LM70880-Q1: Output power issue and input, output capacitor selection question

Hi Jun,

Please use our quickstart calculator to determine the loop stability of your designs by entering as much details into the calculator. My suspicion is that loop is unstable as VIN increases in voltage or you are hitting current limit.

Ben

Thank you for your reply.

In the current circuit, can I improve the stability of the loop by adjusting the capacitor values ​​at the input and output?

Hi Jun,

Yes, the quickstart calculator will show the stability of the loop when changing the output capacitance and the external compensation components.

Ben

Hello Ben, thank you for your reply.

I have changed the capacitance of the board I am currently testing and reflected the contents of the components used in the calculator.

The resulting graph is shown in the photo below.

The phase margin for 12V is 70, and for 24V it is 64.

Depending on the values ​​entered into the calculator for the capacitors of the PCB board being used, three 4.7uF input capacitors were used, and three 47uF output capacitors were used.

In the case of the 12V output PCB, the output was 12V normally, but there was a problem with the inductor. When I checked with a thermal imaging camera, the temperature rose to around 80 degrees, and there was a symptom that the temperature rose little by little over time.

Hi Jun,

What is the output current for the 24V output PCB? It is possible the part is going into thermal shutdown. What is the thermal resistance of the PCB?

Ben 

Hello Ben, thank you for your reply.

The current reflected in the design is 8A, but there is nothing connected to the actual PCB.

Is there a way I can check if it is in thermal shutdown state?

Additionally, how can I measure and tell you the thermal resistance value of my PCB?

Thnak you.

Hello Ben, Let me tell you about the additional testing.

When 45V input power is applied to a 24V PCB, 24V output is produced.

At this time, if you measure the temperature using a thermal imaging camera, the temperature is 35 to 38 degrees.

When 60V power is input to the 24V PCB, the output power is 200mV.

At this time, when measured with a thermal imaging camera, the temperature is 35 to 38 degrees, as before.

Hi Jun,

jun cha said:

Is there a way I can check if it is in thermal shutdown state?

Below describes what happens to the operation of the IC during thermal shutdown.

jun cha said:

Additionally, how can I measure and tell you the thermal resistance value of my PCB?

If you have a thermal imagining camera or a thermocoupler, you can roughly measure the temperature of the IC and its surrounding area. Then you can use the equation below:

Tj will be the measured IC and its surrounding area.

Ta will be the temperature of the environment

Power is the power dissipation of the IC

R(thetaJA) is what you are solving to determine the thermal resistance of the PCB.

Ben

Hi Jun,

jun cha said:

When 45V input power is applied to a 24V PCB, 24V output is produced.

At this time, if you measure the temperature using a thermal imaging camera, the temperature is 35 to 38 degrees.

If you continue to increase the output load, you will see that the thermal imaging camera will show that the temp of the IC will increase. If the temp of the IC approaches the thermal shutdown temperature, the IC will behave as discussed in my earlier post.

Ben

Hi Ben, I looked into this because you told me about the symptoms that appear when you have a Thermal Shutdown.

It is stated that the VCC Regulator is turned off when the Thermal Shutdown state is reached.

I measured the voltages of VCC and VDD and found that they were as follows:

VCC -> 5V / VDDA -> 8V

Since VCC is measured at 5V, I don't think it is in the thermal shutdown state you mentioned.

Is there another way to figure out the problem?

Hi Jun,

What was the PG signal? Did the part stop switching? Please provide some oscilloscope photos of the SW node for the 24V output. It is difficult to determine why the part is not outputting the desired voltage without more information.

Ben

Hello Ben, thank you for your reply.

Below are photos of the oscilloscopes of the PG and SW Nodes you mentioned.

PG Signal

SW Node

Is there any additional information you need?

thank you.

Hi Jun,

Does the PG pin have a pull-up voltage? The PG pin shows that it is low which means the output voltage is below the operating range.

If the design is for 400kHz, the SW node plot doesn't appear correct. Are you able to zoom in on the SW node?

If possible, can you load the output to 2A/24V and recapture the SW and PG signal?

Ben

Hello Ben, thank you for your reply.

