DRV8814_16 Datasheet, PDF (14/26 Pages) Texas Instruments – DC Motor Driver IC

DRV8814
SLVSAB9E – MAY 2010 – REVISED NOVEMBER 2015
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8.2.2 Detailed Design Procedure
8.2.2.1 Current Regulation
In a stepper motor, the set full-scale current (I_FS) is the maximum current driven through either winding. This
quantity depends on the xVREF analog voltage and the sense resistor value (R_SENSE). During stepping, I_FS
defines the current chopping threshold (I_TRIP) for the maximum current step. The gain of DRV8814 is set for 5
V/V.
I_FS (A)
xVREF(V)
A_v u R_SENSE (:)
xVREF(V)
5 u R_SENSE (:)
(2)
To achieve I_FS = 1.25 A with R_SENSE of 0.2 Ω, xVREF should be 1.25 V.
8.2.2.2 Decay Modes
The DRV8814 supports two different decay modes: slow decay and fast decay. The current through the motor
windings is regulated using a fixed-frequency PWM scheme. This means that after any drive phase, when a
motor winding current has hit the current chopping threshold (I_TRIP), the DRV8814 places the winding in one of
the two decay modes until the PWM cycle has expired. Afterward, a new drive phase starts. The blanking time,
t_BLANK, defines the minimum drive time for the current chopping. I_TRIP is ignored during t_BLANK, so the winding
current may overshoot the trip level.
8.2.2.3 Sense Resistor
The power dissipated by the sense resistor equals Irms² × R. For example, if the rms motor current is 2-A and a
100-mΩ sense resistor is used, the resistor dissipates 2 A² × 0.1 Ω = 0.4 W. The power quickly increases with
greater current levels.
Resistors typically have a rated power within some ambient temperature range, along with a derated power curve
for high ambient temperatures. When a PCB is shared with other components generating heat, margin should be
added. It is always best to measure the actual sense resistor temperature in a final system, along with the power
MOSFETs, as those are often the hottest components.
Because power resistors are larger and more expensive than standard resistors, it is common practice to use
multiple standard resistors in parallel, between the sense node and ground. This distributes the current and heat
dissipation.
8.2.3 Application Curves
Figure 8. Direction Change
Figure 9. Current Limiting
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