SNVAA16 May   2021 TPS548A28 , TPS548A29 , TPS548B28 , TPS54JA20 , TPS54JB20

 

  1.   Trademarks
  2. 1DC/DC Converters with Internal LDO
  3. 2DC/DC Converters with External Bias
  4. 3Efficiency and Power Loss Calculations
  5. 4Power Loss of Multi-Rail Point-of-Load System
  6. 5Summary

3 Efficiency and Power Loss Calculations

The power efficiency formula is shown in Equation 1 and the power loss formula is shown in Equation 2. Power loss results in Watts are easier to compare than efficiency numbers, especially in multi-rail systems because a watt is more specific than an efficiency percentage when considering power dissipation.

Equation 1. Efficiency = VOUT x IOUT / VIN x IVIN
Equation 2. Power Loss = VIN x IVIN - VOUT x IOUT

Equation 3 and Equation 4 show the power losses of the internal LDO and demonstrate that additional losses are present depending on the efficiency of the internal LDO, using 25% efficiency as an example.

Equation 3. Efficiency=(VOUT ×IOUT)/(VIN×IVIN+(VCC×IVCC)/(0.25))
Equation 4. Power Loss=VIN×IVIN+(VCC×IVCC)/0.25-VOUT ×IOUT

When the VCC pin is connected to an external source that is greater than the internal LDO output voltage, the internal LDO is turned off, power is drawn from the external source, and the internal LDO power loss (VIN-VLDO)*IVCC is zero resulting in increased efficiency of the converter. However, if the IBIAS current is provided by an additional DC/DC converter, its own power losses need to be considered into the total system efficiency. If the additional DC/DC converter is a synchronous buck converter, the power losses will be lower than the internal LDO power losses. The internal LDO also provides the voltage for boot capacitor CBOOT to provide the gate-drive voltage to drive the power MOSFETs. When the external VCC is higher than the internal LDO voltage, the gate-drive voltage increases, lowering the RDS(on) of the MOSFETs and increasing the efficiency, especially at higher output current.