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 power MOSFET R_DS(ON) (conduction) losses, the power MOSFET switching losses and the quiescent supply current dissipation. While other factors may contribute additional power losses, these other items are typically insignificant compared to the three main items.

P_TOT = P_COND + P_SW + P_Q

For loads connected to VM, assuming that all the outputs are loaded with same current, total conduction loss can be expressed as -

P_COND = 4 x (I_OUT)² x R_DS(ONL)

As the high-side and low-side MOSFETs of the DRV8935 have the same on-resistance, the conduction loss will be independent of the duty cycle of the input PWM or the amount of PWM off-time. 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.

P_COND = 4 x (0.6-A)² x 0.45-Ω = 0.648-W

P_SW can be calculated from the nominal supply voltage (VM), regulated output current (I_OUT), switching frequency (f_PWM) and the device output rise (t_RISE) and fall (t_FALL) time specifications.

Assuming that all the four outputs are switching simultaneously -

P_SW = 4 x (P_{SW_RISE} + P_{SW_FALL})

P_{SW_RISE} = 0.5 x VM x I_OUT x t_RISE x f_PWM

P_{SW_FALL} = 0.5 x VM x I_OUT x t_FALL x f_PWM

P_{SW_RISE} = 0.5 x 24 V x 0.6 A x 100 ns x 40 kHz = 0.0288 W

P_{SW_FALL} = 0.5 x 24 V x 0.6 A x 100 ns x 40 kHz = 0.0288 W

P_SW = 4 x (0.0288W + 0.0288W) = 0.2304 W

P_Q can be calculated from the nominal supply voltage (VM) and the I_VM current specification.

P_Q = VM x I_VM = 24 V x 5 mA = 0.12 W

The total power dissipation (P_TOT) is calculated as the sum of conduction loss, switching loss and the quiescent power loss.

P_TOT = P_COND + P_SW + P_Q = 0.648-W + 0.2304-W + 0.12-W = 0.9984-W

For an ambient temperature of T_A and total power dissipation (P_TOT), the junction temperature (T_J) is calculated as

T_J = T_A + (P_TOT x R_θJA)

Considering a JEDEC standard 4-layer PCB, the junction-to-ambient thermal resistance (R_θJA) is 33 °C/W for the HTSSOP package and 43 °C/W for the VQFN package.

Assuming 25°C ambient temperature, the junction temperature for the HTSSOP package is calculated as -

T_J = 25°C + (0.9984-W x 33°C/W) = 57.95 °C

The junction temperature for the VQFN package is calculated as -

T_J = 25°C + (0.9984-W x 43°C/W) = 67.93 °C

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