13
FN9056.11
September 29, 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 19:
where Q
GATE
is the maximum total gate charge of the upper
MOSFET, C
BOOT
is the bootstrap capacitance, V
BOOT1
is
the bootstrap voltage immediately before turn-on, and
V
BOOT2
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 Q
LOWER
is negligible, V
BOOT1
is
simply V
CPVOUT
- V
D
. 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 V
BOOT2
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, Q
g
, 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, Q
RR
, 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:
NOTE:
V
G-S
V
CC
CBOOT
DBOOT
Q
UPPER
Q
LOWER
+
-
FIGURE 7. UPPER GATE DRIVE BOOTSTRAP
V
G-S
V
CC
-V
D
+
V
D
-
CPVOUT
ISL6527A
Q
GATE
C
BOOT
V
BOOT1
V
BOOT2
–=
(EQ. 19)
C
BOOT
Q
GATE
V
BOOT1
V–
BOOT2
-----------------------------------------------------
(EQ. 20)
ISL6527, ISL6527A
