Dual Full-Bridge Motor Driver
A4970
7
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Current Sensing
To minimize current sensing inaccuracies caused by ground trace
IR drops, each current-sensing resistor should have a separate
return to the ground terminal of the device. For low-value sense
resistors, the IR drops in the PCB can be signifi cant and should
be taken into account. The use of sockets should be avoided as
their contact resistance can cause variations in the effective value
of R
S
.
Generally, larger values of R
S
reduce the aforementioned effects
but can result in excessive heating and power loss in the sense
resistor. The selected value of R
S
should not cause the maximum
operating voltage of 0.75 V (V
REF
(max)/10), for the VE terminal,
to be exceeded. The recommended value of R
S
is in the range of:
R
S
= 0.50 / I
TRIP
(max) .
If desired, the reference input voltage can be fi ltered by placing
a capacitor from VREF to ground. The ground return for this
capacitor as well as the bottom of any resistor divider used
should be independent of the high-current power-ground trace to
avoid changes in VREF due to IR drops.
Thermal Considerations
For normal operation it is recommended that the maximum
operating junction temperature be 145°C, which is below the
operating range of the TSD system. The junction temperature can
be measured best by attaching a thermocouple to the batwing of
the device, and measuring the pin temperature, T
PIN
. The junction
temperature can then be approximated by using the formula:
T
J
= T
PIN
+ (2 × I
LOAD
× V
F
× R
θJT
) ,
where V
F
can be chosen from the electrical specifi cation table
for the given level of I
LOAD
. The value for R
θJT
is approximately
6°C/W.
The power dissipation of the batwing package can be improved
20% to 30% by adding a section of printed circuit board copper
(typically 6 to 18 square centimeters) connected to the batwing
terminals of the device.
The thermal performance in applications that run at high load
currents, high duty cycles, or both, can be improved by adding
external diodes from each output to ground in parallel with the
internal diodes. Fast-recovery (≤200 ns) diodes should be used to
minimize switching losses.
Load Supply Terminal
The load supply terminal, VBB, should be decoupled with an
electrolytic capacitor (≥47 μF is recommended), placed as close
to the device as is physically practical. To minimize the effect of
system ground IR drops on the logic and reference input signals,
the system ground should have a low-resistance return to the load
supply voltage.
Fixed Off-Time Selection
With increasing values of t
OFF
, switching losses decrease, low-
level load current regulation improves, EMI reduces, PWM
frequency decreases, and ripple current increases. The value of
t
OFF
can be chosen for optimization of these parameters. For
applications where audible noise is a concern, typical values of
t
OFF
should be chosen in the range of 15 to 35 μs.
APPLICATION NOTES
