LMR36503-Q1: Same input voltage, the output is different

Hi Wang,

What is the load current in your application? Also, could you clarify what the signals are in the oscilloscope shots?

I assumed that the yellow curve is the battery voltage, blue is Vout and green is Vin. It looks like Vin is dropping below its lower threshold in the FPWM mode while it stays above it in AUTO mode. This could be because higher current is drawn from the input in FPWM mode. To confirm this could you probe and send the SW node waveforms too in both modes?

Thanks and regards,

Niranjan

Hi  Niranjan,

Why does the input voltage drop when the mode changes from FPWM to AUTO?

The BU(Yellow line) keeps when mode changes, why does the BU_3.3V_IN (Green line) drop?

The drop causes that the DCDC doesn't work. 

We need to know the reason.

Test point as below:

Best Regards!

Wang Jingkun

Hi Wang,

You will see the VIN drop in AUTO mode also, if you keep BU low for a longer time. In FPWM mode, since the converter continues to operate at the same frequency, more current will be drawn from the input compared to AUTO. This causes the input to drop faster in FPWM mode than in AUTO mode. This is why VIN hits its lower threshold before the BU goes high again in FPWM mode.

The BU lower threshold seems to be lower than required after taking into account the diode drop. Just to confirm, what is the lower level of BU? This should be increased to be above VIN_min of IC + the diode drop to have proper output when BU drops.

Regards,

Niranjan

Hi Niranjan,

In AUTO mode, BU keeps low for 60 seconds, we can't see the drop.

The BU drops to 3.4V. This is our customer's requirement of Power Supply Fluctuation Test.

So it can't increased BU input level.

Now we only want to know why does the input voltage drop when the mode changes from FPWM to AUTO?

We also have the countermeasure to solve the issue.

But we need to the reason to explain to our customer.

Best Regards!

Wang Jingkun

Hi Wang,

I have ordered an EVM to test this. To recreate the issue, can you tell me the load current? I will get back to you in a week or so, once I have the results.

Can you also send the zoomed in switch node and boot to SW waveforms at the points I've marked? I want to know the switching frequency at these points in both modes.

Thanks and regards,

Niranjan

Hi Niranjan,

The load current:

When MODE pin= High,  it is about 8mA.

When MODE pin= Low, it is about 0.2mA.

Best Regards!

Wang Jingkun

Hi Wang,

I will look into this and get back to you.

Regards,

Niranjan

Hi Wang,

Sorry for the delay in responding, I haven't gotten the EVM yet. I suspect that this is a boot UVLO issue.

In the AUTO mode the device would enter into pulse skipping at light load, while in FPWM mode it keeps switching at the same frequency. Since it switches more often in the FPWM mode it takes more energy from the boot capacitor and cause it to hit the UVLO threshold after some time, when operating at low Vin. This is why the output is regulated in the AUTO mode but falls after some time in FPWM mode.

I will confirm this once I get the EVM in 4-5 days. Meanwhile, if you have a differential probe could you probe the voltage across boot capacitor (boot-SW) to get a clearer picture?

Thanks and regards,

Niranjan