O F F

C H O P

V

S E N S E

9 5 ( ) ± 9

I

A

R



K

GD

Q

IDRIVE

RT



VCP

G

PWM

I

Q



¦



u

PWM

OFF

BLANK

1

¦

 N+]

t

t



|



VCP

G

PWM

I

Q

¦



DRV8701

www.ti.com

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:

where

•

fPWM is the maximum desired PWM frequency to be applied to the DRV8701 inputs or the current chopping

frequency, whichever is larger.

•

IVCP is the charge pump capacity, which depends on VM.

(5)

The internal current chopping frequency is at most:

(6)

Example: If a system at VM = 7 V (IVCP = 8 mA) uses a maximum PWM frequency of 40 kHz, then the DRV8701

will support QG < 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:

(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 tDRIVE. 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 (QGD) and desired rise time (RT), select IDRIVE based on:

(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-k

Ω 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 AV is 20 V/V and VOFF is typically 50 mV.

Example: If the desired chopping current is 3 A,

Set RSENSE = 50 mΩ

(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 k

Ω; set R1 = 2 kΩ.

Copyright © 2015, Texas Instruments Incorporated

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Product Folder Links: DRV8701