DRV8332-HT Datasheet, PDF (20/26 Pages) Texas Instruments – THREE PHASE PWM MOTOR DRIVER

DRV8332-HT
SLES274B – AUGUST 2013 – REVISED JANUARY 2014
www.ti.com
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 DRV8332. 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 DRV8332. 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.
THERMAL INFORMATION
The thermally enhanced package provided with the DRV8332 is designed to interface directly to heat sink using
a thermal interface compound in between, (e.g., Ceramique from Arctic Silver, TIMTronics 413, etc.). The heat
sink then absorbs heat from the ICs and couples it to the local air.
R_θJA is a system thermal resistance from junction to ambient air. As such, it is a system parameter with the
following components:
• R_θJC (the thermal resistance from junction to case, or in this example the power pad or heat slug)
• Thermal grease thermal resistance
• Heat sink thermal resistance
The thermal grease thermal resistance can be calculated from the exposed power pad or heat slug area and the
thermal grease manufacturer's area thermal resistance (expressed in °C-in ²/W or °C-mm²/W). The approximate
exposed heat slug size is as follows:
• DRV8332, 44-pin DDV …… 0.055 in² (35.6 mm ²)
The thermal resistance of a thermal pad is considered higher than a thin thermal grease layer and is not
recommended. Thermal tape has an even higher thermal resistance and should not be used at all. Heat sink
thermal resistance is predicted by the heat sink vendor, modeled using a continuous flow dynamics (CFD) model,
or measured.
Thus the system R_θJA = R_θJC + thermal grease resistance + heat sink resistance.
See the TI application report, IC Package Thermal Metrics (SPRA953A), for more thermal information.
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