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I am attempting to design an Ideal Diode Controller for application in a 12 V automotive circuit using an LTC4359 as the controller. I want to implement the shutdown facility available with the controller to turn it off using the ignition line of the car to turn on the controller.

As per the datasheet,

SHDN: Shutdown Control Input. The LTC4359 can be shut down to a low current mode by pulling the SHDN pin below 0.6V. Pulling this pin above 2V or disconnecting it allows an internal 2.6μA current source to turn the part on. Maintain board leakage to less than 100nA for proper operation. The SHDN pin can be pulled up to 100V or down to –40V with respect to VSS without damage. If the shutdown feature is not used, connect SHDN to IN.

Based on the above knowledge, I deduced that whenever the ignition voltage falls below 2 V, it should put the controller in shutdown (low current) mode. To test the hypothesis, I tried attempting to simulate the circuit using LTspice. The simulated circuit is as shown below:

Schematic of LTC4359 as an Ideal Diode Controller with Shutdown Mode

However, when I simulate the circuit, I do not see the output being turned off (low current mode). The voltage continues to float above 11.4 V. I was expecting to see a transition at 0.6 V on the leading and trailing edge. The simulated output is as follows:

Spice output traces

As is visible from above, despite the input toggling from 0 V to 12.5 V, the output voltage only toggles from 11.26 V to 11.97 V.

While I understand that a relay is a better alternative as it will not have any leakage current, I want to use the shutdown functionality in the controller and tie it up with the ignition.

I want to know where I am I going wrong and what changes I have to make to make the functionality available.

ocrdu
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Ishan Karve
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    When in "shutdown" you're only eliminating the gate drive for the MOSFET, but its body diode will still conduct. You should read the "Shutdown Mode" & "Load Switching and Inrush Control" sections of the datasheet which discuss this and how you need to add a 2nd MOSFET to get what you want. – Ste Kulov Dec 31 '22 at 10:28
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    @SteKulov Thank you for your inputs. I did add the second MOSFET in B2B config and got the desired results. Thanks also for clearing my doubts. – Ishan Karve Dec 31 '22 at 13:32

1 Answers1

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I want to know, where I am I going wrong and what amends do I have to make to make the functionality available.

Think about the body diode inside the MOSFET and what it brings to the party: -

enter image description here

You need to use back-to-back MOSFETs in series to get load-switching control. Like this: -

enter image description here

Image from Primer on PowerPath Controllers, Ideal Diodes & Prioritizers.

Andy aka
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    Thank you for your inputs and reference links. They are surely insightful. I simulated the circuit using two MOSFETS in B2B config and got the desired results, – Ishan Karve Dec 31 '22 at 13:34