DRV8461SPWPR datasheet(103/117 Pages) TI

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8.2.4 Thermal Application

This section presents the power dissipation calculation and junction temperature estimation of the device.

8.2.4.1 Power Dissipation

The total power dissipation constitutes of three main components - conduction loss (PCOND), switching loss

(PSW) and power loss due to quiescent current consumption (PQ).

8.2.4.2 Conduction Loss

The current path for a motor connected in full-bridge is through the high-side FET of one half-bridge and

low-side FET of the other half-bridge. The conduction loss (PCOND) depends on the motor rms current (IRMS) and

high-side (RDS(ONH)) and low-side (RDS(ONL)) on-state resistances as shown in Equation 16.

PCOND = 2 x (IRMS)2 x (RDS(ONH) + RDS(ONL))

(16)

The conduction loss for the typical application shown in Table 8-1 is calculated in Equation 17.

PCOND = 2 x (IRMS)2 x (RDS(ONH) + RDS(ONL)) = 2 x (3-A / √2)2 x (0.3-Ω) = 2.7-W

(17)

Note

This power calculation is highly dependent on the device temperature which significantly effects

the high-side and low-side on-resistance of the FETs. For more accurate calculation, consider the

dependency of on-resistance of FETs with device temperature.

8.2.4.3 Switching Loss

The power loss due to the PWM switching frequency depends on the output voltage rise/fall time (tRF), supply

voltage, motor RMS current and the PWM switching frequency. The switching losses in each H-bridge during

rise-time and fall-time are calculated as shown in Equation 18 and Equation 19.

PSW_RISE = 0.5 x VVM x IRMS x tRF x fPWM

(18)

PSW_FALL = 0.5 x VVM x IRMS x tRF x fPWM

(19)

The DRV8461 features two values of output rise/fall time (tRF) - 140 ns and 70 ns. The smaller rise/fall time

obviously results in lesser switching loss. Assuming tRF = 140 ns and 30-kHz PWM frequencyfor this exercise,

and after substituting the values of various parameters, the switching losses in each H-bridge are calculated as

shown below -

PSW_RISE = 0.5 x 24-V x (3-A / √2) x (140 ns) x 30-kHz = 0.107-W

(20)

PSW_FALL = 0.5 x 24-V x (3-A / √2) x (140 ns) x 30-kHz = 0.107-W

(21)

The total switching loss for the stepper motor driver (PSW) is calculated as twice the sum of rise-time (PSW_RISE)

switching loss and fall-time (PSW_FALL) switching loss as shown below -

PSW = 2 x (PSW_RISE + PSW_FALL) = 2 x (0.107-W + 0.107-W) = 0.428-W

(22)

Note

The output rise/fall time (tRF) is expected to change based on the supply-voltage, temperature and

device to device variation.

The switching loss is directly proportional to the PWM switching frequency. The PWM frequency in an

application will depend on the supply voltage, inductance of the motor coil, back emf voltage and OFF

time or the ripple current (for smart tune ripple control decay mode).

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DRV8461

SLOSE98 – DECEMBER 2022

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