There is currently no load on the PCB board.

The PG pin is pulled up to the VDDA voltage through a 100K Ohm resistor.

Below are pictures of each node with a load of 2A.

PG Signal

SW Node

Thank you.

Hi Jun,

The PG signal does not look correct. What is the measured output voltage when the PG signal was measured?

Ben

I think it's roughly 1V.

Thank you.

Hi Jun,

The PG pin is working correctly. The scope shot was taken while the output load current was set at 2A? The conditions are 48V to 24V with IOUT=2A, correct? Is the current from the power supply set properly?

Ben

Hello Ben.

The current PCB inputs and outputs are as follows:

Input : 60V   /   Output : 24 V, 2A

Are there any other additional issues?

Thank you.

HI Jun,

Are you able to get 24V at the output with 2A of load current?

Ben

When the input power exceeds 45V on a 24V PCB, 24V output will not be generated under either no load or loaded(2A) conditions.

Thank you.

What are the ratings of the input capacitors? 100V?

Ben

The input capacitor is rated at 100 V and the output capacitor is rated at 50 V.

Hi Jun,

Are you able to replace the part with another unit that is good? I was able to output 24V without an issue but it seems that the unit you have may not not be operating correctly.

Ben

Is the part you are replacing an IC? Is it an input capacitor?

Hi Jun, 

I am suggesting that the IC on your setup should be replaced.

Ben

Hello, Ben.

I tried replacing the IC chip with a new one, but the symptoms were the same.

I built a new board and tested it using an internal compensator instead of an external one.

According to the datasheet, the EXTCOMP pin and the VDDA pin should be connected through a 100K Ohm.

But the converter doesn't work properly. I connected VCC instead of VDDA and the board worked.

I tested it by connecting a load of 1~2A, and after about 5 minutes, the 12V PCB worked normally, but the 24V PCB was damaged and caused a spark.

Is there anything wrong with the board I built for 24V output?

Thank you.

Hi Jun,

jun cha said:

According to the datasheet, the EXTCOMP pin and the VDDA pin should be connected through a 100K Ohm.

But the converter doesn't work properly. I connected VCC instead of VDDA and the board worked

Connecting the EXTCOMP pin with a 100kohm resistor to VCC is not correct. The datasheet is correct about connecting the 100kohm resistor to VDDA. Connecting the EXTCOMP pin to VCC will exceed the ABS MAX rating of the pin and will damage the IC which you saw when the output was set to 24V. See below:

Have you ordered our EVM?  I am starting to suspect the issue may be the PCB itself or the soldering of the IC to the PCB is causing your issue.

Ben

Hello, Ben.

Yesterday you said that the device you were using was outputting 24V normally.

I'd like to do a comparison test between the board I built and the device you used.

Did you test it by changing the output to 24V on the EVM Board? Or did you use a different device?

If you used another device, could you please provide the schematic and information about the components used?

Thank you.

Hi Jun,

I have taken efficiency measurements at 48V input and 24V output. I have taken the output to 36V without any issues. The efficiency measurements were taken on an EVM. Please see the LM70880-Q1 schematic in the user guide. The EVM originally is setup for a fixed 5V output but I modified the EVM for a 24V output.

Ben

Hello, ben.

When testing with an EVM board, can I change the output voltage by simply changing the feedback resistor value and inductor resistance?

Are there any other additional changes that need to be made?

If I use an internal compensator, will there be a big difference in functionality compared to when I configure an external compensator?

Thank you.

Hi Jun,

Which EVM do you have? LM70880-Q1 or LM706A0-Q1?

Ben

HI Jun,

Are there any updates?

Ben

Hello, ben.

Currently the EVM board is not in use, but I plan to build a new PCB and test it.

I'm going to use the same design as the EVM board, and I'm going to use the internal compensator. If I use the internal compensator, will there be any problems or disadvantages in functionality?

Thank you.

Hi Jun,

There are no disadvantages in functionality but using external compensation gives you the flexibility to fine tune the loop compensation. 

Please get the EVM working with 24V output first before building a new PCB. It is best to iron out the issues before going to a new board design.

Ben