ISL6557A

FN9068 Rev 3.00

Page 9 of 18

July 2004

indefinitely as shown in Figure 6 as long as the controller is

enabled or until the overcurrent condition resolves.

VOLTAGE REGULATION

The ISL6557A uses a digital to analog converter (DAC) to gen-

erate a reference voltage based on the logic signals at pins

VID4 to VID0. The DAC decodes the a 5-bit logic signal (VID)

into one of the discrete voltages shown in Table 1. Each VID

input offers a 20

A pull up to 2.5V for use with open-drain out-

puts. External pull-up resistors or active-high output stages can

augment the pull-up current sources, but a slight accuracy error

can occur if they are pulled above 2.9V.

.

The DAC-selected reference voltage is connected to the non-

inverting input of the error amplifier, and the output of the differ-

ential remote-sense amplifier usually gets connected to the

error amplifier as shown in Figure 7. The remote-sense ampli-

fier eliminates voltage differences between local and remote

ground to provide a more accurate means of sensing output

voltage.

The integrating compensation network shown in Figure 7

assures that the steady-state error in the output voltage is

limited to the error in the reference voltage (output of the DAC)

plus offset errors in the remote-sense and error amplifiers.

Intersil specifies the guaranteed tolerance of the ISL6557A and

all Intersil controllers to include all variations in the amplifiers

and reference so that the output voltage remains within the

specified system tolerance.

0A

0V

5ms/DIV

OUTPUT VOLTAGE,

OUTPUT CURRENT, 20A/DIV

FIGURE 6. OVERCURRENT BEHAVIOR IN HICCUP MODE

500mV/DIV

FIGURE 7. OUTPUT-VOLTAGE AND LOAD-LINE

REGULATION

-

+

REFERENCE

EXTERNAL CIRCUIT

ISL6557A INTERNAL CIRCUIT

COMP

FB

IOUT

VDIFF

VSEN

RGND

-

+

ERROR AMPLIFIER

-

+

REMOTE

GROUND

DIFFERENTIAL

REMOTE-SENSE

AMPLIFIER

VOLTAGE

TABLE 1. VOLTAGE IDENTIFICATION CODES

VID4

VID3

VID2

VID1

VID0

VDAC

111

11

Off

111

10

0.800

111

01

0.825

111

00

0.850

110

11

0.875

110

10

0.900

110

01

0.925

110

00

0.950

101

11

0.975

101

10

1.000

101

01

1.025

101

00

1.050

100

11

1.075

100

10

1.100

100

01

1.125

100

00

1.150

011

11

1.175

011

10

1.200

011

01

1.225

011

00

1.250

010

11

1.275

010

10

1.300

010

01

1.325

010

00

1.350

001

11

1.375

001

10

1.400

001

01

1.425

001

00

1.450

000

11

1.475

000

10

1.500

000

01

1.525

000

00

1.550