A3941 Allegro MicroSystems, Inc., A3941 Datasheet - Page 14

• Manufacturer Part Number: A3941
• Description: Automotive Full Bridge MOSFET Driver
• Manufacturer: Allegro MicroSystems, Inc.

A3941
schemes, the PHASE input is still required to reverse the load
current.
Although fast decay with diode rectification provides a higher
degree of current control than slow decay schemes, it still may
not provide sufficient control for servo systems where full four-
quadrant control is required. This is only possible using fast
decay with synchronous rectification. By applying the PWM sig-
nal to the PHASE input, and holding PWMH and PWML and SR
high (figure 2B), the load current can be controlled in both direc-
tions with a single PWM signal. Because all four MOSFETs in
the bridge change state, the supply can be directly applied to the
load in either direction. The effect is: when the PWM duty cycle
is less than 50%, the average current flows from B to A; when
greater than 50%, the average current flows from A to B; and
when at 50%, the average current is zero. This allows the load
current to be independent of any back EMF voltage generated, for
example by a rotating motor, and effectively allowing the applied
torque to work with or against a motor in either direction.
Synchronous Rectification
reduce power dissipation in the external MOSFETs. As described
above, the A3941 can be instructed to turn on the appropriate
low-side and high-side driver during the load current recirculation
PWM off-cycle. During the decay time, synchronous rectifica-
tion allows current to flow through the selected MOSFET, rather
than through the source-drain body diode. The body diodes of the
recirculating power MOSFETs will conduct only during the dead
time that occurs at each PWM transition.
Dead Time
To prevent cross-conduction (shoot through) in any phase of
the power FET bridge, it is necessary to have a dead time delay,
t
complementary turn-on event. The potential for cross-conduc-
tion occurs when any complementary high-side and low-side pair
of FETs are switched at the same time; for example, when using
synchronous rectification or after a bootstrap capacitor charg-
ing cycle. In the A3941, the dead time for both phases is set by
a single dead-time resistor, R
AGND pins.
DEAD
, between a high-side or low-side turn-off and the next
Synchronous rectification is used to
DEAD
, between the RDEAD and
Automotive Full Bridge MOSFET Driver
Figure 2. Fast decay power bridge current paths
Figure 3. Dead time versus R
PWMH
PWML
PHASE
SR
PWMH
PWML
PHASE
SR
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
GHA
GHA
GLA
GLA
0
Inputs
Inputs
0
(B) Fast decay, SR active, full four-quadrant control
50
Driving
1
1
1
0
Driving
1
1
1
1
LOAD
LOAD
Phase
GHx
GLx
Phase
GHx
GLx
(A) Fast decay, diode recirculation
100
Outputs
Outputs
150
A
A
H
H
GHB
GLB
L
GHB
GLB
L
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
H
H
B
L
B
L
DEAD
200
R
DEAD
, (full range)
250
PWMH
PWML
PHASE
SR
PWMH
PWML
PHASE
SR
(kΩ)
GHA
GHA
GLA
GLA
Inputs
Inputs
300
Recirculating
Recirculating
0
0
1
0
1
1
0
1
LOAD
LOAD
350
Phase
GHx
GLx
Phase
GHx
GLx
Outputs
Outputs
400
A
A
GHB
GLB
GHB
GLB
H
L
L
L
450
B
B
H
L
L
L
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