DRV8801-Q1_16 Datasheet, PDF (16/28 Pages) Texas Instruments – DMOS Full-Bridge Motor Drivers

DRV8801-Q1
SLVSAS7B – FEBRUARY 2011 – REVISED JANUARY 2016
www.ti.com
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.
8.2.2.2 Power Dissipation
The power dissipation of the DRV8801-Q1 is a function of the RMS motor current and the each output’s FET
resistance (R_DS(ON)).
Power ≈ I_RMS2 x (High-Side R_DS(ON) + Low-Side R_DS(ON))
(2)
For this example, the ambient temperature is 35°C, and the junction temperature reaches 65°C. At 65°C, the
sum of RDS(ON) is about 1 Ω. With an example motor current of 0.8 A, the dissipated power in the form of heat
will be 0.8 A²x 1 Ω = 0.64 W.
The temperature that the DRV8801-Q1 reaches will depend on the thermal resistance to the air and PCB. It is
important to solder the device PowerPAD to the PCB ground plane, with vias to the top and bottom board layers,
to dissipate heat into the PCB and reduce the device temperature. In the example used here, the DRV8801-Q1
had an effective thermal resistance R_θJA of 47°C/W, and:
T_J = T_A + (P_D x R_θJA) = 35°C + (0.64 W x 47°C/W) = 65°C
(3)
8.2.2.3 Motor Current Trip Point
When the voltage on pin SENSE exceeds V_TRIP (0.5 V), overcurrent is detected. The R_SENSE resistor should be
sized to set the desired I_TRIP level.
R_SENSE = 0.5 V / I_TRIP
(4)
To set I_TRIP to 2.5 A, R_SENSE = 0.5 V / 2.5 A = 0.2 Ω.
To prevent false trips, I_TRIP must be higher than regular operating current. Motor current during startup 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 is beneficial to limit startup current by using series inductors on the DRV8801-Q1 output, as that allows I_TRIP to
be lower, and it may decrease the system’s required bulk capacitance. Startup current can also be limited by
ramping the forward drive duty cycle.
8.2.2.4 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
8.2.2.5 Drive Current
This current path is through the high-side sourcing DMOS driver, motor winding, and low-side sinking DMOS
driver. Power dissipation I²R loses in one source and one sink DMOS driver, as shown in Equation 5.
P_D = I² (r_DS(on)Source + r_DS(on)Sink )
(5)
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