DRV8813

www.ti.com

SLVSA72C

– APRIL 2010 – REVISED MAY 2011

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 2 as case 1. The current flow direction shown

indicates the state when the xENBL pin is high.

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 2 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 2 as case 3.

Figure 2. Decay Mode

The DRV8813 supports fast decay, slow decay and a mixed decay mode. Slow, fast, or mixed decay mode is

selected by the state of the DECAY pin - logic low selects slow decay, open selects mixed decay operation, and

logic high sets fast decay mode. The DECAY pin has both an internal pullup resistor of approximately 130 k

Ω

and an internal pulldown resistor of approximately 80 k

Ω. This sets the mixed decay mode if the pin is left open

or undriven. Note that the DECAY pin sets the decay mode for both H-bridges.

Mixed decay mode begins as fast decay, but at a fixed period of time (75% of the PWM cycle) switches to slow

decay mode for the remainder of the fixed PWM period.

Blanking Time

After the current is enabled in an H-bridge, the voltage on the xISEN pin is ignored for a fixed period of time

before enabling the current sense circuitry. This blanking time is fixed at 3.75

μs. Note that the blanking time also

sets the minimum on time of the PWM.

Copyright

© 2010–2011, Texas Instruments Incorporated

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