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DRV8821DCA Datasheet(PDF) 8 Page - Texas Instruments |
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DRV8821DCA Datasheet(HTML) 8 Page - Texas Instruments |
8 / 22 page 5 xOUT1 xOUT2 3 1 2 3 Drive current Slow decay (brake) Fast decay (reverse) VM 1 2 DRV8821 SLVS912C – JANUARY 2009 – REVISED JANUARY 2010 www.ti.com Current Regulation The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor connected to the xISEN pins, multiplied by a factor of 5, with a reference voltage. The reference voltage is input from the xVREF pin. The full-scale (100%) chopping current is calculated as follows: (1) Example: If a 0.22- Ω sense resistor is used and the VREFx pin is 3.3 V, the full-scale (100%) chopping current is 3.3 V/(5 * 0.22 Ω) = 1.875 A. The reference voltage is also scaled by an internal DAC that allows torque control for fractional stepping of a bipolar stepper motor, as described in the "microstepping indexer" section below. Decay Mode During PWM current chopping, the H-bridge is enabled to drive 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 positive current flow in the step table below. 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 8 Submit Documentation Feedback Copyright © 2009–2010, Texas Instruments Incorporated Product Folder Link(s): DRV8821 |
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