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

2.2.3 Case 5: Precision current reference

If Figure 2-2 describes the LDO architecture, VREF can be written as Equation 21 or Equation 22. Use Equation 5 and Equation 7 with Equation 21 for calculations during the fast charge time. Use Equation 13, Equation 15, Equation 22 and Equation 23 for calculations after the changeover event occurs.

When t ≤ tCO use Equation 21 to calculate VREF.

Equation 21. V R E F = I F C ( t ) × R N R / S S

When t > tCO use Equation 22 to calculate VREF.

Equation 22. V R E F = I N R / S S ( t ) × R N R / S S
Equation 23. t C O = - τ N R / S S × ln ⁡ 1 - V C O I F C × R N R / S S

Figure 2-18 shows the rise time for the TPS7A96 (and the lower current version, the TPS7A94) which uses a precision current reference with an NR/SS pin. During startup the LDO uses a fast charge circuit to rapidly turn on VOUT. The TPS7A94 and TPS7A96 have a unique feature in that VCO is programmable using the FB_PG pin and external resistor dividers. In this test using an EVM, VCO is programmed using the external FB_PG resistors and is set to 97% × VOUT = 1.164 V. These LDO regulators operate in unity gain feedback, thus VTOP = 0V.

Figure 2-19 shows the rise time for TPS7H1111 which is similar to the TPS7A94 and TPS7A96, except the TPS7H1111 is optimized for power devices in a space environment. In this test using an EVM, VCO is programmed using the external FB_PG resistors and is set to VOUT = 1.626 V. These LDO regulators operate in unity gain feedback, thus VTOP = 0V.

Figure 2-20 shows the test using a TPS7A57 EVM. VCO is internally set to 97% × VOUT = 1.164 V. These LDO regulators operate in unity gain feedback, thus VTOP = 0V.

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 TPS7A96 rise time
RNR/SS = 8.06kΩ VOUT = 1.2V
IFC = 2.1mA INR/SS = 150µA
CNR/SS = 4.7µF TPS7A96 EVM
Figure 2-18 TPS7A96 rise time
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 TPS7A57 rise time
RNR/SS = 24kΩ IFC = 200µA
INR/SS = 50µA VOUT = 1.2V
CNR/SS = 3.8µF TPS7A57
Figure 2-20 TPS7A57 rise time
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 TPS7H1111 rise time
RNR/SS = 18kΩ IFC = 2.1mA
INR/SS = 100µA VOUT = 1.8V
CNR/SS = 4.7 µF TPS7H1111 EVM
Figure 2-19 TPS7H1111 rise time