DRV8312 Datasheet, PDF (19/31 Pages) Texas Instruments – Three Phase PWM Motor Driver

DRV8312
DRV8332
www.ti.com
SLES256 – MAY 2010
Output Inductor Selection
For normal operation, inductance in motor (assume larger than 10 µH) is sufficient to provide low di/dt output
(e.g. for EMI) and proper protection during overload condition (CBC current limiting feature). So no additional
output inductors are needed during normal operation.
However during a short condition, the motor (or other load) could be shorted, so the load inductance might not
present in the system anymore; the current in short condition can reach such a high level that may exceed the
abs max current rating due to extremely low impendence in the short circuit path and high di/dt before oc
detection circuit kicks in. So a ferrite bead or inductor is recommended to utilize the short circuit protection
feature in DRV8312/32. With an external inductor or ferrite bead, the current will rise at a much slower rate and
reach a lower current level before oc protection starts. The device will then either operate CBC current limit or
OC shut down automatically (when current is well above the current limit threshold) to protect the system.
For a system that has limited space, a power ferrite bead can be used instead of an inductor. The current rating
of ferrite bead has to be higher than the RMS current of the system at normal operation. A ferrite bead designed
for very high frequency is NOT recommended. A minimum impedance of 10 Ω or higher is recommended at 10
MHz or lower frequency to effectively limit the current rising rate during short circuit condition.
The TDK MPZ2012S300A and MPZ2012S101A (with size of 0805 inch type) have been tested in our system to
meet short circuit conditions in the DRV8312. But other ferrite beads that have similar frequency characteristics
can be used as well.
For higher power applications, such as in the DRV8332, there might be limited options to select suitable ferrite
bead with high current rating. If an adequate ferrite bead cannot be found, an inductor can be used.
The inductance can be calculated as:
Loc
_ min
=
PVDD ×Toc _ delay
Ipeak - Iave
(1)
Where Toc_delay = 250 nS, Ipeak = 15 A (below abs max rating).
Because an inductor usually saturates pretty quickly after reaching its current rating, it is recommended to use an
inductor with a doubled value or an inductor with a current rating well above the operating condition.
PCB LAYOUT RECOMMENDATION
PCB Material Recommendation
FR-4 Glass Epoxy material with 2 oz. copper on both top and bottom layer is recommended for improved thermal
performance (better heat sinking) and less noise susceptibility (lower PCB trace inductance).
Ground Plane
Because of the power level of these devices, it is recommended to use a big unbroken single ground plane for
the whole system / board. The ground plane can be easily made at bottom PCB layer. In order to minimize the
impedance and inductance of ground traces, the traces from ground pins should keep as short and wide as
possible before connected to bottom ground plane through vias. Multiple vias are suggested to reduce the
impedance of vias. Try to clear the space around the device as much as possible especially at bottom PCB side
to improve the heat spreading.
Decoupling Capacitor
High frequency decoupling capacitors (100 nF) should be placed close to PVDD_X pins and with a short ground
return path to minimize the inductance on the PCB trace.
AGND
AGND is a localized internal ground for logic signals. A 1-Ω resistor is recommended to be connected between
GND and AGND to isolate the noise from board ground to AGND. There are other two components are
connected to this local ground: 0.1-µF capacitor between VREG to AGND and Roc_adj resistor between
OC_ADJ and AGND. Capacitor for VREG should be placed close to VREG and AGND pins and connected
without vias.
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): DRV8312 DRV8332
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