DRV8701_15 Datasheet, PDF (29/42 Pages) Texas Instruments – DRV8701 Brushed DC Motor Full-Bridge Gate Driver

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DRV8701
SLVSCX5B – MARCH 2015 – REVISED JULY 2015
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 External FET Selection
The DRV8701 FET support is based on the charge pump capacity and output PWM frequency. For a quick
calculation of FET driving capacity, use the following equations when drive and brake (slow decay) are the
primary modes of operation:
Q_G

I_VCP
¦_PWM
where
• f_PWM is the maximum desired PWM frequency to be applied to the DRV8701 inputs or the current chopping
frequency, whichever is larger.
• I_VCP is the charge pump capacity, which depends on VM.
(5)
The internal current chopping frequency is at most:
¦_PWM

t_OFF
1
 t_BLANK
|  N+]
(6)
Example: If a system at VM = 7 V (I_VCP = 8 mA) uses a maximum PWM frequency of 40 kHz, then the DRV8701
will support Q_G < 200 nC FETs.
If the application will require a forced fast decay (or alternating between drive and reverse drive), the maximum
FET driving capacity is given by:
Q_G

I_VCP
 u ¦_PWM
(7)
8.2.1.2.2 IDRIVE Configuration
Select IDRIVE based on the gate charge of the FETs. Configure this pin so that the FET gates are charged
completely during t_DRIVE. If the designer chooses an IDRIVE that is too low for a given FET, then the FET may
not turn on completely. TI suggests to adjust these values in-system with the required external FETs and motor
to determine the best possible setting for any application.
For FETs with a known gate-to-drain charge (Q_GD) and desired rise time (RT), select IDRIVE based on:
IDRIVE  ^QGD
RT
(8)
Example: If the gate-to-drain charge is 2.3 nC, and the desired rise time is around 100 to 300 ns,
IDRIVE1 = 2.3 nC / 100 ns = 23 mA
IDRIVE2 = 2.3 nC / 300 ns = 7.7 mA
Select IDRIVE between 7.7 and 23 mA
Select IDRIVE as 12.5-mA source (25-mA sink)
Requires a 33-kΩ resistor from the IDRIVE pin to GND
8.2.1.2.3 Current Chopping Configuration
The chopping current is set based on the sense resistor value and the analog voltage at VREF. Calculate the
current using Equation 9. The amplifier gain A_V is 20 V/V and V_OFF is typically 50 mV.
Example: If the desired chopping current is 3 A,
Set R_SENSE = 50 mΩ
I_{C H O P}

95() ± 9_OFF
A _V K R_SENSE
(9)
VREF would have to be 3.05 V.
Create a resistor divider from AVDD (4.8 V) to set VREF ≈ 3 V
Set R2 = 3.3 kΩ; set R1 = 2 kΩ.
Copyright © 2015, Texas Instruments Incorporated
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