LTC3895 blows up randomly

If I was you I would look of the cause of the blowing up:

1. by high output current of the chip

2. by blowed up output mosfets 

3. load on the output is too high

4. input voltage is to high 

5. Input voltage instability or high ripple

What is your load is it dynamic or static, does it change too frequently? 

kind regards

Hello, thank you for your message.

Here a brief description of my application:

I have to drive a laser diodes array at 5A (and a voltage around 25V, from its I-V characteristic). Since I need to fine tune the power (in other words the output current) I have added an external control loop to limit the output current at a precise value (by limiting ITH pin voltage) and I programmed the output resistor (VFB pin of LTC3895) to generate an output voltage a bit larger of my operating point (and so around 27V).

But, in order to better understand the problem, I made some tests replacing the array with a fixed load of 5 Ohm and deactivating the added control loop (so the output voltage stays fixed at 27V, given by the output voltage divider). 

So here the answers to your question:

1. What do you mean, the output current? If it is the case see point 3. The output current should flow only trough top or bottom mosfet and not across the IC, shouldn't it?

2. Even if the  failure occurs, mosfet are always ok. If I replace the IC the board start working properly again (if I "guess" a compliant IC).

3. Maximum load current (measured with hall probe current connected to an oscilloscope) get from zero to 5A (or 27/5=5.4A in the simplified setup), the observed current step is clear and no overshoots are noticed. 

4.I monitored input voltage given by an off the shelf AC-DC: it stays fixed at 48V without spikes or similar things.

5. see point 4

As explained before, the problem randomly arises also with a fixed 5 Ohm load. When it happens, the IC goes instantly to 150°C (so, in my opinion, is a "voltage related problem" not given by "excessive power dissipation" since it is immediate and doesn't require time) and it happens at first start with the application of RUN or when I release the current limit to 5A after having applied the RUN.

Anyway, what make me crazy is that when I get an IC that is ok, the board works properly indefinitely....otherwise the IC get killed instantly.

Regards,

Massimo

Hi Massimo,

It would hugely help, if you could share the schematics. Understandably that may be prohibited due to confidentiality issues.

However, could you explain how have you connected the EXTVcc pin (32)?

Cheers, heke

Hi Heke,

the EXTVcc pin is connected to ground. I would try to share some parts of the schematic, but I must check if it is allowed.

Anyway, since the buck converter is mounted over a second board (with a strip "male-female" connection between the two boards, not only for Vin but also for the other control signals: RUN,PGOOD,PLLIN, etc...) and given the "random" occurences of the issue, my suspects are focused also on ESD problems, that may occur during the assembly of the whole system. Have you ever experienced similar issues with this converter?

The datasheet says nothing about the need of external protection and if there are some pins that are more subsceptible to ESD...

Regards,

Massimo

Hi Massimo,

ESD could be the origin of the problem, but to my understanding the PMIC's have typically quite hefty internal ESD protection even for the digital I/O. Would say that the module assembly is hardly the weak point in this case. However, you could perhaps try e.g. bridge the grounds of the main board and the DC-DC board with a crocodile jumper before plugging the DC-DC board in, to see if that make difference. I have not worked myself with this particular chip, but in general have not had any ESD issues (I have once had trouble with another LT/LTC converter, burning 10 chips without finding out the reason, but would say, it was not due to ESD).

There are a few other things to consider (forgive if these are trivial to you)

1. As EXTVcc is grounded, do you have the resistor from pin SS to INTVcc installed (datasheet page 22)?

2. Are all the boards assembled by an assembly house and did they do the component procurement? (In some cases the assembler may be inclined to improve the margin by using grey market, refurbished or counterfeit parts).

3. Do you use an external FET for the DRVcc regulation?

All in all, I'd suspect that either some pin is operating outside the maximum (voltage) rating, or that the internal regulator gets overloaded (not sure if there is any over current protection on the internal LDO)

Cheers, heke