a4935 Allegro MicroSystems, Inc., a4935 Datasheet - Page 19

• Manufacturer Part Number: a4935
• Description: Automotive 3-phase Mosfet Driver
• Manufacturer: Allegro MicroSystems, Inc.

A4935
tor (>1000 nF). If the bootstrap capacitor voltage does not reach
the threshold within approximately 200 μs, an undervoltage fault
will be flagged.
VREG Capacitor Selection
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 FET is turned on, the gate-drive
circuit will provide the high transient current to the gate that is
necessary to turn on the FET quickly. This current, which can be
several hundred milliamperes, cannot be provided directly by the
limited output of the VREG regulator, and must be supplied by an
external capacitor connected to VREG.
The turn-on current for the high-side FET is similar in value to
that for the low-side FET, but is mainly supplied by the boot-
strap capacitor. However the bootstrap capacitor must then be
recharged from the VREG regulator output. Unfortunately the
bootstrap recharge can occur a very short time after the low-
side turn-on occurs. This requires that the value of the capacitor
connected between VREG and AGND should be high enough to
minimize the transient voltage drop on VREG for the combina-
tion of a low-side FET turn-on and a bootstrap capacitor recharge.
A value of 20 × C
working voltage will never exceed V
rated as low as 15 V. This capacitor should be placed as close as
possible to the VREG pin.
Supply Decoupling
Because this is a switching circuit, there are current spikes from all
supplies at the switching points. As with all such circuits, the power
supply connections should be decoupled with a ceramic capacitor,
typically 100 nF, between the supply pin and ground. These capaci-
tors should be connected as close as possible to the device supply
pins VBB and VDD, and the power ground pin, PGND.
Power Dissipation
In applications where a high ambient temperature is expected, the
on-chip power dissipation may become a critical factor. Careful
BOOT
is a reasonable value. The maximum
REG
, so the capacitor can be
Automotive 3-Phase MOSFET Driver
attention should be paid to ensure the operating conditions allow
the A4935 to remain in a safe range of junction temperature.
The power consumed by the A4935, P
given:
where:
Braking
The A4935 can be used to perform dynamic braking by either
forcing all low-side FETs on and all high-side FETs off or, con-
versely, by forcing all low-side FETs off and all high-side FETs
on. This will effectively short-circuit the back EMF of the motor,
creating a breaking torque.
During braking, the load current can be approximated by:
where V
resistance of the phase winding.
Care must be taken during braking to ensure that the maximum
ratings of the power FETs are not exceeded. Dynamic braking is
equivalent to slow decay with synchronous rectification and all
phases enabled.
P
SWITCHING
N is the number of FETs switching during a PWM cycle, and
Ratio
I
AV
P
P
CPUMP
=
BIAS
=
BEMF
Q
P
GATE
D
R
GATE
=
=
=
=
=
or
is the voltage generated by the motor and R
[( 2 V
[V
P
V
Q
10
× N × f
BB
BIAS
BB
GATE
+ 10
– V
× I
BB
I
+ P
BRAKE
× V
BB
REG
PWM
) – V
CPUMP
.
] I
REG
;
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
REG
,
=
AV
× N × f
] I
+ P
V
BEMF
R
AV
SWITCHING
L
PWM
D
, can be estimated by:
, for V
, for V
,
× Ratio ;
BB
BB
,
< 15 V,
≥ 15 V,
L
is the
(10)
(6)
(7)
(8)
(9)
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