DRV8841_15 Datasheet, PDF (15/28 Pages) Texas Instruments – Dual H-Bridge Driver IC

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DRV8841
SLVSAC0F – MAY 2010 – REVISED DECEMBER 2015
Typical Application (continued)
8.2.1 Design Requirements
Table 3 lists the design parameters for this design example.
DESIGN PARAMETER
Supply Voltage
Motor Winding Resistance
Motor Winding Inductance
Sense Resistor Value
Target Full-Scale Current
8.2.2 Detailed Design Procedure
Table 3. Design Parameters
REFERENCE
VM
R_L
I_L
RSENSE
I_FS
EXAMPLE VALUE
24 V
3.9 Ω
2.9 mH
200 mΩ
1.25 A
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 DRV8841 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 DRV8841 supports three different decay modes: slow decay, fast decay, and mixed 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 DRV8841 will place the
winding in one of the three 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
For optimal performance, it is important for the sense resistor to be:
• Surface-mount
• Low inductance
• Rated for high enough power
• Placed closely to the motor driver
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 will dissipate 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.
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