CHOP

ISENSE

2.75 V

TORQUE

I

256

ISGAIN

R

u

u

u

13

DRV8704

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SLVSD29 – OCTOBER 2015

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7.3.3 Current Regulation

The current through the motor windings is regulated by an adjustable fixed-off-time PWM current regulation

circuit. When an H-bridge is enabled, current rises through the winding at a rate dependent on the DC voltage

and inductance of the winding and the magnitude of the back EMF present. Once the current hits the current

chopping threshold, the bridge disables the current for a fixed period of time, which is programmable between

525 ns and 128 µs by writing to the TOFF bits in the OFF register. After the off time expires, the bridge is re-

enabled, starting another PWM cycle.

Note that the decay mode is set by DECMOD bits in the DECAY register. Slow, fast, mixed, or auto mixed decay

modes are available.

The chopping current is set by a comparator which compares the voltage across a current sense resistor

connected to the xISENx pins, multiplied by the gain of the current sense amplifier, with a reference voltage. The

current sense amplifier is programmable in the CTRL register.

When driving in PWM mode, the chopping current is calculated as follows:

where

•

TORQUE is the setting of the TORQUE bits

•

ISGAIN is the programmed gain of the ISENSE amplifiers (5, 10, 20, or 40).

(1)

7.3.4 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. This is shown in the diagram below 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. If the winding current approaches zero, the bridge is disabled

to prevent any reverse current flow. Fast decay mode is shown in the diagram below as case 2.

In slow decay mode, winding current is recirculated by enabling both of the low-side FETs in the bridge. This is

shown as case 3 in Figure 9.