22

DRV8874

SLVSF66A – AUGUST 2019 – REVISED DECEMBER 2019

www.ti.com

Submit Documentation Feedback

Copyright © 2019, Texas Instruments Incorporated

PVM = VM x IVM

(9)

PVM = 0.096 W = 24 V x 4 mA

(10)

PSW can be calculated from the nominal supply voltage (VM), average output current (IRMS), switching frequency

(fPWM) and the device output rise (tRISE) and fall (tFALL) time specifications.

PSW = PSW_RISE + PSW_FALL

(11)

PSW_RISE = 0.5 x VM x IRMS x tRISE x fPWM

(12)

PSW_FALL = 0.5 x VM x IRMS x tFALL x fPWM

(13)

PSW_RISE = 0.018 W = 0.5 x 24 V x 0.5 A x 150 ns x 20 kHz

(14)

PSW_FALL = 0.018 W = 0.5 x 24 V x 0.5 A x 150 ns x 20 kHz

(15)

PSW = 0.036 W = 0.018 W + 0.018 W

(16)

PRDS can be calculated from the device RDS(on) and average output current (IRMS)

PRDS = IRMS

2 x (R

DS(ON)_HS + RDS(ON)_LS)

(17)

It should be noted that RDS(ON) has a strong correlation with the device temperature. A curve showing the

normalized RDS(on) with temperature can be found in the Typical Characteristics curves. Assuming a device

temperature of 85 °C it can be expected that RDS(on) will see an increase of ~1.25 based on the normalized

temperature data.

PRDS = 0.0625 W = (0.5 A)

2 x (100 mΩ x 1.25 + 100 mΩ x 1.25)

(18)

By adding together the different power dissipation components it can be verified that the expected power

dissipation and device junction temperature is within design targets.

PTOT = PVM + PSW + PRDS

(19)

PTOT = 0.194 W = 0.096 W + 0.036 W + 0.0625 W

(20)

The device junction temperature can be calculated with the PTOT, device ambient temperature (TA), and package

thermal resistance (RθJA). The value for RθJA is heavily dependent on the PCB design and copper heat sinking

around the device.

TJ = (PTOT x RθJA) + TA

(21)

TJ = 92°C = (0.194 W x 35 °C/W) + 85°C

(22)

It should be ensured that the device junction temperature is within the specified operating region. Other methods

exist for verifying the device junction temperature depending on the measurements available.

Additional information on motor driver current ratings and power dissipation can be found in Thermal

Performance and Related Documentation.

8.2.1.2.3

Thermal Performance

The datasheet-specified junction-to-ambient thermal resistance, RθJA, is primarily useful for comparing various

drivers or approximating thermal performance. However, the actual system performance may be better or worse

than this value depending on PCB stackup, routing, number of vias, and copper area around the thermal pad.

The length of time the driver drives a particular current will also impact power dissipation and thermal

performance. This section considers how to design for steady-state and transient thermal conditions.

The data in this section was simulated using the following criteria:

•

2-layer PCB, standard FR4, 1-oz (35 mm copper thickness) or 2-oz copper thickness.

•

Top layer: DRV887x HTSSOP package footprint and copper plane heatsink. Top layer copper area is

varied in simulation.

•

Bottom layer: ground plane thermally connected through vias under the thermal pad for DRV887x. Bottom

layer copper area varies with top copper area. Thermal vias are only present under the thermal pad (grid

pattern with 1.2mm spacing).

•

4-layer PCB, standard FR4. Outer planes are 1-oz (35 mm copper thickness) or 2-oz copper thickness.

•

Top layer: DRV887x HTSSOP package footprint and copper plane heatsink. Top layer copper area is

varied in simulation. Inner planes were kept at 1-oz.

•

Mid layer 1: GND plane thermally connected to DRV887x thermal pad through vias. The area of the

ground plane is 74.2 mm x 74.2 mm.

•

Mid layer 2: power plane, no thermal connection.