SLVSDS7B August 2019 – November 2019 DRV8876
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.
PVM can be calculated from the nominal supply voltage (VM) and the IVM active mode current specification.
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.
PRDS can be calculated from the device RDS(on) and average output current (IRMS)
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 increases of ~1.25 based on the normalized temperature data.
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.
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.
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.