Seeking LTC4365 advice

[bryce1]

Hi guys,

for the first time, I'm trying to implement proper circuit protection. I chose the LTC4365, and I think I calculated all the values correctly.
But some questions remain:

- Is it going to work with 12V/25A?
- What would be the best mosfets to use for this 12V/25A circuit? I put the IRLB8748PbF on there because I use them in other places as well. Could you recommend any?
- Do you have any advice for the layout side of things?

Thanks a lot!

[bryce1]

anyone? 

[HalfSpace]

Hi bryce1,

Have you used LTSpice? This device is listed under “Special Functions”. Maybe a simulation of your circuit in LTSpice may answer some of your questions.

What are you protecting with this circuit?

HalfSpace 

[miguelvp]

To help you out a bit with LTSpice I modified the Jig for the LT4365 to match your circuit. I had to disconnect the electrolytic cap because with it connected it wouldn't simulate at all. and sorry if I used the US representations most of the circuit was already using those so I left them all like that.

I selected BSF050N03MQ3 as mosfets because it was the closest to the ones you have (30V 5mOhm 15nC) so they match the IRLB8748PbF (30V 4.8mOhm 15nc)

I changed the resistors values to match yours and modified the fault so it all matches.
I didn't remove the jig load.

You might want to load the default jig as well if you use LTSpice. You probably want to put values for the LEDs I just selected the generic one so that won't match.

One thing, without load the power LEDs show 20V going through them (D3 and D4 in the schematic attached)

I attached the LTSpice model in the zip.

Edit: when I told it to open the test fixture (that creates the Test Jig) it selected Si4850EY Mosfets. (60V, 31mOhm 18nc) the datasheet for the LTC4365 uses all kinds of different ones depending on the configuration. But they do have a lot of sample applications.

[miguelvp]

I closed LTSpice and asked me to save the file so I'm not sure if what I uploaded had all that was needed, so I'm attaching it again.

[bryce1]

Thx, I'll have to get into ltspice.

What are you protecting with this circuit?

It is supposed to protect a circuit that switches 12 loads (low side n mosfet) that each draw between 0.2 and 5A.

[HalfSpace]

Thx, I'll have to get into ltspice.

What are you protecting with this circuit?

It is supposed to protect a circuit that switches 12 loads (low side n mosfet) that each draw between 0.2 and 5A.

Hi bryce1,

Are any of the loads inductive? ?  Motors, solenoids etc.

Your mosfets are only 30V and any inductive switched load may produce voltage spikes which could damage them.

HalfSpace 

[bryce1]

There are two pumps, yes. But I use flyback diodes for every load. Wouldn't that prevent such spikes?

For the LTC4365, would you recommend different ones?

[HalfSpace]

There are two pumps, yes. But I use flyback diodes for every load. Wouldn't that prevent such spikes?

For the LTC4365, would you recommend different ones?

Sometimes the spikes from DC motors can be in the positive direction. The mosfets should be ok if a transient voltage suppressor diode with a clamp voltage of 22-25v was added across your flyback diodes.

HalfSpace 

[bryce1]

Ok that, or I will use different mosfets. Thanks a lot!

That still leaves me with the question what mosfets to use for the LTC4365.

[miguelvp]

Ok that, or I will use different mosfets. Thanks a lot!

That still leaves me with the question what mosfets to use for the LTC4365.

some from page 10 of the datasheet:
http://cds.linear.com/docs/en/datasheet/4365fa.pdf

Code: [Select]

Table 1. Dual MOSFETs for Various Supply Ranges
VIN  MOSFET VTH(MAX) VGS(MAX) VDS(MAX)
2.5V SiB914 0.8V     5V       8V
3.3V Si5920 1.0V     5V       8V
5V   Si7940 1.5V     8V       12V
?30V Si4214 3.0V     20V      30V
?60V Si9945 3.0V     20V      60V

But they have a lot of detail and applications information from page 8 on. But I would read the whole thing if I was you.
Also on page 1 they have a 12V automotive application using an Si4946

[bryce1]

I would have used them, but they are mostly for sub 5A. I will need at least 25A.