ISL6526, ISL6526A

FN9055 Rev 12.00

Page 9 of 18

Sep 30, 2015

reference voltage and feed it back to the inverting input of

the error amplifier; see Figure 2. However, since the value of

R1 affects the values of the rest of the compensation

components, it is advisable to keep its value less than 5k

.

R4 can be calculated based Equation 2:

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

to 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 -

3.3V Input” on page 4 and “Typical Application - 5V Input” on

page 4). An internal 20µA (typical) current sink develops a

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

Equation 3 with:

temperature.

,

where

I is the output inductor ripple current.

For the ripple current, see Equation 11 in “Output Inductor

Selection” on page 12.

A small ceramic capacitor should be placed in parallel with

presence of switching noise on the input voltage.

R4

R1 0.8V



–

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

=

(EQ. 2)

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



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

=

(EQ. 3)



I



2

----------

+

