DRV8701 datasheet(16/42 Pages) TI

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9_9

BLANK

ICHOP

tOFF

9_9

BLANK

tOFF

VREF

Drive

Brake / Slow Decay

Drive

Brake / Slow Decay

REF

OFF

CHOP

SENSE

± 9

DRV8701

SLVSCX5B – MARCH 2015 – REVISED JULY 2015

www.ti.com

7.3.3 Current Regulation

The maximum current through the motor winding is regulated by a fixed off-time PWM current regulation, or

current chopping. When an H-bridge is enabled in forward or reverse drive, current rises through the winding at a

rate dependent on the DC voltage and inductance of the winding. After the current hits the current chopping

threshold, the bridge enters a brake (low-side slow decay) mode until tOFF has expired.

Note that immediately after the current is enabled, the voltage on the SP pin is ignored for a period of time

(tBLANK) before enabling the current sense circuitry.

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

connected to the SP pin, multiplied by a factor of AV, with a reference voltage from the VREF pin. The factor AV

is the shunt amplifier gain, which is 20 V/V in the DRV8701.

The chopping current is calculated as follows:

(1)

Example: If a 50 m

Ω sense resistor is used and VREF = 3.3 V, the full-scale chopping current will be 3.25 A. AV

is 20 V/V and VOFF is assumed to be 50 mV in this example.

For DC motors, current regulation is generally used to limit the start-up and stall current of the motor. If the

current regulation feature is not needed, it can be disabled by tying VREF directly to AVDD and tying SP and SN

to GND.

Figure 21. Sense Amplifier and Current Chopping Operation

During brake mode (slow decay), current is recirculated through the low-side FETs. Because current is not

flowing through the sense resistor, SO does not represent the motor current.

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