8

Diode Emulation

Diode emulation allows for higher converter efficiency under

light-load situations. With diode emulation active

(FCCM = LO), the ISL6608 will detect the zero current

crossing of the output inductor and turn off LGATE. This

ensures that discontinuous conduction mode (DCM) is

achieved. This prevents the low side MOSFET from sinking

current, and no negative spike at the output is generated

during pre-biased startup (See Figure 7 on page 7). The

LGATE has a minimum ON time of 400ns in DCM mode.

Diode emulation is asynchronous to the PWM signal.

Therefore, the ISL6608 responds to the FCCM input

immediately after it changes state. Refer to Figures 2 to 7 on

page 7 for details.

Intersil does not recommend Diode Emulation used with the

topology. The turn-OFF of the low side MOSFET forces the

forward current going through the body diode of the

MOSFET. If the current sampling circuit of the controller is

activated during the body diode conduction, a diode voltage

drop, is sampled. This will falsely trigger the over current

protection function of the controller.

The ISL6608 works with DCR, upper MOSFET, or power

resistor current sensing topologies to start up from pre-

biased load with no problem.

Three-State PWM Input

A unique feature of the ISL6608 and other Intersil drivers is

the addition of a shutdown window to the PWM input. If the

PWM signal enters and remains within the shutdown window

for a set holdoff time (typically 160ns), the output drivers are

disabled and both MOSFET gates are pulled and held low.

The shutdown state is removed when the PWM signal

moves outside the shutdown window. Otherwise, the PWM

rising and falling thresholds outlined in the ELECTRICAL

SPECIFICATIONS determine when the lower and upper

gates are enabled.

Adaptive Shoot-Through Protection

Both drivers incorporate adaptive shoot-through protection

to prevent upper and lower MOSFETs from conducting

simultaneously and shorting the input supply. This is

accomplished by ensuring the falling gate has turned off one

MOSFET before the other is allowed to turn on.

During turn-off of the lower MOSFET, the LGATE voltage is

monitored until it reaches a 1V threshold, at which time the

UGATE is released to rise. Adaptive shoot-through circuitry

monitors the upper MOSFET gate-to-source voltage during

UGATE turn-off. Once the upper MOSFET gate-to-source

voltage has dropped below a threshold of 1V, the LGATE is

allowed to rise.

Internal Bootstrap Diode

This driver features an internal bootstrap Schottky diode.

Simply adding an external capacitor across the BOOT and

PHASE pins completes the bootstrap circuit. The bootstrap

capacitor must have a maximum voltage rating above VCC +

5V and its capacitance value can be chosen from the

following equation:

control MOSFETs. The

allowable droop in the rail of the upper drive. The previous

relationship is illustrated in Figure 8.

As an example, suppose an upper MOSFET has a gate

the drive voltage over a PWM cycle is 200mV. One will find

that a bootstrap capacitance of at least 0.125

µF is required.

The next larger standard value capacitance is 0.15

µF. A

good quality ceramic capacitor is recommended.

∆V

BOOT

------------------------

≥

•

•

=

FIGURE 8. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE

VOLTAGE

20nC

2.0

1.6

1.4

1.0

0.8

0.6

0.4

0.2

0.0

0.3

0.0

0.1

0.2

0.4

0.5

0.6

0.9

0.7

0.8

1.0

1.2

1.8

ISL6608