DRV8821

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SLVS912C – JANUARY 2009 – REVISED JANUARY 2010

The DRV8821 also supports a mixed decay mode. Mixed decay mode begins as fast decay, but after a period of

time (75% of the PWM cycle) switches to slow decay mode for the remainder of the fixed PWM period.

Mixed decay mode is only active if the current through the winding is decreasing (per the indexer step table); if

the current is increasing, then slow decay is always used.

Slow or mixed decay mode is selected by the state of the xDECAY pins - logic low selects slow decay, and logic

high selects mixed decay operation.

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 ms. Note that the blanking time also

sets the minimum on time of the PWM.

Microstepping Indexer

Built-in indexer logic in the DRV8821 allows a number of different stepping configurations. The xUSM1 and

xUSM0 pins are used to configure the stepping format as shown in the table below:

xUSM1

xUSM0

STEP MODE

0

0

Full step (2-phase excitation)

0

1

½ step (1-2 phase excitation)

1

0

1/4 step (W1-2 phase excitation)

1

1

Eight microsteps/steps

The following table shows the relative current and step directions for different settings of xUSM1 and xUSM0. At

each rising edge of the xSTEP input, the indexer travels to the next state in the table. The direction is shown with

the DIR pin high; if the xDIR pin is low the sequence is reversed. Positive current is defined as xOUT1 = positive

with respect to xOUT2.

Note that the home state is 45 degrees. This state is entered at power-up or application of xRESETn.

Motor AB and motor CD act independently, and their indexer logic functions identically.

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Product Folder Link(s): DRV8821