ISL6534CVZ datasheet(14/29 Pages) RENESAS

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ISL6534

FN9134 Rev 2.00

Page 14 of 29

Nov 18, 2005

Once the power is “Good”, PGOOD will pull low if any of the

3 SS/EN pins is pulled low. Also, if a short is detected on

either switcher, then the PGOOD will pull low, for as long as

the condition is there. If VOUT1 or VOUT2 has a short

detected which stays there for 1-2 clock pulses, then all

three regulators will shut down, and wait for a power-down

and up cycle to reset (either VCC or VCC12 (or both) must

power down and up). If the short-circuit is not there long

enough to shut down, it may still cause PGOOD to go low

momentarily. If this causes a system issue, a filter capacitor

could be tried; it should be at least several nF to be effective.

Note that this is not a full-feature PGOOD; it is not directly

monitoring if the VOUT1 or VOUT2 drops below a set UV

level; it only checks for the simple short-circuit condition, via

the COMP pins. And it is not monitoring VOUT3 at all. So it

is a good indication that all three outputs have ramped up,

but it is less useful as a monitor from that point on.

Since PGOOD is an open-drain pull-down device, it usually

requires an external pull-up resistor; however, if the pin is not

used, no resistor is necessary. A value in the range of 1k

 to

10k

 is typical.

POR

Both the VCC (5V) and VCC12 (12V) are monitored for

Power-On-Reset, as shown in the Specification Table. The

two POR outputs are logically gated together, such that both

have to be above their rising trip points to enable the SS/EN

ramps to start (if they are not held low) and then enable each

output. Either POR output can go below its falling trip point to

disable all outputs, and then back to restart the enable

operation.

Shunt Regulator

The ISL6534 must have both a 12V (for VCC12) and a 5V

power supply (for VCC); both must be above their respective

POR rising trip points to enable the outputs to start

switching. The shunt regulator (nominal 5.8V) was designed

for those systems that do not have a 5V supply available; the

range of the shunt (5.6V to 6.0V) was designed not to

overlap the usual 4.75V to 5.25V range of typical power

supplies. An external resistor between VCC12 and VCC is

required; a typical value of 150

 is the recommended

starting value (it may change due to other factors, such as

VCC12 voltage, VBS voltages, oscillator frequency, etc.).

The dissipation of the resistor is approximately 1/4W; it

needs to be sized accordingly. For example, 12V - 5.8V =

6.2V across the 150

 resistor is 41mA; P = IV = 0.256W.

Several low-power resistors in parallel can also be used.

See Figure 11.

In either case, both VCC and VCC12 pins have small

decoupling capacitors (typically 1.0 to 10.0µF); they should

each be located near their pin, with a via to the GND plane.

Short-Circuit Protection

There is no current sensing or rDS(ON) sensing or

undervoltage sensing on the ISL6534. However, if either

Channel 1 or 2 output is shorted while active, there is a

simple detection on the error amp COMP output that implies

either overcurrent or undervoltage; the PGOOD pin goes low

immediately. If the condition persists for 1-2 internal clock

cycles (3-6µs at 300kHz), then ALL 3 Outputs are latched

off, requiring either a VCC or VCC12 POR to restart. The

protection was not designed to work for the case of powering

up an output into a short-circuit, and there are limitations on

detecting applied shorts. Note that the linear regulator has

no short-circuit protection.

See Application Considerations for more details.

Oscillator

The internal oscillator is nominally 300kHz (±20% tolerance)

with no external components required (tie FS_SYNC pin to

GND), as measured at either of the LG or UG pins. To run

faster, a resistor from FS_SYNC pin to GND will speed up

the frequency. See Figure 12 for a curve that shows the

frequency versus resistor value. Since the curve is steep as

it approaches 300kHz, operation in this area is not

recommended if the exact frequency is important.

Since this pin has several functions muxed onto it, it is

important that they do not interfere with each other. Thus,

the circuit that looks for the resistor will shut off (and default

to the 300kHz) if it doesn’t see a current in the expected

range. There should not be any excessive capacitive loading

on the pin either, and if a resistor is used, it should be

located very close to the FS_SYNC pin, and its connection

to GND should be near the IC GND, and away from the

output stage power GND, to minimize jitter.

FIGURE 11. SHUNT REGULATOR AND DECOUPLING

CAPACITORS FOR VCC AND VCC12 PINS

(VCC) = 5.8V

VCC12

VCC PIN

VCC12 PIN

OPTIONAL R FOR

SHUNT REGULATOR;

VCC = 5.8V

VCC

VCC12

VCC PIN

VCC12 PIN

NON-SHUNT

MODE; SEPARATE

5V AND 12V

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