ISL6558IBZ datasheet(10/17 Pages) RENESAS

10 / 17 page

ISL6558

FN9027 Rev 13.00

Page 10 of 17

August 28, 2015

The PWM drive signals are switched out of phase. The PWM

drive signal phase relationship is 360° divided by the number

of active channels. Figure shows the PWM drive signals for

a four channel converter running at 125kHz. Each PWM

drive signal is 90° out of phase with the other.

Frequency Setting

A resistor, RT, connected between the FS/EN pin and

ground sets the frequency of the internal oscillator. Tying the

FS/EN pin to ground disables the oscillator, thus shutting

down the converter. The resistor can be calculated given the

desired channel switching frequency, FSW.

Figure 5 provides a graph of oscillator frequency vs RT. The

maximum recommended channel frequency is 1.5MHz.

Reference Voltage

An internal 0.8V reference is used for both PWM duty cycle

determination as well as output voltage protection. The

reference is trimmed such that the system, including

amplifier offset voltages, is accurate to ±

1% over

temperature range.

Fault Protection

The ISL6558 protects the load device from damaging stress

levels. The overcurrent trip point is integral in preventing

output shorts of varying degrees from causing current spikes

which would damage a load device. The output voltage

detection features insure a safe window of operation for the

load device.

Overcurrent

The RISEN resistor scales the voltage sampled across the

lower MOSFET and provides current feedback proportional

to the output current of each active channel. The ISEN

currents from all the active channels are averaged together

to form a scaled version of the total output current, ITOTAL.

PWM1, 5V/DIV

FIGURE 4. FOUR ACTIVE CHANNEL PWM DRIVE SIGNALS

1ms/DIV

PWM3, 5V/DIV

PWM4, 5V/DIV

PWM2, 5V/DIV

10.9 1.1

FSW

log

–

(EQ. 5)

100

200

500 1,000 2,000

100

200

500

1,000

CHANNEL OSCILLATOR FREQUENCY, FSW, [kHz]

FIGURE 5. OSCILLATOR FREQUENCY vs RT

ITOTAL

ISEN1

ITRIP

82.5

A

FIGURE 6. INTERNAL OVERCURRENT DETECTION

CIRCUITRY



n = ACTIVE CHANNELS

ISEN4

ISL6558

ISEN3

ISEN2

FIGURE 7. OVERCURRENT OPERATION

10ms/DIV

IOUT (5A/DIV)

VOUT (1V/DIV)

SHORT APPLIED

SHORT REMOVED

PGOOD (5V/DIV)

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