DRV8832-Q1 Datasheet, PDF (13/20 Pages) Texas Instruments – Low-Voltage Motor Driver IC

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DRV8832-Q1
SLVSBW9C – APRIL 2013 – REVISED DECEMBER 2015
Typical Application (continued)
8.2.1 Design Requirements
Table 4 lists the design parameters of the DRV8832-Q1.
Table 4. Design Parameters
DESIGN
PARAMETER
Motor voltage
Motor RMS current
Motor start-up
Motor current trip
point
REFERENCE
V_CC
I_RMS
I_START
I_LIMIT
EXAMPLE VALUE
5V
0.3 A
1.3 A
0.9 A
8.2.2 Detailed Design Procedure
8.2.2.1 Motor Voltage
The motor voltage to use will depend on the ratings of the motor selected and the desired RPM. A higher voltage
spins a brushed DC motor faster with the same PWM duty cycle applied to the power FETs. A higher voltage
also increases the rate of current change through the inductive motor windings.
For the DRV8832-Q1, TI recommends to set a motor voltage at the lowest system VCC. This will maintain a
constant RPM across varying VCC conditions.
For example if the VCC voltage can vary from 4.5V to 5.5V, setting the VSET voltage to 1.125 V will compensate
for power supply variation. The DRV8832-Q1 will set the motor voltage at 4.5 V, even if VCC is 5.5 V.
8.2.2.2 Motor Current Trip Point
When the voltage on pin ISENSE exceeds VILIM (0.2 V), overcurrent is detected. The RSENSE resistor should
be sized to set the desired ILIMIT level.
R_ISENSE = 0.2 V / I_LIMIT
(2)
To set I_ILIMIT to 0.5 A, R_ISENSE = 0.2 V / 0.9 A = 0.22 Ω.
To prevent false trips, I_LIMIT must be higher than regular operating current. Motor current during start-up is
typically much higher than steady-state spinning, because the initial load torque is higher, and the absence of
back-EMF causes a higher voltage and extra current across the motor windings.
It can be beneficial to limit start-up current by using series inductors on the DRV8832-Q1 output, as that allows
I_LIMIT to be lower, and it may decrease the system’s required bulk capacitance. Start-up current can also be
limited by ramping the forward drive duty cycle.
8.2.2.3 Sense Resistor Selection
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 x R. For example, if peak motor current is 1 A, RMS
motor current is 0.7 A, and a 0.4-Ω sense resistor is used, the resistor will dissipate 0.7 A² x 0.4 Ω = 0.2 W. The
power quickly increases with higher current levels.
Resistors typically have a rated power within some ambient temperature range, along with a de-rated 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.
Copyright © 2013–2015, Texas Instruments Incorporated
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