21

DRV8874

www.ti.com

SLVSF66A – AUGUST 2019 – REVISED DECEMBER 2019

Submit Documentation Feedback

Copyright © 2019, Texas Instruments Incorporated

Typical Application (continued)

8.2.1.1 Design Requirements

Table 8. Design Parameters

REFERENCE

DESIGN PARAMETER

EXAMPLE VALUE

VM

Motor and driver supply voltage

24 V

VCC

Controller supply voltage

3.3 V

IRMS

Output RMS current

0.5 A

fPWM

Switching frequency

20 kHz

ITRIP

Current regulation trip point

1 A

AIPROPI

Current sense scaling factor

455 µA/A

RIPROPI

IPROPI external resistor

5.5 kΩ

VREF

Current regulation reference voltage

2.5 V

VADC

Controller ADC reference voltage

2.5 V

RREF1

VREF external resistor

16 kΩ

RREF2

VREF external resistor

50 kΩ

TA

PCB ambient temperature

–20 to 85 °C

TJ

Device max junction temperature

150 °C

RθJA

Device junction to ambient thermal resistance

35 °C/W

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Current Sense and Regulation

The DRV887x family of devices provide integrated regulation and sensing out the output current.

The current sense feedback is configured by scaling the RIPROPI resistor to properly sense the scaled down

output current from IPROPI within the dynamic voltage range of the controller ADC. An example of this is shown.

RIPROPI <= VADC / (ITRIP x AIPROPI)

(4)

RIPROPI = 5.5 kΩ <= 2.5 V / (1 A x 455 µA/A)

(5)

If VADC = 2.5 V, ITRIP = 1 A, and AIPROPI = 455 µA/A then to maximize the dynamic IPROPI voltage range an

RIPROPI of approximately 5.5 kΩ should be selected.

The accuracy tolerance of RIPROPI can be selected based on the application requirements. 10%, 5%, 1%, 0.1%

are all valid tolerance values. The typical recommendation is 1% for best tradeoff between performance and cost.

The output current regulation trip point (ITRIP) is configured with a combination of VREF and RIPROPI. Since RIPROPI

was previously calculated and AIPROPI is a constant, all the remains is to calculate VREF.

VREF = RIPROPI x (ITRIP x AIPROPI)

(6)

VREF = 2.5 V = 5.5 kΩ x (1 A x 455 µA/A)

(7)

If RIPROPI = 5.5 kΩ, ITRIP = 1 A, and AIPROPI = 455 µA/A then VREF should be set to 2.5 V.

VREF can be generated with a simple resistor divider (RREF1 and RREF2) from the controller supply voltage. The

resistor sizing can be achieved by selecting a value for RREF1 and calculating the required value for RREF2.

8.2.1.2.2

Power Dissipation and Output Current Capability

The output current and power dissipation capabilities of the device are heavily dependent on the PCB design and

external system conditions. This section provides some guidelines for calculating these values.

Total power dissipation for the device is composed of three main components. These are the quiescent supply

current dissipation, the power MOSFET switching losses. and the power MOSFET RDS(on) (conduction) losses.

While other factors may contribute additional power losses, these other items are typically insignificant compared

to the three main items.

PTOT = PVM + PSW + PRDS

(8)

PVM can be calculated from the nominal supply voltage (VM) and the IVM active mode current specification.