8

FN9055.8

March 20, 2007

If the output voltage desired is 0.8V, simply route the output

back to the FB pin through R1, but do not populate R4.

Overcurrent Protection

The overcurrent function protects the converter from a shorted

monitor the current. This method enhances the converter’s

efficiency and reduces cost by eliminating a current sensing

resistor.

The overcurrent function cycles the soft-start function in a

programs the overcurrent trip level (see Typical Application

diagrams on pages 3 and 4). An internal 20

μA (typical) current

overcurrent function initiates a soft-start sequence.

Figure 3 illustrates the protection feature responding to an

overcurrent event. At time t0, an overcurrent condition is

sensed across the upper MOSFET. As a result, the regulator

is quickly shutdown and the internal soft-start function begins

producing soft-start ramps. The delay interval seen by the

output is equivalent to three soft-start cycles. The fourth

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

the equation above 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’.

A small ceramic capacitor should be placed in parallel with

presence of switching noise on the input voltage.

Current Sinking

The ISL6526, ISL6526A incorporate a MOSFET shoot-

through protection method which allows a converter to sink

current as well as source current. Care should be exercised

when designing a converter with the ISL6526, ISL6526A

when it is known that the converter may sink current.

FIGURE 2. OUTPUT VOLTAGE SELECTION

+

R1

+3.3V

R4

ISL6526,

C4

Q1

FB

UGATE

VCC

BOOT

COMP

D1

R2

C2

C1

R3

C3

PHASE

LGATE

Q2

CPVOUT

VIN

ISL6526A

FIGURE 3. OVERCURRENT PROTECTION RESPONSE

0V

TIME

T1

T0

T2

INTERNAL SOFT-START FUNCTION

DELAY INTERVAL

I

PEAK

I

r

DS ON

()

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

=

(EQ. 3)

I

PEAK

I

OUT MAX

()

ΔI

()

2

----------

+

>

ISL6526, ISL6526A