ISL6527IBZ datasheet(13/16 Pages) INTERSIL

13 / 16 page

FN9056.10

November 18, 2008

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 19:

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 Equation 19 and solving for CBOOT in

Equation 20.

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.

ISL6527, ISL6527A 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

ISL6527,

GND

LGATE

UGATE

PHASE

BOOT

VIN

NOTE:

VG-S ≈ VCC

CBOOT

DBOOT

QUPPER

QLOWER

FIGURE 7. UPPER GATE DRIVE BOOTSTRAP

VG-S ≈ VCC -VD

CPVOUT

ISL6527A

GATE

BOOT

BOOT1

BOOT2

–

()

(EQ. 19)

BOOT

GATE

BOOT1

–

BOOT2

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

≥

(EQ. 20)

ISL6527, ISL6527A

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