11

FN9099.5

May 5, 2008

internal soft-start cycle initiates a normal soft-start ramp of

the output, at time T1. The output is brought back into

regulation by time T2 as long as the overcurrent event has

cleared.

Had the cause of the overcurrent still been present after the

delay interval, the overcurrent condition would be sensed

and the regulator would be shut down again for another

delay interval of three soft-start cycles. The resulting hiccup

mode style of protection would continue to repeat indefinitely.

The overcurrent function will trip at a peak inductor current

typical). The OC trip point varies mainly due to the MOSFET

Equation 3 with:

temperature.

,where

ΔI is

the output inductor ripple current.

For an equation for the ripple current, see the section under

component guidelines titled “Output Inductor Selection” on

page 14.

A small ceramic capacitor should be placed in parallel with

presence of switching noise on the input voltage.

Overvoltage and Undervoltage Protection

All three regulators are protected from faults through internal

Overvoltage and Undervoltage detection circuitry. If the any

rail falls below 85% of the targeted voltage, then an

undervoltage event is tripped. An undervoltage will disable

all three regulators for a period of 3 soft-start cycles, after

which a normal soft-start is initiated. If the output is still under

85% of target, the regulators will continue to be disabled and

soft-started in a hiccup mode until the fault is cleared. This

protection feature works much the same as the VDDQ PWM

overcurrent protection works. See Figure 3.

If the any rail exceeds 115% of the targeted voltage, then all

three outputs are immediately disabled. The ISL6532A will

not re-enable the outputs until either the bias voltage is

toggled in order to initiate a POR or the S5 signal is forced

LOW and then back to HIGH.

Thermal Protection (S0/S3 State)

If the ISL6532A IC junction temperature reaches a nominal

temperature of +140°C, all regulators will be disabled. The

ISL6532A will not re-enable the outputs until the junction

temperature drops below +110°C and either the bias voltage

is toggled in order to initiate a POR or the SLP_S5 signal is

forced LOW and then back to HIGH.

Shoot-Through Protection

A shoot-through condition occurs when both the upper and

lower MOSFETs are turned on simultaneously, effectively

shorting the input voltage to ground. To protect from a shoot-

through condition, the ISL6532A incorporates specialized

circuitry, which insures that complementary MOSFETs are

not ON simultaneously.

regulator looks at the lower gate drive pin, LGATE, and the

upper gate drive pin, UGATE, to determine whether a

MOSFET is ON or OFF. If the voltage from UGATE or from

LGATE to GND is less than 0.8V, then the respective

MOSFET is defined as being OFF and the other MOSFET is

regulator to both source and sink current.

Since the voltage of the MOSFET gates are being measured

to determine the state of the MOSFET, the designer is

encouraged to consider the repercussions of introducing

external components between the gate drivers and their

respective MOSFET gates before actually implementing

such measures. Doing so may interfere with the shoot-

through protection.

TIME

T1

T0

T2

500mV/DIV

RESPONSES

INTERNAL SOFT-START FUNCTION

DELAY INTERVAL

I

PEAK

I

r

DS ON

()

-----------------------------------------------------

=

(EQ. 3)

I

PEAK

I

OUT MAX

()

ΔI

()

2

----------

+

>

ISL6532A