A89503
80 V Automotive Half-Bridge MOSFET Driver
APPLICATION INFORMATION
Bootstrap Capacitor Selection
CBOOT × ∆V
tCHARGE
=
The A89503 requires a bootstrap capacitor C. To simplify this
description of the bootstrap capacitor selection criteria, generic
naming is used here. So, for example, CBOOT, QBOOT, and VBOOT
refer to the bootstrap capacitor, and QGATE refers to the high-side
MOSFET. CBOOT must be correctly selected to ensure proper
operation of the device—too large and time will be wasted charg-
ing the capacitor, resulting in a limit on the maximum duty cycle
and PWM frequency; too small and there can be a large volt-
age drop at the time the charge is transferred from CBOOT to the
MOSFET gate.
100
where CBOOT is the value of the bootstrap capacitor in nF and ∆V
is the required voltage of the bootstrap capacitor. At power up and
when the drivers have been disabled for a long time, the bootstrap
capacitor can be completely discharged. In this case, ∆V can be
considered to be the full high-side drive voltage, 12 V. Otherwise,
∆V is the amount of voltage dropped during the charge transfer,
which should be 400 mV or less. The capacitor is charged when-
ever the S terminal is pulled low and current flows from the capaci-
tor connected to the VREG terminal through the internal bootstrap
To keep the voltage drop due to charge sharing small, the charge
in the bootstrap capacitor, QBOOT, should be much larger than
QGATE, the charge required by the gate:
diode circuit to CBOOT
.
VREG Capacitor Selection
QBOOT >> QGATE
A factor of 20 is a reasonable value, so
QBOOT = CBOOT × VBOOT = QGATE × 20
The internal reference, VREG, supplies current for the low-side
gate-drive circuits and the charging current for the bootstrap
capacitors. When a low-side MOSFET is turned on, the gate-
drive circuit will provide the high transient current to the gate that
is necessary to turn the MOSFET on quickly. This current, which
can be several hundred milliamperes, cannot be provided directly
by the limited output of the VREG regulator but must be supplied
by an external capacitor, CREG, connected between the VREG
terminal and GND.
or
QGATE × 20
CBOOT
=
VBOOT
where VBOOT is the voltage across the bootstrap capacitor.
The turn-on current for the high-side MOSFET is similar in value
but is mainly supplied by the bootstrap capacitor. However, the
bootstrap capacitor must then be recharged from CREG through
the VREG terminal. Unfortunately, the bootstrap recharge can
occur a very short time after the low-side turn on occurs. This
means that the value of CREG between VREG and GND should
be high enough to minimize the transient voltage drop on
VREG for the combination of a low-side MOSFET turn on and
a bootstrap capacitor recharge. For block commutation control
(trapezoidal drive), where only one high side and one low side
are switching during each PWM period, a minimum value of 20 ×
CBOOT is reasonable. For sinusoidal control schemes, a minimum
value of 40 × CBOOT is recommended. As the maximum work-
ing voltage of CREG will never exceed VREG, the part’s voltage
rating can be as low as 15 V. However, it is recommended that
a capacitor rated to at least twice the maximum working volt-
age should be used to reduce any impact operating voltage may
have on capacitance value. For best performance, CREG should be
ceramic rather than electrolytic. CREG should be mounted as close
to the VREG terminal as possible.
The voltage drop, ∆V, across the bootstrap capacitor as the MOS-
FET is being turned on, can be approximated by:
QGATE
∆V
=
CBOOT
so for a factor of 20, ∆V will be 5% of VBOOT
.
The maximum voltage across the bootstrap capacitor under
normal operating conditions is VREG (max). However in some
circumstances the voltage may transiently reach a maximum of
18 V, which is the clamp voltage of the Zener diode between the
C terminal and the S terminal. In most applications with a good
ceramic capacitor, the working voltage can be limited to 16 V.
Bootstrap Charging
It is necessary to ensure the high-side bootstrap capacitor is com-
pletely charged before a high-side PWM cycle is requested. The time
required to charge the capacitor, tCHARGE, in µs, is approximated by:
40
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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