ISL6527IBZ datasheet(8/17 Pages) RENESAS

8 / 17 page

ISL6527, ISL6527A

FN9056 Rev 11.00

Page 8 of 17

Sep 29, 2015

Output Voltage Selection

The output voltage can be programmed to any level between

VIN and the supplied external reference. An external resistor

divider is used to scale the output voltage relative to the

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 on Equation 3:

If the output voltage desired is VREF, 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

output by using the upper MOSFET ON-resistance, rDS(ON),

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

hiccup mode to provide fault protection. A resistor (ROCSET)

programs the overcurrent trip level (see Typical Application

diagrams on page 3 and page 4). An internal 20µA (typical)

current sink develops a voltage across ROCSET that is

referenced to VIN. When the voltage across the upper

MOSFET (also referenced to VIN) exceeds the voltage across

ROCSET, the 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

(IPEAK) determined by Equation 4:

where IOCSET is the internal OCSET current source (20µA

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

rDS(ON) variations. To avoid overcurrent tripping in the normal

operating load range, find the ROCSET resistor from the

Equation 4 with:

1. The maximum rDS(ON) at the highest junction

temperature.

2. The minimum IOCSET from the “Specification Table” on

page 7.

3. Determine IPEAK for

where

I is the output inductor ripple current.

R1 VREF



VOUT1 VREF

–

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

(EQ. 3)

FIGURE 2. OUTPUT VOLTAGE SELECTION

COUT

+3.3V

VOUT

LOUT

ISL6527,

UGATE

VCC

BOOT

COMP

PHASE

LGATE

CPVOUT

VIN

REF_IN

VREF

ISL6527A

FIGURE 3. OVERCURRENT PROTECTION RESPONSE

TIME

VOUT (2.5V)

INTERNAL SOFT-START FUNCTION

DELAY INTERVAL

IPEAK

IOCSET x ROCSET

rDS ON



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

(EQ. 4)

IPEAK IOUT MAX



I



----------



ISL6527IBZ Datasheet(PDF) 8 Page - Renesas Technology Corp

ISL6527IBZ Datasheet(HTML) 8 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL