ISL6545, ISL6545A

FN6305 Rev 7.00

Page 14 of 17

October 7, 2015

BOOTSTRAP Considerations

Figure 11 shows the upper gate drive (BOOT pin) supplied by a

bootstrap circuit from VCC. The boot capacitor, CBOOT,

develops a floating supply voltage referenced to the PHASE pin.

The supply is refreshed to a voltage of VCC less the boot diode

drop (VD) each time the lower MOSFET, Q2, turns on. Check

that the voltage rating of the capacitor is above the maximum

VCC voltage in the system; a 16V rating should be sufficient for a

12V system. A value of 0.1µF is typical for many systems driving

single MOSFETs.

If VCC is 12V, but VIN is lower (such as 5V), then another

option is to connect the BOOT pin to 12V, and remove the

BOOT cap (although, you may want to add a local cap from

BOOT to GND). This will make the UGATE VGS voltage equal

to (12V - 5V = 7V). That should be high enough to drive most

MOSFETs, and low enough to improve the efficiency slightly.

Do NOT leave the BOOT pin open, and try to get the same

effect by driving BOOT through VCC and the internal diode;

this path is not designed for the high current pulses that will

result.

For low VCC voltage applications where efficiency is very

important, an external BOOT diode (in parallel with the internal

one) may be considered. The external diode drop has to be

lower than the internal one; the resulting higher VG-S of the

upper FET will lower its rDS(ON). The modest gain in efficiency

should be balanced against the extra cost and area of the

external diode.

+VCC

ISL6545x

GND

LGATE/OCSET

UGATE

PHASE

BOOT

VCC

+VIN

VG-S  VCC - VD

VG-S  VCC

CBOOT

Q1

Q2

+

-

FIGURE 11. UPPER GATE DRIVE BOOTSTRAP

VCC

+ VD -

ISL6545, ISL6545A