LDO power dissipation calculator
Power burned in a linear regulator and the junction temperature it produces, with the highest ambient and output current that keep the junction under its limit.
- Junction temperature
- 82.53 °C
- θJA used
- 50 °C/W
- Highest θJA that meets the limit
- 99.94 °C/W
- Maximum dissipation at this ambient
- 1.7 W
- Maximum output current at this ambient
- 999.7 mA
- Maximum ambient at this load
- 82.47 °C
- Efficiency
- 66 %
- Junction temperature
- Limit
How this is calculated
P_D = (V_IN − V_OUT) · I_OUT + V_IN · I_Q T_J = T_A + P_D · θJA θJA = θJC + θCS + θSA P_D(max) = (T_J(max) − T_A) / θJA I_OUT(max) = (P_D(max) − V_IN · I_Q) / (V_IN − V_OUT)
- Steady state
- Power ratings and θJA are steady-state figures. Junction to case settles in seconds, junction to ambient can take minutes (SLVA118A section 3), so short pulses can exceed the average without exceeding T_J.
- θJA
- Depends on board copper, layer count and airflow as much as on the package. Use the figure for your board, not the JEDEC test board, where the datasheet gives both.
- Quiescent current
- Included as V_IN · I_Q. PMOS LDOs draw a near-constant I_Q; bipolar parts draw more as load rises, so use the ground current at your load.
- TI SLVA118A, Linear Regulator Design Guide for LDOsApril 2003, revised June 2008, Equations 3, 5 to 8, 11 and 16
- TI SLVA462, Understanding Thermal Dissipation and Design of a HeatsinkMay 2011, Equations 1 and 2
Questions
The voltage across it times the load current, plus input voltage times quiescent current. 5 V to 3.3 V at 500 mA is 0.85 W. TI SLVA118A works 5 V to 1.8 V at 100 mA: 320 mW.
Divide the temperature budget by the power. For 320 mW, a 125 °C limit and 70 °C ambient, θJA must be under about 172 °C/W, which rules out a SOT-23 that SLVA118A compares and allows an SO-8.
Find the power the package can shed, (T_J(max) − T_A) / θJA, and divide by the headroom. SLVA462 gives 2.25 W at 25 °C for θJA 44.5 °C/W and 125 °C, so with 1 V across the part that is about 2.25 A.
When the dissipation forces a package or heatsink you cannot fit. Efficiency is roughly V_OUT / V_IN for an LDO, so 5 V to 1.8 V is about 36 %, and the rest is heat.