LM60440AQEVM: Evalboard only starting at higher input voltage after turning off and on again

Hi Franz,

A few things I need to ask:

1. Is the issue repeatable? Does it only start up at 16V+ every time?
2. Does this issue occur at load or no load? 
3. How did customer connect EN to VIN? Easiest way to do that is to short out R5 (REN1) on the LM60440EVM. A shunt jumper between Pin2 and Pin3 of J3 will short EN to VINS. 
4. Do you have another EVM that you can test and remodify for 3.45V to observe the same potential failure? 

Normally if the EN pin is shorted to VIN, the device will turn once the enable input rising threshold of 1.23V is cleared and start switching once enough input voltage is applied to sustain the 3.45V output regulation.

Regards,

Jimmy 

Hi, Olaf here for the answers:

1. Yes 

2. Both 

3. Connection on the board between VINS and EN 

4. No

Thanks :-)

There is no J3 on this board. I did a wire connection between test-pins VINS and EN. I can measure VIN @ EN.

Hi Olaf,

Can you provide a marked up schematic with the changes you made on the EVM? 

I don't see why the LM60440 device would have any issues with a 3.45V (close to typical applications of 3.3V). 

Can you also provide a scope shot of VOUT, SW and VIN with increasing VIN? A timescale of 1s/div with VIN slowly increasing from 4Vin will help show when the device begins switching and regulating.

Also what input power supply are you using and is it stable? 

Regards,

Jimmy 

Jimmy, Olaf here. Sorry - i´m on vacation next week. The power source is a professional Rohde&Schwarz HMC8041. Power Supply does´nt matter - the effect is present @ no load, power supply shows 6mA output current. I just asked Franz to send a new one, the scope shots have to wait until i´m back. But from the power supply display- i can tell you that the board will start instant @ 16V - no boating or pulsing before.

Correction - i have a R&S 8043

Jimmy, in my project i will "receive" an input voltage from a power over cable connection - and because the1st voltage-rail i need, a wonderful TI-LDO :-) will produce 3.3VA. Just for the understanding of 3.45V.

I just changed the FB-resistor for 3.45V - nothing else. The EVM did work correct (@5Vin) after this!

I can exclude an ESD issue - since 32 years of my hardware development live i never destroyed anything ;-) I know the rules... Mostly i touch the earth of a mains socket before i take a pcb in my hands!

Hi Olaf,

Thanks for the detailed response. The EVM should be robust enough to not get damaged that easily so I am not worried about the handling. Though I too am surprised that this is causing the issue you are seeing.

I'll need to order an EVM on my end to also test this out as well but I do think this may be a one off issue. It's good that you will be on vacation next week since it will take roughly a week to get the EVM anyways. 

When the EVM comes in (roughly 3-5 business days) I will test and provide feedback on what I see. Expect to hear back from me on this forum earliest by 5/13. 

Regards,

Jimmy 

Thanks Jimmy - so long.

Will reply to you soon. Thank you. 

Hi Olaf,

Just wanted to give you an update. I believe due to the pandemic situation, package shipment was slightly delayed.

I've attached the FEDEX tracking number (787016400192) in case you want to know the progress of the LM60440EVMs.

Hopefully these EVMs come in by end of today so I can start testing and providing you some feedback.

Sorry for the delays and thank you for waiting. 

Regards,

Jimmy