DRV8711 Datasheet, PDF (14/34 Pages) Texas Instruments – STEPPER MOTOR CONTROLLER IC

DRV8711
SLVSC40 – JUNE 2013
www.ti.com
Example:
If a 0.1-Ω sense resistor is used, ISGAIN is set to 0 (gain of 5), and TORQUE is set to 255, the full-scale (100%)
chopping current will be (2.75V * 255) / (256 * 5 * 0.1Ω) = 5.5A.
Decay Modes
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 3, Item 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. If synchronous rectification is enabled (SRn pin logic low), the
opposite FETs are turned on; as the winding current approaches zero, the bridge is disabled to prevent any
reverse current flow. If SRn is high, current is recirculated through the body diodes, or through external Schottky
diodes. Fast-decay mode is shown in Figure 3, Item 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 3, Item 3.
VM
PWM
ON
PWM OFF
Slow Decay
1
xOUT1
3
2
xOUT2
1 Drive Current
2 Fast decay (reverse)
3 Slow decay (brake)
Fast Decay
Mixed Decay
TBLANK
TDECAY
TOFF
Itrip
Figure 3. Decay Modes
The DRV8711 supports fast decay and slow decay modes in both indexer and direct PWM modes. In addition, in
indexer mode only, it supports fixed mixed decay and auto mixed decay modes. Decay mode is selected by the
DECMOD bits in the DECAY register.
Mixed decay mode begins as fast decay, but after a programmable period of time (set by the TDECAY bits in the
DECAY register) switches to slow decay mode for the remainder of the fixed off time. Even if mixed decay is
selected, if the current is increasing or remaining the same (per the step table), then slow decay is used.
Auto mixed decay mode samples the current level at the end of the blanking time, and if the current is above the
Itrip threshold, immediately changes the H-bridge to fast decay. During fast decay, the (negative) current is
monitored, and when it falls below the Itrip threshold (and another blanking time has passed), the bridge is
switched to slow decay. Once the fixed off time expires, a new cycle is started.
If the bridge is turned on and at the end of TBLANK the current is below the Itrip threshold, the bridge remains on
until the current reaches Itrip. Then slow decay is entered for the fixed off time, and a new cycle begins.
Refer to Figure 4 and Figure 5.
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