DRV8307_15 Datasheet, PDF (23/34 Pages) Texas Instruments – DRV8307 Brushless DC Motor Controller

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9.2.1 Design Requirements
DRV8307
SLVSCK2 – APRIL 2014
Design Parameter
Supply voltage
PWM frequency
PWM duty cycle
Current limiter
External FETs
Bulk supply capacitance
Value
8.5 to 32 V
16 to 50 kHz
0% to 100%
V_LIMITER / R_ISENSE
N-channel MOSFETs
2 to 4 µF per watt
9.2.2 Detailed Design Procedure
When designing a system with the DRV8307, determine an operating motor voltage between 8.5 to 32 V. Higher
voltages directly scale motor speed, with the same PWM input.
The frequency of the input clock (PWM) must be between 16 and 50 kHz. Note that this frequency does not
affect the pre-driver output frequency, which is fixed at 25 kHz (typical).
The PWM duty cycle controls motor speed and can be set either to a fixed value or varied while the motor is
spinning. If it is changed while spinning, use gradual steps (for example, 1% increments), because a large
change in the commanded duty cycle can cause a large step in commutation, which can lock up the motor. This
behavior is typical with other industry devices.
The DRV8307 device constantly monitors motor current and reduces FET drive when necessary, to keep current
within V_LIMITER / R_ISENSE. This feature reduces the requirements of power supply current capacity and bulk
capacitance to maintain a stable voltage, especially during motor startup. The designer should target a peak
current limit and size R_ISENSE appropriately. V_LIMITER is fixed at 0.25 V (typical).
R_ISENSE = 0.25 V / I_PEAK
(1)
For example, if 4-A peak is desired, then a 0.06-Ω resistor should be chosen as in Equation 2.
0.06 Ω = 0.25 V / 4 A
(2)
When selecting the power FETs, use six N-channel MOSFETs. They must support V_GS > 10 V (since the
DRV8307 device drives 10 V V_GS). They must also support V_DS > VM, and TI recommends to have 1.5× to 2×
margin, to prevent FET damage during transient voltage spikes that can occur when motors change speeds.
It is important to use large bulk capacitance on VM, and the required size depends on the power of the motor. Of
course, power = voltage × current. A general recommendation is to use 2 to 4 µF per watt. If a motor system
uses 24 V and 3 A, a reasonable choice is 144 to 288 µF.
9.2.3 Application Performance Plot
ENABLE
LOCKn
RPM
Figure 14. Typical Spinup Profile
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