Datasheet
I =
CHOP
V
REFX
5 · R
ISENSE
¾
DRV8840
SLVSAB7C –MAY 2010–REVISED AUGUST 2013
www.ti.com
Current Regulation
The maximum current through the motor winding is regulated by a fixed-frequency PWM current regulation, or
current chopping. When the H-bridge is enabled, current rises through the winding at a rate dependent on the
DC voltage and inductance of the winding. Once the current hits the current chopping threshold, the bridge
disables the current until the beginning of the next PWM cycle.
For DC motors, current regulation is used to limit the start-up and stall current of the motor. Speed control is
typically performed by providing an external PWM signal to the ENBLx input pins.
If the current regulation feature is not needed, it can be disabled by connecting the ISENSE pins directly to
ground and the VREF pins to V3P3.
The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor
connected to the ISEN pin, multiplied by a factor of 5, with a reference voltage. The reference voltage is input
from the VREF pin, and is scaled by a 5-bit DAC that allows current settings of zero to 100% in an approximately
sinusoidal sequence.
The full-scale (100%) chopping current is calculated in Equation 1.
(1)
Example:
If a 0.25-Ω sense resistor is used and the VREFx pin is 2.5 V, the full-scale (100%) chopping current will be
2.5 V / (5 x 0.25 Ω) = 2 A.
Five input pins (I0 - I4) are used to scale the current in the bridge as a percentage of the full-scale current set by
the VREF input pin and sense resistance. The I0 - I4 pins have internal pulldown resistors of approximately
100 kΩ. The function of the pins is shown in Table 3.
Table 3. Pin Functions
RELATIVE CURRENT
I[4..0]
(% FULL-SCALE CHOPPING CURRENT)
0x00h 0%
0x01h 5%
0x02h 10%
0x03h 15%
0x04h 20%
0x05h 24%
0x06h 29%
0x07h 34%
0x08h 38%
0x09h 43%
0x0Ah 47%
0x0Bh 51%
0x0Ch 56%
0x0Dh 60%
0x0Eh 63%
0x0Fh 67%
0x10h 71%
0x11h 74%
0x12h 77%
0x13h 80%
0x14h 83%
0x15h 86%
0x16h 88%
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