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

2.1.3 Fast-charge circuitry

The NR/SS filter significantly improves the power supply rejection ratio (PSRR) and noise in LDO regulators. [21]

The time constant for the NR/SS filter may result in longer than desired turn-on times for some applications. Modern LDO regulators may include a fast charge circuit to reduce the turn-on time of the filtered reference supply, and by extension, the output voltage. The fast-charge circuit operates while VNR/SS measures less than the changeover voltage (VCO), when the steady state filter values are used.

Figure 2-5 shows typical turn-on behavior of an LDO regulator using fast charge.

For LDO regulators that use a voltage reference, this fast-charge circuit is either a parallel current source or parallel resistor with the NR/SS resistor as shown in Figure 2-1. For LDO regulators that use a current reference, the fast-charge circuit modifies the IREF current to be a larger value as shown in Figure 2-2. Equation 11 calculates the time when the changeover voltage occurs (tCO). Entering τ = tCO into Equation 7 yields the initial condition (VCO_FF) on VTOP just after the changeover voltage event.

If a fast-charge current source is across RNR/SS, use Equation 12 instead of Equation 11 to calculate tCO.

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 Changeover voltage vs time Figure 2-5 Changeover voltage vs time
Equation 11. t C O = - τ N R / S S × ln ⁡ 1 - V C O V R E F
Equation 12. t C O = - τ N R / S S × ln ⁡ 1 - V C O V R E F + I F C × R N R / S S

Common values of VCO are 95% to 97% of VREF. Use Equation 5 or Equation 6 to calculate VFB(t) during fast-charge operation, but after the fast-charge function completes, use Equation 13.

Equation 13. V F B t = V R E F + V C O - V R E F × e -   t - t C O τ N R / S S

If the LDO uses a precision current source, (as shown in Figure 2-2) use Equation 14.

Equation 14. V R E F = I N R / S S × R N R / S S

Use Equation 15 to calculate VTOP after the changeover event. Equation 13 defines VFB(t).

Equation 15. V T O P t = V F B t × R T O P R B O T T O M + V C O _ F F - V F B t × R T O P R B O T T O M × e -   t - t C O τ F F