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ZHCSLJ5D February 2022 – March 2023 TIOL112 , TIOL1123 , TIOL1125

PRODUCTION DATA

9.2.2.1 Maximum Junction Temperature Check

For a 200 mA current limit:

Choose driver output current limit, I_O(LIM) = 250 mA (allowing for current limit tolerance); R_SET = 27 kΩ
The maximum voltage drop across the high-side switch at 250 mA current is V_DS(ON) = 1.1 V.

This causes a power consumption of:

Equation 1.

{P D}_{O P} = V_{D S (O N)} \times I_{O (L I M)} = 1.1 V \times 250 m A = 275 m W

For a 5 mA LDO current output,

Equation 2.

{P D}_{L D O} = (V_{L +} - V_{V C C_{O U T}}) \times I_{V C C_O U T} = (30 - 5) V \times 5 m A = 125 m W

Total power dissipation,

Equation 3.

P D = {P D}_{L D O} + {P D}_{O P} = 275 m W + 125 m W = 400 m W

Multiply this value with the Junction-to-ambient thermal resistance of θ_JA = 45.9 °C/W (taken from the Thermal Information table table) to receive the difference between junction temperature, T_J, and ambient temperature, T_A:

Equation 4.

∆ T = T_{J} - T_{A} = P D \times θ_{J A} = 400 m W \times 45.9 \frac{℃}{W} = 18.36 ℃

Add this value to the maximum ambient temperature of T_A = 105°C to receive the final junction temperature:

Equation 5.

T_{J} = T_{A} + ∆ T = T_{A} + P D \times θ_{J A} = 105 ℃ + 400 m W \times 45.9 \frac{℃}{W} = 105 ℃ + 18.36 ℃ = 123.36 ℃

As long as T_J is below the recommended maximum value of 150°C, no thermal shutdown will occur. However, the junction temperature is closer to T_WRN and thermal warning may be generated if the junction temperature rises above T_WRN.

Note that the modeling of the complete system may be necessary to predict junction temperature in smaller PCBs and/or enclosures without air flow.