TPS2121: TPS2121 Inrush Current for loading

Hello,

Could you send a schematic?

Also could you describe the test in a little more detail:

Is this a hot plug event? If it is a hot plug event where a power source is immediately connected to the system you will have an inductive surge through the wires.  

Does the device attempt to restart? There is a 25ms delay for a restart condition if the device has hit thermal shutdown. If you increase the range, you can be able to see whether the device restarts. 

Since this current is occurring within the 250us window, the current limiting is not likely to trigger.

Best Regards, 

Elizabeth 

Hi Elizabeth,

Thanks for your reply.

I have attached the schematic file.

For your questions:

1. No, it is not hot plug. 

2. No.

Let me describe the measurement method in detail:

The test sample removes all loads and only keeps the TPS2121 and the buck/boost circuit.

Step:

1. Solder a wire to both ends of C7 and connect to the output terminal of the DC power supply.

2. Solder a wire to M13 or M18 and connect to the positive terminal of C7.

2. Connect the passive probe of the oscilloscope to the wire in the first step. (Measure the input voltage and set this signal is trigger signal)

3. Hook the oscilloscope current probe to the positive terminal of the input power supply.

4. DC power supply output power and get the signal by oscilloscope.

20210706_Main Board_WL-頁面-7-8.pdf

Hi Elizabeth,

Thanks for your information.

According to my literal understanding from your reply:

According to the current circuit design, since the current surge current is tens of us, the TPS2121 will not be triggered to limit this surge current. Is my understanding correct?

If my understanding is correct, I found that when the power is input from TPS2121, the BUCK and BOOST circuits cannot work normally, but when the power is supplied from the output terminal of TPS2121 to the BUCK and BOOST circuits, the BUCK and BOOST circuits can work normally, what is the reason?

If TPS2121 triggers to limit the inrush current, according to the current design, is it necessary to add protection circuits to both input terminals of TPS2121 to suppress the inrush current from triggering the protection of TPS2121?

Hi Wesley, 

It is possible the short-circuit protection is kicking in which is described below and in the datasheet:

This would mean the device would turn off at a current 2.4 times your programmed ILM, and after 25ms, will attempt to restart. What resistance are you using on your ILM pin? 

Please replicate this test showing waveforms for Vin, Vout, Iout, and ST with 5-10ms per division. This will give us more data to see if the device is attempting to restart. 

It appears your voltage ramp up is very fast at around 1V/ 10us and due to the large capacitive load of the buck and boost circuitry, it is understandable you will see a large current spike. 

Best Regards, 

Elizabeth 

Hi Elizabeth,

For first question: The ILM resistor I use 28k ohm (Default), new resistor will change to 18.7k ohm.

For second: Sorry I can't measure it now, because the PCB samples are tested on the firmware by my colleague.

But try to measure the waveform by use POC sample (without TPS2121).

You can see the picture are below:

CH1 (Yellow): Power input from DC power supply, DC +5.0V / 6A.

CH2 (Green): Boost output, DC +5.4V.

CH3 (Blue): Buck output, DC +3.3V.

CH4 (Pink): Power input current (You can think of this current as the TPS2121 output.).

Picture 1: Take a look at the input and output of the system. The time of oscilloscope is 2 sec / Div. You will find that it takes a long time for the output to output the set voltage.

Picture 2: I changed the time of oscilloscope to 10ms/div.

Picture 3: I zoom in the waveform and focus first inrush current waveform when power on.

From TPS2121 datasheet and ILM setting, the current  (typ) is 3.7A, the inrush current can pass to 3.7A*2.4=8.88A. If the inrush current over 8.88A the the TPS2121 will shutdown and restart at 25 ms. Am I right ?

After the TPS2121 restarts, if the new inrush current exceeds the limit, will it cause the TPS2121 to restart again? This causes the TPS2121 to restart again until the inrush current is less than the limit.

Hi Wesley, 

After an over current fault, when the current exceeds around 2.4*ILM, the over current protection circuit holds the internal FET off for about 25 ms; after which, the device turns back on. If the short is still present, the current-limit loop will regulate the output current to ILM and behave in a manner similar to a power up into a short. 

With your original ILM resistor of 28k ohm, you would expect around a 3-4.5A current limit. This could mean a 10.8A over current threshold to trigger overcurrent protection.

With a ILM resistor of 18.7k ohm, you would expect a current limit 4.6-5.8A. This would mean a 11.04A to 13.94A current needed to trigger overcurrent protection. 

Can you confirm if your device is attempting to restart after 25ms? If it doesn't it's possible the overcurrent is not triggering and a lower current limit threshold is needed so that 8A is within the overcurrent limit threshold. 

Best Regards, 

Elizabeth 

Hi Wesley, 

Do you have any updates regarding this issue?

Please let me know if you have any further questions. 

Best Regards, 

Elizabeth 

Hi Wesley,

Since this thread has gone on two weeks without a reply, I am closing/locking it. If there is another update here, please make a new thread.

Best Regards, 

Elizabeth