DRV8848_16 Datasheet, PDF (12/27 Pages) Texas Instruments – Dual H-Bridge Motor Driver

DRV8848
SLLSEL7A – OCTOBER 2014 – REVISED NOVEMBER 2015
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The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor,
connected to the xISEN pin, with a reference voltage. The reference voltage is derived from the voltage applied
to the VREF pin and it is V_VREF / 6.6. The VREF pin can be tied, on board, to the 3.3 V – VINT pin, or it can be
externally forced to a desired VREF voltage.
The full scale chopping current in a winding is calculated as follows:
I_FS
V_VREF
6.6 u R_ISENSE
where
• I_FS is the regulated current.
• V_VREF is the voltage on the VREF pin.
• R_ISENSE is the resistance of the sense resistor.
(1)
Example: If V_VREF is 3.3 V and a 500-mΩ sense resistor is used, the full-scale chopping current is 3.3 V / (6.6 ×
500 mΩ) = 1 A.
Note that if the current control is not needed, the xISEN pins may be connected directly to ground. In this case,
VREF should be connected to VINT.
7.3.5 Current Recirculation and Decay Modes
During PWM current chopping, the H-bridge is enabled to drive current through the motor winding until the PWM
current chopping threshold is reached (see case 1 in Figure 8).
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. This is called recirculation current. To handle
this recirculation current, the DRV8848 H-bridge operates in mixed decay mode.
Mixed decay is a combination of fast and slow decay modes. In fast decay mode, the opposite drivers are turned
on to allow the current to decay (see case 2 in Figure 8). If the winding current approaches zero, while in fast
decay, the bridge is disabled to prevent any reverse current flow. In slow decay mode, winding current is
recirculated by enabling both of the low-side FETs in the bridge (see case 3 in Figure 8). Mixed decay starts with
fast decay, then goes to slow decay. In DRV8848, the mixed decay ratio is 25% fast decay and 75% slow decay
(as shown in Figure 9).
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