needed

to supply the output load, the comparator threshold voltage

is increased, increasing the inductor current to

accommodate the increased load demands.

Circuit Operation

Figure 1 will be used to describes operation of the controller.

A transconductance error amplifier provides the major

voltage control function. The error amplifier’s positive input is

connected to an internal DAC that is programmed via a 5-bit

code from the microprocessor. Regulation is accomplished

by the amplifier attempting to make both inputs equal. This

does not happen because of the limited loop gain and

provides the bases for droop compensation mentioned

earlier and described below.

Figure 2 shows a curve of the current comparator threshold

voltage as a function of the error amplifier output voltage,

PWM1

PWM2

+

GATE DRIVER

and

OUTPUT FETs

GATE DRIVER

and

OUTPUT FETs

+

-

gm

Error

Amplifier

+

-

Comparator

Current

OUTPUT

DRIVERS

GATE

Logic

and

OSCILLATOR

SET

RESET

INPUT

VOLTAGE

Voltage

Reference

ISL6560

D/A

VID4

VID3

VID2

VID1

VID0

CS+

CS-

CT

COMP

REF

FB

GND

FIGURE 1. FUNCTIONAL SYSTEM BLOCK DIAGRAM SHOWING MAJOR COMPONENTS

0

20

40

60

80

100

120

140

160

0.5

1.0

1.5

2.0

2.5

3.0

0

12.

5V

/V

{

Output

Disable

Threshold

FIGURE 2. CURRENT COMPARATOR THRESHOLD VOLTAGE

AS A FUNCTION OF VCOMP

ISL6560