ISL8117

15

FN8666.3

June 4, 2015

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Tracking Operation

The ISL8117 can be set up to track an external supply. To

implement tracking, a resistive divider is connected between the

external supply output and ground. The center point of the divider

shall be connected to the SS/TRK pin of ISL8117. The resistive

divider ratio sets the ramping ratio between the two voltage rails.

To implement coincident tracking, set the tracking resistive

divider ratio exactly the same as the ISL8117 output resistive

divider given by Equation 5. Make sure that the voltage at

SS/TRK is greater than 0.6V when the master rail reaches

regulation.

To minimize the impact of the 2µA soft-start current on the

tracking function, it is recommended to use resistors of less than

10kΩ for the tracking resistive divider.

When overcurrent protection (OCP) is triggered, the internal

minimum soft-start circuit determines the OCP soft-start hiccup.

Light-load Efficiency Enhancement

When MOD/SYNC is tied to VCC5V, the ISL8117 operates in high

efficiency diode emulation mode and pulse skipping mode in

light-load condition. The inductor current is not allowed to reverse

(discontinuous operation). At very light-loads, the converter goes

into diode emulation and triggers the pulse skipping function. In

pulse skipping mode, the upper MOSFET remains off until the

output voltage drops to the point the error amplifier output goes

the pulse skipping mode is 60ns.

Prebiased Power-up

The ISL8117 has the ability to soft-start with a prebiased output.

The output voltage would not be yanked down during prebiased

start-up. The PWM is not active until the soft-start ramp reaches

the output voltage times the resistive divider ratio.

Overvoltage protection is alive during soft-starting.

Frequency Selection

Switching frequency selection is a trade-off between efficiency

and component size. Low switching frequency improves

efficiency by reducing MOSFET switching loss. To meet the output

ripple and load transient requirements, operation at a low

switching frequency would require larger inductance and output

capacitance. The switching frequency of the ISL8117 is set by a

resistor connected from the RT pin to GND according to

Equation 1.

The frequency setting curve shown in Figure 30 assists in

Frequency Synchronization

The MOD/SYNC pin may be used to synchronize ISL8117 to an

external clock or the CLKOUT pin of another ISL8117. When the

MOD/SYNC pin is connected to the CLKOUT pin of another

ISL8117, the two controllers operate in synchronization.

When the MOD/SYNC pin is connected to an external clock,

ISL8117 will synchronize to this external clock frequency. For

than the external clock frequency.

When frequency synchronization is in action, the controllers will

enter forced continuous current mode at light-load.

CLKOUT pin outputs a clock signal with 280ns pulse width. The

signal frequency is the same as the frequency set by the resistor

from RT pin to ground. The signal rising edge is in line with the

PWM falling edge.

Gate Control Logic

The gate control logic translates the PWM signal into gate drive

signals providing amplification, level shifting and shoot-through

protection. The gate driver has circuitry that helps optimize the IC

performance over a wide range of operational conditions. As

MOSFET switching times can vary dramatically from type to type

and with input voltage, the gate control logic provides adaptive

dead time by monitoring real gate waveforms of both the upper

and the lower MOSFETs. Shoot-through control logic provides a

16ns dead time to ensure that both the upper and lower

MOSFETs will not turn on simultaneously causing a shoot-through

condition.

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