ISL6526AEVAL2 datasheet(14/18 Pages) RENESAS

14 / 18 page

ISL6526, ISL6526A

FN9055 Rev 12.00

Page 14 of 18

Sep 30, 2015

Just after the PWM switching cycle begins and the charge

transfer from the bootstrap capacitor to the gate capacitance is

complete, the voltage on the bootstrap capacitor is at its lowest

point during the switching cycle. The charge lost on the

bootstrap capacitor will be equal to the charge transferred to

the equivalent gate-source capacitance of the upper MOSFET

as shown in Equation 18:

where QGATE is the maximum total gate charge of the upper

MOSFET, CBOOT is the bootstrap capacitance, VBOOT1 is the

bootstrap voltage immediately before turn-on, and VBOOT2 is

the bootstrap voltage immediately after turn-on.

The bootstrap capacitor begins its refresh cycle when the gate

drive begins to turn-off the upper MOSFET. A refresh cycle ends

when the upper MOSFET is turned on again, which varies

depending on the switching frequency and duty cycle.

The minimum bootstrap capacitance can be calculated by

rearranging the previous equation and solving for CBOOT.

Typical gate charge values for MOSFETs considered in these

types of applications range from 20 to 100nC. Since the

voltage drop across QLOWER is negligible, VBOOT1 is simply

VCPVOUT - VD. A Schottky diode is recommended to minimize

the voltage drop across the bootstrap capacitor during the on-

time of the upper MOSFET. Initial calculations with VBOOT2 no

less than 4V will quickly help narrow the bootstrap capacitor

range.

For example, consider an upper MOSFET is chosen with a

maximum gate charge, Qg, of 100nC. Limiting the voltage drop

across the bootstrap capacitor to 1V results in a value of no

less than 0.1µF. The tolerance of the ceramic capacitor should

also be considered when selecting the final bootstrap

capacitance value.

A fast recovery diode is recommended when selecting a

bootstrap diode to reduce the impact of reverse recovery

charge loss. Otherwise, the recovery charge, QRR, would have

to be added to the gate charge of the MOSFET and taken into

consideration when calculating the minimum bootstrap

capacitance.

ISL6526, ISL6526A DC/DC Converter

Application Circuit

Figure 8 shows an application circuit of a DC/DC Converter.

Detailed information on the circuit, including a complete Bill of

Materials and circuit board description, can be found in

Application Note AN1021:

http://www.intersil.com/data/an/an1021.pdf.

QGATE

CBOOT VBOOT1 VBOOT2

–





(EQ. 18)

CBOOT

QGATE

VBOOT1 V

–

BOOT2

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

(EQ. 19)

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