SLVAFX0 October   2024 TLV702 , TLV703 , TLV755P , TPS74401 , TPS7A13 , TPS7A14 , TPS7A20 , TPS7A21 , TPS7A49 , TPS7A52 , TPS7A53 , TPS7A53B , TPS7A54 , TPS7A57 , TPS7A74 , TPS7A83A , TPS7A84A , TPS7A85A , TPS7A91 , TPS7A92 , TPS7A94 , TPS7A96 , TPS7H1111-SP

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction to linear regulator turn-on time
  5. 2What impacts the LDO rise time?
    1. 2.1 Simple Use Cases
      1. 2.1.1 Case 1: LDO with an NR filter but without CFF capacitance
      2. 2.1.2 Case 2: NR filter with a CFF capacitance
      3. 2.1.3 Fast-charge circuitry
      4. 2.1.4 Non-ideal LDO behavior
        1. 2.1.4.1 Applied voltage bias
        2. 2.1.4.2 Fast charge current tolerance
        3. 2.1.4.3 Internal error amplifier offset voltage
        4. 2.1.4.4 Temperature impacts the fast-charge current source
        5. 2.1.4.5 Error amplifier common mode voltage
        6. 2.1.4.6 Reference voltage (VREF) ramp time dominates the turn-on time
        7. 2.1.4.7 Start-up during dropout mode
        8. 2.1.4.8 Large values of COUT induce internal current limit
        9. 2.1.4.9 Limitations of large-signal LDO bandwidth
    2. 2.2 Specific Use Cases and Examples
      1. 2.2.1 Case 3: Precision voltage reference with RNR/SS and parallel IFC fast charge
      2. 2.2.2 Case 4: Precision voltage reference with IFC fast charge and no RNR/SS
      3. 2.2.3 Case 5: Precision current reference
      4. 2.2.4 Case 6: Soft-start timing
  6. 3System Considerations
    1. 3.1 Inrush current calculation
    2. 3.2 Inrush current analysis
    3. 3.3 Maximum slew rate
  7. 4LDO regulators referenced in this paper
  8. 5Conclusion
  9. 6References

3.2 Inrush current analysis

We can use the previous equations to quickly calculate the inrush current through the LDO regulator if we know the output capacitance and load during the turn-on period. Figure 3-3 provides an example using the TPS7A20 (recall that this device includes an NR filter that also controls the turn-on time). Two 1µF capacitors are installed on the output which provide an effective capacitance of 1.4µF.

Figure 3-3 provides an example using the TPS7A84A. Recall that the TPS7A84A uses a fast-charge current source (without an NR resistor in parallel) and switches in the NR filter for steady state operation. The analysis used COUT = 67µF and the peak current (IPEAK) is well within the tolerance of the capacitors.

TPS7A20, TPS7A21, TPS7A13, TPS7A14, TPS7A49, TPS7A91, TPS7A92, TLV702, TLV703, TLV755P, TPS7A52, TPS7A53, TPS7A53B, TPS7A54, TPS7A83A, TPS7A84A, TPS7A85A, TPS7A57, TPS7A94, TPS7A96, TPS7H1111-SP, TPS74401, TPS7A74, TPS74701, TPS74801, TPS74901 TPS7A20 turn-on with inrush current analysis
TPS7A20 COUT = 1µF || 1µF
Figure 3-2 TPS7A20 turn-on with inrush current analysis
TPS7A20, TPS7A21, TPS7A13, TPS7A14, TPS7A49, TPS7A91, TPS7A92, TLV702, TLV703, TLV755P, TPS7A52, TPS7A53, TPS7A53B, TPS7A54, TPS7A83A, TPS7A84A, TPS7A85A, TPS7A57, TPS7A94, TPS7A96, TPS7H1111-SP, TPS74401, TPS7A74, TPS74701, TPS74801, TPS74901 TPS7A84A turn-on with inrush current analysis
TPS7A84A COUT = 47µF || 10µF || 10µF
(A.) VCM induced current spike
Figure 3-3 TPS7A84A turn-on with inrush current analysis