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I am building a step down converter using LTC3892. It will have 12V output sourcing 50A in each of two interleaved phases.

Problem is that LTC3892 has fixed current limit value at 75mV. This means that for 50A current limit, I would need a current sense resistor of 1.5mOhm which is a bit odd value.

I wonder whether I could use a 2mOhm resistor and then use a resistor divider to reduce the signal voltage from 100mV to 75mV? Has anyone had any experience with such method?

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    \$\begingroup\$ It is by no means an odd value, even on amazon you get lots of shunts for that. 75mV is one of the "universal" values shunts are manufactured to, and 50A is by no means an odd value to have a shunt for. Have you actually looked around where to buy? \$\endgroup\$
    – PlasmaHH
    May 18, 2017 at 13:24
  • \$\begingroup\$ Well, you are right about 1.5mOhm being common, but I have 2mOhm shunts in the shop, so I would like to use them. \$\endgroup\$ May 18, 2017 at 13:28
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    \$\begingroup\$ There's a differential amplifier inside the chip (You can see it from the block diagram given in the datasheet). So, you may need to use dividers on either side of the sense pins, but this may lead to to some measurement errors inside the chip. \$\endgroup\$ May 18, 2017 at 13:39
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    \$\begingroup\$ The divider would be differential not ground referenced. \$\endgroup\$
    – sstobbe
    May 18, 2017 at 14:09

2 Answers 2

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Maybe you could, but I suggest simply connecting Ilim to change the limit voltage to 100mV which happens to be the same ratio as 1.5:2, then no additional parts required.

enter image description here

Edit: Since you have the version without the configurable limit, you could try dividing it however you are in unknown territory in some ways. Suggest leaving the 1n cap across the 3m ohm resistor and dividing the resistor with something like 1\$\Omega\$/4.99\$\Omega\$, with the 1 ohms at the Vout side. You might consider a few pF across the sense inputs, but it's probably not necessary. 1% is overkill, but Samsung ones cost 33 cents for 10 so it won't break the bank. The higher you go in resistance for the divider the further you are away from the designers' concept so unforseen problems may occur.

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  • \$\begingroup\$ I have the LTC3892-1, which has fixed value of 75mV. \$\endgroup\$ May 18, 2017 at 18:19
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If you have sufficient 2 mOhm resistors, simply parallel 4 of them and place these in series with two more in parallel. That will give you 1.5 mOhm.

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  • \$\begingroup\$ PCB space is also a consideration, so it would be a bit wasteful to have 6 resistors just for measuring current. \$\endgroup\$ May 18, 2017 at 18:24

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