ISL6532A

FN9099 Rev 6.00

Page 9 of 18

Sep 12, 2013

NCH is an open-drain output that controls the MOSFET

states. A 2k

 or larger resistor is to be tied between the 12V

rail and the NCH pin. Until the voltage on the NCH pin reaches

the NCH trip level, the PWM is disabled.

If NCH is not actively utilized, it still must be tied to the 12V rail

through a resistor. For systems using 5V dual as the input to

the switching regulator, a time constant, in the form of a

capacitor, can be added to the NCH pad to delay start of the

PWM switcher until the 5V dual has switched from 5VSBY to

5VATX.

PGOOD (Pin 21)

Power Good is an open-drain logic output that changes to a

logic low if any of the three regulators are out of regulation in

S0/S1/S2 state. PGOOD will always be low in any state other

than S0/S1/S2.

SLP_S5# (Pin 24)

This pin accepts the SLP_S5# sleep state signal.

SLP_S3# (Pin 23)

This pin accepts the SLP_S3# sleep state signal.

FB2 (Pin 18)

Connect the output of the external linear regulator to this pin

through a properly sized resistor divider. The voltage at this pin

is regulated to 0.8V. This pin is monitored for under and

overvoltage events.

DRIVE2 (Pin 19)

Connect this pin to the gate terminal of an external

N-Channel MOSFET transistor. This pin provides the gate

voltage for the linear regulator pass transistor. It also provides

a means of compensating the error amplifier for applications

requiring the transient response of the linear regulator to be

optimized.

Functional Description

Overview

The ISL6532A provides complete control, drive, protection and

ACPI compliance for a regulator powering DDR memory

systems. It is primarily designed for computer applications

powered from an ATX power supply. A 250kHz Synchronous

Buck Regulator with a precision 0.8V reference provides the

proper Core voltage to the system memory of the computer. An

internal LDO regulator with the ability to both sink and source

current and an externally available buffered reference that

voltage. The ISL6532A also features an LDO regulator for 1.5V

AGP Video and Core voltage.

ACPI compliance is realized through the SLP_S3 and SLP_S5

sleep signals and through monitoring of the 12V ATX bus.

Initialization

The ISL6532A automatically initializes upon receipt of input

power. Special sequencing of the input supplies is not

necessary. The Power-On Reset (POR) function continually

monitors the input bias supply voltages. The POR monitors the

bias voltage at the 5VSBY and P12V pins. The POR function

initiates soft-start operation after the bias supply voltages

exceed their POR thresholds.

ACPI State Transitions

COLD START (S4/S5 TO S0 TRANSITION)

At the onset of a mechanical start, the ISL6532A receives it’s

bias voltage from the 5V Standby bus (5VSBY). As soon as the

SLP_S3 and SLP_S5 have transitioned HIGH, the ISL6532A

starts an internal counter. Following a cold start or any

subsequent S4/S5 state, state transitions are ignored until the

system enters S0/S1. None of the regulators will begin the soft-

start procedure until the 5V Standby bus has exceeded POR,

trip level.

Once all of these conditions are met, the PWM error amplifier

will first be reset by internally shorting the COMP pin to the FB

pin. This reset lasts for 2048 clock cycles, which is typically

sequence will then begin.

The PWM error amplifier reference input is clamped to a level

proportional to the soft-start voltage. As the soft-start voltage

slews up, the PWM comparator generates PHASE pulses of

increasing width that charge the output capacitor(s). The

internal VTT LDO will also soft-start through the reference that

tracks the output of the PWM regulator. The reference for the

AGP LDO controller will rise relative to the soft-start reference.

The soft-start lasts for 2048 clock cycles, which is typically

8.2ms. This method provides a rapid and controlled output

voltage rise.

FIGURE 1. TYPICAL COLD START

12VATX 2V/DIV

5VSBY

S3

S5

1V/DIV

500mV/DIV

500mV/DIV

500mV/DIV

12V POR

SOFT-START

INITIATES

SOFT-START ENDS

PGOOD COMPARATOR

ENABLED

2048 CLOCK

CYCLES

2048 CLOCK

CYCLES

PGOOD

5V/DIV