SLVSDS7B August 2019 – November 2019 DRV8876

PRODUCTION DATA.

- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
- 8 Application and Implementation
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information

Refer to the PDF data sheet for device specific package drawings

- RGT|16
- PWP|16

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 R_{DS(on)} (conduction) losses. While other factors may contribute additional power losses, these other items are typically insignificant compared to the three main items.

Equation 8. P_{TOT} = P_{VM} + P_{SW} + P_{RDS}

P_{VM} can be calculated from the nominal supply voltage (V_{M}) and the I_{VM} active mode current specification.

Equation 9. P_{VM} = V_{M} x I_{VM}

Equation 10. P_{VM} = 0.096 W = 24 V x 4 mA

P_{SW} can be calculated from the nominal supply voltage (V_{M}), average output current (I_{RMS}), switching frequency (f_{PWM}) and the device output rise (t_{RISE}) and fall (t_{FALL}) time specifications.

Equation 11. P_{SW} = P_{SW_RISE} + P_{SW_FALL}

Equation 12. P_{SW_RISE} = 0.5 x V_{M} x I_{RMS }x t_{RISE} x f_{PWM}

Equation 13. P_{SW_FALL} =0.5 x V_{M} x I_{RMS }x t_{FALL} x f_{PWM}

Equation 14. P_{SW_RISE} = 0.018 W = 0.5 x 24 V x 0.5 A x 150 ns x 20 kHz

Equation 15. P_{SW_FALL} = 0.018 W = 0.5 x 24 V x 0.5 A x 150 ns x 20 kHz

Equation 16. P_{SW} = 0.036 W = 0.018 W + 0.018 W

P_{RDS} can be calculated from the device R_{DS(on)} and average output current (I_{RMS})

Equation 17. P_{RDS} = I_{RMS}^{2} x (R_{DS(ON)_HS} + R_{DS(ON)_LS})

It should be noted that R_{DS(ON)} has a strong correlation with the device temperature. A curve showing the normalized R_{DS(on)} with temperature can be found in the Typical Characteristics curves. Assuming a device temperature of 85 °C it can be expected that R_{DS(on)} will see an increases of ~1.25 based on the normalized temperature data.

Equation 18. P_{RDS} = 0.219 W = (0.5 A)^{2} x (350 mΩ x 1.25 + 350 mΩ x 1.25)

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.

Equation 19. P_{TOT} = P_{VM} + P_{SW} + P_{RDS}

Equation 20. P_{TOT}= 0.351 W = 0.096 W + 0.036 W + 0.219 W

The device junction temperature can be calculated with the P_{TOT}, device ambient temperature (T_{A}), and package thermal resistance (R_{θJA}). The value for R_{θJA} is heavily dependent on the PCB design and copper heat sinking around the device.

Equation 21. T_{J} = (P_{TOT} x R_{θJA}) + T_{A}

Equation 22. T_{J} = 97°C = (0.351 W x 35 °C/W) + 85°C

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.