Hi team,
My customer is using TPS7A26 in their project. The operation condition is shown as below.
- Vin=5V (from DCDC)
- Vout=3.3V
- Cin=0.1+10uF
- Cout=2.2uF
- Load: two MCUs
The TPS7A26 has overshoot when start-up and the peak voltage is about 3.5V. I measure the waveform of Vin, Vout and Iout, as figure.1. The output current is about 2.648A which is large than Icl of TPS7A26 and trigger overcurrent. The overshoot happen because the LDO starts with a current limit. Is it right?
I think the reason of such large start-up current is input cap of MCU, parasite cap and start-up current of MCU. I removed some input cap of MCU and the waveform is shown as figure 2. The start-up current become less but it is still 1.5A and the peak output voltage is about 3.4V.
But after checking with the customer, the input cap of MCU could not be removed. Is there any solution to reduce the start-up overshoot or start-up current?
I have tried to add a 470uH, 0.6Ω inductor behind the LDO output and the start-up overshoot disappear. Adding only 1Ω resistor could also solve this problem. Will it cause stability issue? Which one is recommended, inductor or resistor, and how large it should be? Is there any solution for the overshoot? Thanks.
Figure.1
Figure. 2
