LM3150: output voltage drops when current increases, only when circuit is after LTC4365

Hi Jon,

Are you reading an output voltage dip on the E-load? Some E-loads show a drop in voltage, even when there is none. If this is the case, please measure the output voltage across the C205 capacitor with a scope or a DMM directly to get a more accurate reading of output voltage.

Thank you,

Joshua Austria

Hi - I did measure with a scope and a DMM and confirmed that the voltage does indeed drop when the circuit is connected to the LTC4365. It does not do this in isolation.

Ok a few new observations. When the circuit is connected through the LTC4365, the high side gate drive waveform is very strange - it does not look like this when not connected to the LTC4365. Sorry for the phone pics - couldn't find my USB stick!

Here is with no load connected (ch1 yellow = high gate drive, blue = 5v output of regulator).

Note that input to the LTC4365 is 16V from my bench top supply, which is within the operating region defined by the resistor network. I confirmed that the output of the LTC4365 is stable and there isn't a large drop other than FET Rds_on.

When a load is connected, the high gate drive appears to be a normal square wave (this waveform corresponds to the last pic of the e-load, at 10ohm):

The e-load reads between 100-200mV lower than the scope and DMM once the voltage starts dipping. Prior to that (at ~5.05v, they're nearly identical). Here are a few shots showing decreasing constant resistance (higher current demanded from the power supply) and the output voltage dipping. I confirmed right before I connected it back to the LTC4365 that this circuit works in startup and steady state conditions up to 4A as intended in isolation...

You can see that the regulator begins to drop aggressively as the resistance is decreased the current goes up. 

Hi Jon,

Just to confirm, is there no issue from the LM3150? If the LM3150 works in isolation, then the problem is the with the LTC part, which I cannot comment on. 

Thank you,

Joshua Austria

The LM3150 appears to work fine in isolation. I am asking for input on if there is something in my system design that might be causing the LM3150 to be behaving strangely.

Hi Jon,

In terms of the LM3150, as long as the recommended operating ratings are observed, there should not be an issue. As it seems that the part works in isolation, I believe we can also rule out the LM3150 application. If the LTC part is acting as a supply I would imagine it might have to do with a current limit issue from the supply. For those issues, I would advise reaching out to LTC to see what might be the issue from their design.

Thank you,

Joshua Austria

Hi Joshua - observed a new failure method even in isolation. The 3150 will work fine and then seems to die, and VCC (pin 1) and GND are shorted together. I'm using a pretty good power supply (BK precision 9103) at the input. Testing in the range of 9v to 20v for VIN.

Any thoughts on what could be causing that from past experience? I'm only testing with a constant resistance load, not constant current, and always within design spec (~2A, designed to support 4A though).

I've also noticed that the high gate switching frequency seems to have jitter and switch between two frequencies vs. a single stable one like the EVM. Can you take a look at my schematic? Does anything stand out as obviously incorrect?

Hi Jon, 

Can you please send over this calculator filled with your device's components' specifications. The calculator should be able to quickly validate your solution and catch components should they be out of recommendation for use with the device.

Also another question, is the device being tested fresh or was it from the problematic circuit? Additionally, how did you isolate the LM3150 from the rest of your application for this test?

Thank you,

Joshua Austria