(Understanding the problem) When very large capacitor is placed at the output, we will have very high inrush current during startup (I_inrush = Cout*dv/dt). This may lead to device dissipating lot of power and rise in junction temperature of device. To protect the device, Thermal Shutdown (TSD) will act when the junction temperature reaches TSD threshold (150C). This can result in failed or unexpected startup.
(Solution #1: slower slew rate startup) First solution is to reduce the dv/dt (by setting larger dvdt cap) so that I_inrush is less, and total power dissipation reduces at the device. This will increase the overall time required for the startup but will prevent any TSD during the startup. This method is preferred over solution #2 if startup time is not a concern.
To find the optimum value of dvdt cap, use our design calculator. Here you need must fill the Vin, Cout, ILIM, Rlstart. By changing the value of cell D62 (dvdt cap) and checking the cell D72 for the confirmation. This can help in finding optimum value of dvdt cap such that fastest possible startup happens without TSD.
(Solution #2: hiccup startup) Since we have TSD protection to prevent our device from getting any damage, we can use auto-retry feature of device. Due to high inrush current, the device will go into TSD and then the device will retry and perform startup again and this cycle repeats. During each startup cycle, the output capacitor will get charged and inrush current reduces. Eventually this will allow the device to have full startup. This method is faster compared to solution #1 and is preferred if startup time constraints are there. Follow the fault response table in datasheet section 8.3.8 for the NRTRY and RTRY_DLY setting.
(Demonstration of this with waveforms)
Test conditions used are Vin: 12V, Cout: 8mF, No Resistive load, ILIM: 3A.
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Condition : Result |
Waveform |
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Latchoff mode (RTRY_DLY is SHORT) and DVDT is 10nF (high slew rate) : Startup Failed |
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Latchoff mode (RTRY_DLY is SHORT) and DVDT is 115nF (Calculated from design cal) : Startup Passed (Solution #1)
Startup time: 315ms |
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Infinite retry mode (RTRY_DLY is OPEN and NRTRY is SHORT) DVDT is 10nF : Startup Passed (Solution #2)
Startup time: 220ms |
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For further reading, please refer to App note on charging super cap: Charging Super Capacitor With eFuse (ti.com)
