ISL6552

FN4918 Rev 2.00

Page 10 of 18

July 2004

rise to the three state level in PWM 1 output during first 32

PWM cycles.

Figure 4 shows the waveforms when the regulator is operating

at 200kHz. Note that the Soft-Start duration is a function of the

channel frequency as explained previously. Also note the

pulses on the COMP terminal. These pulses are the current

correction signal feeding into the comparator input (see the

Block Diagram

).

Figure 5 shows the regulator operating from an ATX supply. In

this figure, note the slight rise in PGOOD as the 5V supply

rises. The PGOOD output stage is made up of NMOS and

PMOS transistors. On the rising VCC, the PMOS device

becomes active slightly before the NMOS transistor pulls

“down”, generating the slight rise in the PGOOD voltage.

Note that Figure 5 shows the 12V gate driver voltage available

before the 5V supply to the ISL6552 has reached its threshold

level. If conditions were reversed and the 5V supply was to rise

first, the start-up sequence would be different. In this case the

ISL6552 will sense an over-current condition due to charging

the output capacitors. The supply will then restart and go

through the normal Soft-Start cycle.

Fault Protection

The ISL6552 protects the microprocessor and the entire power

system from damaging stress levels. Within the ISL6552 both

Over-Voltage and Over-Current circuits are incorporated to

protect the load and regulator.

Over-Voltage

The VSEN pin is connected to the microprocessor CORE

voltage. A CORE over-voltage condition is detected when the

VSEN pin goes more than 15% above the programmed VID

level.

The over-voltage condition is latched, disabling normal PWM

operation, and causing PGOOD to go low. The latch can only

be reset by lowering and returning VCC high to initiate a POR

and Soft-Start sequence.

During a latched over-voltage, the PWM outputs will be driven

either low or three state, depending upon the VSEN input.

PWM outputs are driven low when the VSEN pin detects that

the CORE voltage is 15% above the programmed VID level.

This condition drives the PWM outputs low, resulting in the

lower or synchronous rectifier MOSFETs to conduct and shunt

the CORE voltage to ground to protect the load.

If after this event, the CORE voltage falls below the over-

voltage limit (plus some hysteresis), the PWM outputs will

three state. The HIP6601 family drivers pass the three state

information along, and shuts off both upper and lower

MOSFETs. This prevents “dumping” of the output capacitors

back through the lower MOSFETs, avoiding a possibly

destructive ringing of the capacitors and output inductors. If the

PWM 1

PGOOD

5V

OUTPUT

VCC

DELAY TIME

FIGURE 3. START-UP OF 4 PHASE SYSTEM OPERATING AT

500kHz

PGOOD

5V

V COMP

VCC

DELAY TIME

FIGURE 4. START-UP OF 4 PHASE SYSTEM OPERATING AT

200kHz

12V ATX

SUPPLY

PGOOD

5 V ATX

SUPPLY

ATX SUPPLY ACTIVATED BY ATX “PS-ON PIN”

FIGURE 5. SUPPLY POWERED BY ATX SUPPLY

FREQUENCY 200kHz