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4-2. Calculating the power dissipation and junction temperature of an LDO regulator

𝑃 = (𝑉𝐼𝑁 − 𝑉𝑂𝑈𝑇) × 𝐼𝑂𝑈𝑇 + 𝑉𝐼𝑁 × 𝐼𝐵 ･･･(1)

The power dissipation (P) of an LDO regulator can be calculated as follows:

When the term V_IN × I_B is negligibly small, Equation 1 can be approximated as follows:

The junction temperature T_j of the LDO regulator can be calculated as follows:

where R_th(j-a) is the thermal resistance of the LDO regulator when it is mounted on a board. If R_th(j-a) is not shown in the datasheet, you can calculate it from the power dissipation (P_D) value:

𝑇𝑗 = 𝑃 × (𝑇(𝑗(𝑚𝑎𝑥)) − 25) / 𝑃𝐷 + 𝑇𝑎 ･･･(5)

Substituting Equation 4 into Equation 3, we obtain:

𝑇𝑗 = (3.6 − 1.8) × 0.1 × (150 − 25) / 0.6 + 40 = 77.5°C

Suppose that T_j(max)= 150°C, P_D = 600 mW, V_IN = 3.6 V, V_OUT = 1.8 V, I_OUT = 100 mA, T_a = 40°C. Then, T_j is calculated as follows:

Note that the power dissipation and thermal resistance values in the datasheet are the values under the specified board conditions. It is therefore necessary to evaluate LDO regulators carefully to ensure that they work properly under actual usage conditions.

The following application notes also provide a description of the power dissipation and junction temperature of an LDO regulator.

Application note: Basics of Low-Dropout (LDO) Regulator ICs

Application note: Thermal Design to Maximize the Performance of LDO Regulators

Application note: Power Efficiency Optimization and Application Circuits Using Dual-Supply LDO Regulators