SLVAG21 April   2026 TPS61381-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2System Overview
    1.     6
    2. 2.1 Design Considerations
    3. 2.2 System Design Theory
      1. 2.2.1 Supercapacitor Charging Operation
        1. 2.2.1.1 Calculation for Backup Supercapacitor Selection
          1. 2.2.1.1.1 Design Calculations
  6. 3Test Results
    1. 3.1 Standby Operation
    2. 3.2 Charging Operation
    3. 3.3 Boost Operation
  7. 4Summary
  8. 5References

2.2.1.1 Calculation for Backup Supercapacitor Selection

The selection the supercapacitor for the backup operation is majorly dependent on output load requirements, the backup duration and the internal resistance of the supercapacitor in question. The working schematic during the backup and boost mode operation is shown in Figure 2-4.

Effective Discharge During Boost Mode Figure 2-4 Effective Discharge During Boost Mode

.Depending on the required backup time and backup power, the following equation calculates the minimum required value of the backup capacitor as given where:

Equation 1. C m i n = ( 2 × P × T ) / ( η × [ ( V C F ) 2 - ( V C L ) 2 ] )
  • Cmin: Minimum value of the required backup capacitor
  • VCF: Backup capacitor voltage at comparator falling threshold
  • VCL: Minimum discharge voltage of the backup capacitor, given by the minimum input voltage of TPS61381
  • η: Efficiency of the TPS61381
  • T: Backup Time
  • P: Backup Power

However, VCL is greatly affected by supercap internal resistance Rin. The VCL must be calculated by Equation 2

Equation 2. V C L = m i n ( ( V o u t I o u t ) / ( η I l i m ) + I l i m R i n , V B U B U V + ( V o u t I o u t R i n ) / ( η V B U B U V ) )

Where:

  • VBUBUV is the TPS61381 BUB UVLO threshold
  • Ilim is the current set by I2C interface

Supercap internal resistance is usually 5-10 times larger than the IC MOSFET Rdson. So, efficiency mainly depends on the loss on internal resistance. The effect of the internal resistance on the backup boost operation can be calculated. See Automotive E-Latch Systems Design Guideline Using TPS61383 for detailed calculations.