DRV8824-Q1_15 Datasheet, PDF (17/30 Pages) Texas Instruments – DRV8824-Q1 Automotive Motor Controller IC

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DRV8824-Q1
SLVSCH0 – APRIL 2014
8.3.7.3 Heatsinking
The PowerPAD™ package uses an exposed pad to remove heat from the device. For proper operation, this pad
must be thermally connected to copper on the PCB to dissipate heat. On a multi-layer PCB with a ground plane,
this can be accomplished by adding a number of vias to connect the thermal pad to the ground plane. On PCBs
without internal planes, copper area can be added on either side of the PCB to dissipate heat. If the copper area
is on the opposite side of the PCB from the device, thermal vias are used to transfer the heat between top and
bottom layers.
For details about how to design the PCB, refer to TI application report SLMA002, "PowerPAD™ Thermally
Enhanced Package" and TI application brief SLMA004, "PowerPAD™ Made Easy", available at www.ti.com.
In general, the more copper area that can be provided, the more power can be dissipated. It can be seen that the
heatsink effectiveness increases rapidly to about 20 cm², then levels off somewhat for larger areas.
8.4 Device Functional Modes
8.4.1 Decay Mode
During PWM current chopping, the H-bridge is enabled to drive current through the motor winding until the PWM
current chopping threshold is reached. This is shown in Figure 7 as case 1. The current flow direction shown
indicates positive current flow.
Once the chopping current threshold is reached, the H-bridge can operate in two different states, fast decay or
slow decay.
In fast decay mode, once the PWM chopping current level has been reached, the H-bridge reverses state to
allow winding current to flow in a reverse direction. As the winding current approaches zero, the bridge is
disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 7 as case 2.
In slow decay mode, winding current is re-circulated by enabling both of the low-side FETs in the bridge. This is
shown in Figure 7 as case 3.
Figure 7. Decay Mode
The DRV8824-Q1 supports fast decay, slow decay and a mixed decay mode. Slow, fast, or mixed decay mode is
selected by the state of the DECAY terminal - logic low selects slow decay, open selects mixed decay operation,
and logic high sets fast decay mode. The DECAY terminal has both an internal pullup resistor of approximately
130 kΩ and an internal pulldown resistor of approximately 80 kΩ. This sets the mixed decay mode if the terminal
is left open or undriven.
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