DRV8846_15 Datasheet, PDF (17/35 Pages) Texas Instruments – DRV8846 Dual H-Bridge Stepper Motor Driver

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DRV8846
SLLSEK2 – JUNE 2014
8.3.4 Decay Mode
After the chopping current threshold is reached, the drive current is interrupted, but due to the inductive nature of
the motor, current must continue to flow for some period of time (called recirculation current). To handle this
recirculation current, the H-bridge can operate in two different states, fast decay or slow decay (or a mixture of
fast and slow decay).
In fast-decay mode, after the PWM chopping current level is reached, the H-bridge reverses state to allow
winding current to flow through the opposing FETs. As the winding current approaches 0, the bridge is disabled
to prevent any reverse current flow. For fast-decay mode, see number 2 in Figure 7.
In slow-decay mode, winding current is recirculated by enabling both of the low-side FETs in the bridge. For
slow-decay mode, see number 3 in Figure 7.
xVM
xOUT1
1 Drive Current
1
2 Fast decay
xOUT2 _{3 Slow decay}
2
3
Figure 7. Decay Modes
The DRV8846 supports fast, slow, mixed, and adaptive decay modes. With stepper motors, the decay mode is
chosen for a given stepper motor and operating conditions to minimize mechanical noise and vibration.
In mixed decay mode, the current recirculation begins as fast decay, but at a fixed period of time (determined by
the state of the DEC1 and DEC0 pins shown in Table 7) the current recirculation switches to slow decay mode
for the remainder of the fixed PWM period. Note that the DEC1 and DEC0 pins are tri-level inputs; these pins
can be driven logic low, logic high, or high-impedance (Z).
Figure 8 shows the current waveforms in slow, fast, and 25% and 1 t_BLANK mixed decay modes.
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