DRV8426PWPR datasheet(33/49 Pages) TI1

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8.2.3 Thermal Application

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

8.2.3.1 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).

8.2.3.1.1

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 2.

PCOND = 2 x (IRMS)

2 x (R

DS(ONH) + RDS(ONL))

(2)

The conduction loss for the typical application shown in Table 14 is calculated in Equation 3.

PCOND = 2 x (IRMS)

2 x (R

DS(ONH) + RDS(ONL)) = 2 x (1-A / √2)

2 x (0.45-Ω + 0.45-Ω) = 0.9-W

(3)

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.

8.2.3.1.2

Switching Loss

The power loss due to the PWM switching frequency depends on the slew rate (tSR), 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 4 and Equation 5.

PSW_RISE = 0.5 x VVM x IRMS x tRISE_PWM x fPWM

(4)

PSW_FALL = 0.5 x VVM x IRMS x tFALL_PWM x fPWM

(5)

Both tRISE_PWM and tFALL_PWM can be approximated as VVM/ tSR. After substituting the values of various

parameters, and assuming 30-kHz PWM frequency, the switching losses in each H-bridge are calculated as

shown below -

PSW_RISE = 0.5 x 24-V x (1-A / √2) x (24-V / 240 V/µs) x 30-kHz = 0.025-W

(6)

PSW_FALL = 0.5 x 24-V x (1-A / √2) x (24-V / 240 V/µs) x 30-kHz = 0.025-W

(7)

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 (0.025-W + 0.025-W) = 0.1-W

(8)

NOTE

The rise-time (tRISE) and the fall-time (tFALL) are calculated based on typical values of the

slew rate (tSR). This parameter 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).

8.2.3.1.3

Power Dissipation Due to Quiescent Current

The power dissipation due to the quiescent current consumed by the power supply is calculated as shown below

PQ = VVM x IVM

(9)

Substituting the values, quiescent power loss can be calculated as shown below -

PQ = 24-V x 5-mA = 0.12-W

(10)

DRV8426PWPR Datasheet(PDF) 33 Page - Texas Instruments

DRV8426PWPR Datasheet(HTML) 33 Page - Texas Instruments

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