DRV8821_16 Datasheet, PDF (14/32 Pages) Texas Instruments – Dual Stepper Motor Controller and Driver

DRV8821
SLVS912J – JANUARY 2009 – REVISED JANUARY 2016
www.ti.com
7.3.6 Protection Circuits
The DRV8821 is fully protected against undervoltage, overcurrent and overtemperature events.
7.3.6.1 Overcurrent Protection (OCP)
All of the drivers in DRV8821 are protected with an OCP (Over-Current Protection) circuit.
The OCP circuit includes an analog current limit circuit, which acts by removing the gate drive from each output
FET if the current through it exceeds a preset level. This circuit will limit the current to a level that is safe to
prevent damage to the FET.
A digital circuit monitors the analog current limit circuits. If any analog current limit condition exists for longer than
a preset period, all drivers in the device will be disabled.
The device is re-enabled upon the removal and re-application of power at the VM pins.
7.3.6.2 Thermal Shutdown (TSD)
If the die temperature exceeds safe limits, all drivers in the device will be shut down.
The device will remain disabled until the die temperature has fallen to a safe level. After the temperature has
fallen, the device may be re-enabled upon the removal and re-application of power at the VM pin.
7.3.6.3 Undervoltage Lockout (UVLO)
If at any time the voltage on the VM pins falls below the undervoltage lockout threshold voltage, all circuitry in the
device will be disabled. Operation will resume when VM rises above the UVLO threshold. The indexer logic will
be reset to its initial condition in the event of an undervoltage lockout.
7.3.6.4 Shoot-Through Current Prevention
The gate drive to each FET in the H-bridge is controlled to prevent any cross-conduction (shoot through current)
during transitions.
7.4 Device Functional Modes
7.4.1 Decay Mode
The DRV8821 supports two different decay modes: slow decay or mixed decay. The mixed decay mode uses
slow decay on increasing steps and mixed decay on decreasing steps. 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.
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 11 as case 1. The current flow direction shown indicates
positive current flow in Figure 11.
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 11 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 11 as case 3.
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