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DRV8889-Q1 Datasheet(PDF) 61 Page - Texas Instruments

Part No. DRV8889-Q1
Description  DRV8889-Q1, DRV8889A-Q1 Automotive Stepper Driver with Integrated Current Sense, 1/256 Micro-Stepping, and Stall Detection
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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DRV8889-Q1 Datasheet(HTML) 61 Page - Texas Instruments

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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).
8.2.4.1.1 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 4.
PCOND = 2 x (IRMS)2 x (RDS(ONH) + RDS(ONL))
(4)
The conduction loss for the typical application shown in Section 8.2.1 is calculated in Equation 5.
PCOND = 2 x (IRMS)2 x (RDS(ONH) + RDS(ONL)) = 2 x (500-mA / √2)2 x (0.45-Ω + 0.45-Ω) = 225-mW
(5)
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.
8.2.4.1.2 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 6 and Equation 7.
PSW_RISE = 0.5 x VVM x IRMS x tRISE_PWM x fPWM
(6)
PSW_FALL = 0.5 x VVM x IRMS x tFALL_PWM x fPWM
(7)
Both tRISE_PWM and tFALL_PWM can be approximated as VVM/ tSR. After substituting the values of various
parameters, and assuming 105 V/µs slew rate and 30-kHz PWM frequency, the switching losses in each H-
bridge are calculated as shown below -
PSW_RISE = 0.5 x 13.5-V x (500-mA / √2) x (13.5-V / 105 V/µs) x 30-kHz = 9.2-mW
(8)
PSW_FALL = 0.5 x 13.5-V x (500-mA / √2) x (13.5-V / 105 V/µs) x 30-kHz = 9.2-mW
(9)
The total switching loss (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 (9.2-mW + 9.2-mW) = 36.8-mW
(10)
www.ti.com
DRV8889-Q1
SLVSEE9C – APRIL 2020 – REVISED AUGUST 2020
Copyright © 2020 Texas Instruments Incorporated
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