A4957 Datasheet, PDF (9/16 Pages) Allegro MicroSystems – The A4957 is a full-bridge controller for use with external N-channel power MOSFETs and is specifically designed for applications with high-power inductive loads such as brush DC motors.

A4957
Full Bridge MOSFET Driver
Application Information
Dead Time
To prevent cross-conduction (shoot-through) in any phase of the
power MOSFET bridge, it is necessary to have a dead time delay,
t_DEAD , between a high-side or low-side turn-off and the next
complementary turn-on event. The potential for cross-conduction
occurs when any complementary high-side and low-side pair
of MOSFETs are switched at the same time; for example, when
using synchronous rectification or after a bootstrap capacitor
charging cycle. In the A4957, the dead time for both phases is set
by a single dead-time resistor, R_DEAD, between the RDEAD and
AGND pins.
For R_DEAD between 3 and 240 kΩ at 25°C, the value of t_DEAD
(ns), can be approximated by:
7200
t _DEAD ≈ 50 + _{1.2 + ( 200 / RDEAD )}
where R_DEAD is in kΩ. Figure 1 illustrates the relationship of
t_DEAD and R_DEAD , with the greatest accuracy obtained for values
of R_DEAD between 6 and 60 kΩ.
The I_DEAD current can be estimated by:
1.2
I_DEAD ≈ _RDEAD
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
50 100 150 200 250 300 350 400
R_DEAD (kΩ)
Figure 1. Dead time versus values of R_DEAD (full range).
The maximum dead time of 6 μs typical can be set by connecting
the RDEAD pin directly to VDD.
Alternatively, the dead time in the A4957 can be disabled by
connecting the RDEAD pin directly to GND. In this case the
required dead time must be supplied by the external controller.
The choice of power MOSFET and external series gate resistance
determine the selection of the dead-time resistor, R_DEAD. The
dead time should be long enough to ensure that one MOSFET
in a phase has stopped conducting before the complementary
MOSFET starts conducting. This should also take into account
the tolerance and variation of the MOSFET gate capacitance, the
series gate resistance, and the on-resistance of the A4957 internal
drives.
Dead time will be present only if the on-command for one
MOSFET occurs within t_DEAD after the off-command for its
complementary MOSFET. In the case where one side of a phase
drive is permanently off, for example when using diode rectifica-
tion with slow decay, then the dead time will not occur. In this
case the gate drive will turn on within the specified propagation
delay after the corresponding phase input goes high. (Refer to the
Gate Drive Timing diagrams.)
Braking
The A4957 can be used to perform dynamic braking by forc-
ing all low-side MOSFETs on and all high-side MOSFETs off
(ALO=BLO=1, AHI=BHI=0) or, conversely, by forcing all low-
side off and all high-side on (ALO=BLO=0, AHI= BHI=1). This
effectively short-circuits the back EMF of the motor, creating a
breaking torque.
During braking, the load current can be approximated by:
V_bemf
I_BRAKE ≈ _RL
where V_bemf is the voltage generated by the motor and R_L is the
resistance of the phase winding.
Care must be taken during braking to ensure that maximum rat-
ings of the power MOSFETs are not exceeded. Dynamic braking
is equivalent to slow decay with synchronous rectification.
Allegro MicroSystems, Inc.
9
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com