LMR33630ADDAEVM: LMR33630A

Hello Richard,

This is unexpected. It is possible there is a short somewhere in the feedback circuitry? Can you share your schematic? Did you design a new board or change the components on the EVM?

Can you remove the bottom feedback resistor, and tie FB directly to VOUT? I would expect VOUT to go to 1V when the device is operating as expected.

Best Regards,
Katelyn

Hello Katelyn,

Thanks a lot for help!

I changed the bottom feedback resistor from 24K to 12K. The output voltage is up to 20V (doubled as before), to maintain the voltage at Pin 5 = 2V.

Did I used a wrong/bad boot capacitor of 0.1uF? Any other reasons to affect the 1V reference voltage?

Best,

Richard

Hello Richard,

I have two recommendations for debug.

1. Replace boot capacitor to verify this capacitor is good.
2. Verify the feedback resistance are all as expected. Remove R4, and replace R2 and R3 with 0-Ohm resistors.

Best Regards,
Katelyn Wiggenhorn

Hello Katelyn,

1. As I said before, the feedback voltage is checked and matched the 1:5 ratio. (when output = 5V, pin 5 is 1V) which said the Rs are correct.

2. I replaced the boot capacitor, no change.

3. When I change input voltage (5 to 10V), the output follows. No regulating at all. (no load)

4. I see there is 133KHz switch at the pin 8, with small off time (duty cycle >=95). When I increase input voltage, the off time reduced (cannot understand this)

Please explain why? I switched new IC (same batch) and a inductor, no luck.

Thanks!

Hello,

In regards to your first point, the buck converter cannot step up the voltage from the input to the output voltage. This means that the output voltage will follow the input voltage until the VIN=Vout(as designed). The FB voltage should not follow the input voltage. The FB voltage should be constant at 1V. However, the output voltage can follow the input until Vout>=Vin. I will need your help to further look into this.

Firstly can you please share your board schematic for review?

Can you please share an oscilliscope waveform of the SW, inductor current, output voltage, and FB voltage:

1. Case #1 from above (output=5v and pin5 is 1V)
2. Case #3 from above Input Voltage = 10V

In regards to your fourth point, the duty cycle should decrease with decreasing input voltage. The off time should increase as you increase input voltage. We need to understand why your output voltage is changing.

Thank you,
Katelyn

Hello,

I want to follow up on your problem. Were you able to take the oscilloscope waveforms?

Thank you,
Katelyn