ISL6569ACR datasheet(15/22 Pages) RENESAS

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ISL6569A

FN9092 Rev 2.00

Page 15 of 22

Dec 29, 2004

impedance state. The Intersil drivers respond by turning off

both upper and lower MOSFETs. If the over-voltage condition

reoccurs, the ISL6569A will again command the lower

MOSFETs to turn on. The ISL6569A will continue to protect

the load in this fashion as long as the over-voltage repeats.

Second, the OVP pin pulls to VCC and can deliver 100mA

into the gate of either a MOSFET or SCR placed on the input

rail (VIN) or VOUT. Turning on the MOSFET or SCR

collapses the power rail and causes a fuse placed further up

stream to blow. The fuse must be sized such that the

MOSFET or SCR will not overheat before the fuse blows.

Once an over-voltage condition is detected, normal PWM

operation ceases and PGOOD remains low until the

ISL6569A is reset. Cycling the voltage on EN below 1.23V or

the bias to VCC below the POR-falling threshold will reset

the controller.

Over-Current Protection

The ISL6569A takes advantage of the proportionality

between the load current and the average current, IAVG, to

detect an over-current condition. See the Channel-Current

Balance section for more detail on how the average current

is created. The average current is continually compared with

a constant 90

A reference current. Once the average

current exceeds the reference current, the comparator

triggers the converter to shutdown. The POR circuit places

all PWM signals in a high-impedance state which commands

the drivers to turn off both upper and lower MOSFETs.

PGOOD pulls low and the system remains in this state while

the controller counts 2048 phase clock cycles. This is

followed by a soft-start attempt (see Soft-Start).

During the soft-start interval, the over-current protection

circuitry remains active. As the output voltage ramps up, if

an over-current condition is detected, the ISL6569A

immediately places all PWM signals in a high-impedance

state. The ISL6569A repeats the 2048-cycle wait period and

follows with another soft-start attempt, as shown in

Figure 12. This hiccup mode of operation repeats up to

seven times. On the eighth soft-start attempt, the part

latches off. Once latched off, the ISL6559 can only be reset

when the voltage on EN is brought below 1.23V or VCC is

brought below the POR falling threshold. Upon completion of

a successful soft-start attempt, operation will continue as

normal, PGOOD will return high, and the over-current latch

counter will reset.

During VID-on-the-fly transitions, the OC comparator output

is blanked. The quality and mix of output capacitors used in

different applications leads to a wide output capacitance

range. Depending upon the magnitude and direction of the

VID change, the change in voltage across the output

capacitors could result in significant current flow. Summing

this instantaneous current with the load current already

present could drive the average current above the reference

current level and cause an OC trip during the transition. By

blanking the OC comparator during the VID-on-the-fly

transition, nuisance tripping is avoided.

General Design Guide

This design guide is intended to provide a high-level

explanation of the steps necessary to create a multi-phase

power converter. It is assumed that the reader is familiar with

many of the basic skills and techniques referenced below. In

addition to this guide, Intersil provides complete reference

designs that include schematics, bills of materials, and

example board layouts for all common microprocessor

applications.

Power Stages

Principally, the designer will be concerned with whether

components can be mounted on both sides of the circuit

board; whether through-hole components are permitted; and

the total board space available for power-supply circuitry.

Generally speaking, the most economical solutions are

those where each phase handles between 15 and 20A. All

surface-mount designs will tend toward the lower end of this

current range and, if through-hole MOSFETs can be used,

higher per-phase currents are possible. In cases where

board space is the limiting constraint, current can be pushed

as high as 30A per phase, but these designs require heat

sinks and forced air to cool the MOSFETs.

MOSFETs

The choice of MOSFETs depends on the current each

MOSFET will be required to conduct; the switching frequency;

the capability of the MOSFETs to dissipate heat; and the

availability and nature of heat sinking and air flow.

LOWER MOSFET POWER CALCULATION

The calculation for heat dissipated in the lower MOSFET is

simple, since virtually all of the heat loss in the lower

MOSFET is due to current conducted through the channel

resistance (rDS(ON)). In Equation 13, IM is the maximum

continuous output current; IPP is the peak-to-peak inductor

5ms/DIV

OUTPUT VOLTAGE,

OUTPUT CURRENT, 20A/DIV

FIGURE 12. OVERCURRENT BEHAVIOR IN HICCUP MODE

500mV/DIV

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