A3946_16 Datasheet, PDF (8/14 Pages) Allegro MicroSystems – Half-Bridge Power MOSFET Controller

A3946
Half-Bridge Power MOSFET Controller
Application Information
Bootstrap Capacitor Selection. C must be cor-
BOOT
rectly selected to ensure proper operation of the device. If
too large, time is wasted charging the capacitor, with the
result being a limit on the maximum duty cycle and PWM
frequency. If the capacitor is too small, the voltage drop can
be too large at the time the charge is transferred from the
C to the MOSFET gate.
BOOT
To keep the voltage drop small:
Q >> Q
BOOT
GATE
where a factor in the range of 10 to 20 is reasonable. Using
20 as the factor:
and
× × Q = C
V =Q
20
BOOT
BOOT
BOOT
GATE
× C = Q
20 / V
BOOT
GATE
BOOT
The voltage drop on the BOOT pin, as the MOSFET is being
turned on, can be approximated by:
At power-up and when the drivers have been disabled for
a long time, the bootstrap capacitor can be completely
discharged. In this case, Delta_v can be considered to be the
full high-side drive voltage, 12 V. Otherwise, Delta_v is the
amount of voltage dropped during the charge transfer, which
should be 400 mV or less. The capacitor is charged whenever
the S pin is pulled low, via a GL PWM cycle, and current
flows from VREG through the internal bootstrap diode
circuit to C_BOOT.
Power Dissipation. For high ambient temperature
applications, there may be little margin for on-chip power
consumption. Careful attention should be paid to ensure that
the operating conditions allow the A3946 to remain in a safe
range of junction temperature.
The power consumed by the A3946 can be estimated as:
P_total = Pd_bias + Pd_cpump + Pd_switching_loss
Delta_v = Q / C
GATE BOOT
For example, given a gate charge, Q_GATE, of 160 nC, and the
typical BOOT pin voltage of 12 V, the value of the Boot
capacitor, C , can be determined by:
BOOT
C
BOOT
=
(160
nC
×
20)
/
12
V
≈
0.266
μF
Therefore, a 0.22 μF ceramic (X7R) capacitor can be chosen
for the Boot capacitor.
In that case, the voltage drop on the BOOT pin, when the
high-side MOSFET is turned on, is:
where:
Pd_bias
=
V
BB
×
I
VBB
,
typically
3
mA,
and
Pd_cpump = (2V_BB – V_REG) I_AVE, for V_BB < 15 V, or
Pd_cpump = (V_BB – V_REG) I_AVE, for V_BB > 15 V,
in either case, where
× × I_AVE = Q_GATE 2 f_PWM
and
Delta_v = 160 nC / 0.22 μF = 0.73 V
Pd_switching_loss = Q_GATE × V_REG × 2 × f_PWM Ratio,
Bootstrap Charging. It is good practice to ensure that the
high-side bootstrap capacitor is completely charged before a
high-side PWM cycle is requested.
where
Ratio = 10 Ω / (R + 10 Ω).
GATE
The time required to charge the capacitor can be approxi-
mated by:
t_CHARGE = C_BOOT (Delta_v / 100 mA)
Allegro MicroSystems, LLC
8
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Worcester, Massachusetts 01615-0036 U.S.A.
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