A3941 Allegro MicroSystems, Inc., A3941 Datasheet - Page 13

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

A3941
Power Bridge Management Using PWM Control
The A3941 provides two PWM control signals, a phase control
for current direction, and the ability to enable or disable syn-
chronous rectification. This allows a wide variety of full bridge
control schemes to be implemented. The six basic schemes are
shown in table 2 and described further below.
Slow Decay
control configuration. Figure 1A shows the path of the bridge
and load current when a PWM signal is applied to PWMH, with
PWML and PHASE tied high, and SR low.
In this case the high-side MOSFETs are switched off during the
current decay time (PWM off-time) and load current recirculates
through the low-side MOSFETs. This is commonly referred to as
high-side chopping or high-side PWM. The recirculating current
flows through the body diode of the low-side MOSFET, which
is complementary to the high-side MOSFET being switched off.
Improved efficiency can be achieved by turning on the comple-
mentary MOSFETs during the PWM off-time to short the reverse
diode and provide synchronous rectification. This can be easily
achieved by taking SR high as shown in figure 1B.
By applying the PWM signal to the PWML pin instead of the
PWMH pin, the low-side MOSFET is turned off during the PWM
off-time and the load current recirculates through the high-side
MOSFETs as in figure 1C.
In the three slow decay configurations shown, the direction of the
average current in the load can be reversed by simply applying a
low level to the PHASE pin. Referring to the slow decay entries
in table 2, when PHASE is high the average current flows from
the phase A connection (SA) to the phase B connection (SB).
When PHASE is low the direction is from B to A.
Fast Decay
over the load current for most simple control systems, it is pos-
sible that current control stability can be affected by, for example,
the back EMF of the load. In these cases, typically actuator posi-
tioning or servo control systems, it may be necessary to use fast
decay to provide continuous control over the load current. The
A3941 can be configured to provide fast decay using either diode
recirculation or synchronous rectification.
Fast decay with diode recirculation is achieved by applying a
PWM signal at the same time to both PWM inputs, PWMH and
PWML, with SR disabled (figure 2A). Because current recircu-
lation is through the body diodes of the MOSFETs, the aver-
age load current cannot be negative so, as for the slow decay
While slow decay usually provides sufficient control
Slow decay is the simplest and most common
Applications Information
Automotive Full Bridge MOSFET Driver
Figure 1. Slow decay power bridge current paths
PWMH
PWML
PHASE
SR
PWMH
PWML
PHASE
SR
PWMH
PWML
PHASE
SR
GHA
GHA
GHA
GLA
GLA
GLA
Inputs
Inputs
Inputs
(A) Slow decay, diode recirculation, high-side PWM
Driving
1
1
1
0
Driving
1
1
1
1
Driving
1
1
1
1
LOAD
LOAD
(B) Slow decay, SR active, high-side PWM
LOAD
(C) Slow decay, SR active, low-side PWM
Phase
GHx
GLx
Phase
GHx
GLx
Phase
GHx
GLx
Outputs
Outputs
Outputs
A
A
A
H
H
H
GHB
GLB
L
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
H
B
L
B
L
B
L
PWMH
PWML
PHASE
SR
PWMH
PWML
PHASE
SR
PWMH
PWML
PHASE
SR
GHA
GHA
GHA
GLA
GLA
GLA
Inputs
Inputs
Inputs
Recirculating
Recirculating
Recirculating
0
1
1
0
0
1
1
1
1
0
1
1
LOAD
LOAD
LOAD
Phase
GHx
GLx
Phase
GHx
GLx
Phase
GHx
GLx
Outputs
Outputs
Outputs
A
A
A
GHB
GLB
H
GHB
GLB
H
GHB
GLB
H
L
L
L
B
H
B
H
B
H
L
L
L
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