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LM74700-Q1: High Voltage Ideal Diode for Reverse Polarity Protection and Reverse Current Blocking

Part Number: LM74700-Q1
Other Parts Discussed in Thread: SM74611

Hi there, 

I need a high side ideal diode for a project I'm working on that requires reverse polarity protection and reverse current blocking. Something like the LM74700 would work fine, but my input voltage range is 15V to 100V (the LM74700 operates from 3.2V to 65V). From what I can tell, there are no off-the-shelf parts that can run at this high of a voltage directly. 

Is it possible to put a zener diode across the ANODE and GND pin and a resistor between the GND pin and circuit ground on the LM74700 to regulate the voltage across the LM74700?

If so, are there any other things I should consider while implementing the LM74700 like this?

If this is not recommended, do you have any other suggestions?

Thanks,

Phillip

  • Hi Phillip

    Thanks for reaching out. 

    If the high voltage is not a continuously applied voltage but only occurs as during a transient condition, the transient can be suppressed using the TVS diode at the input. Please refer to section 10.1.1.4 of the LM74700 datasheet: 

    LM74700-Q1 Low IQ Reverse Battery Protection Ideal Diode Controller datasheet (Rev. G)

    But in case the higher voltages are continuous in nature, then unfortunately the device will not be able to support it.

    The above arrangement with discrete devices will not be able to block reverse currents. The anode voltage with respect to ground is held constant by the zener diode, and the device compares anode voltage wrt cathode voltage (both referenced to GND pin) to turn OFF the device when anode voltage goes below cathode. This means that if the input voltage goes below the output during reverse current condition, then because the anode is not going below cathode but is rather fixed by the zener diode, the device will not turn off. Say the input voltage is 24V and the output cap is also charged to 24V. Now the input starts to go down below 24V, let's say that it becomes 20V, and the breakdown voltage of zener diode is 12V. Then ideally in such a situation, the device must turn off because the input is going below the output and there will be reverse current. But now since the anode voltage wrt ground is being held constant (12V wrt ground)by the zener, it won't be able to detect the input actually going below the output, because the anode voltage wrt ground is still equal to cathode voltage wrt ground. 

    Unfortunately, the device has to operate within it's absolute maximum limits to sustain reliable operation. But that being said, the absolute maximum ratings are with respect to ground. If we raise the voltage on the ground pin by using a discrete solution, say to 12 V, the device can operate between (3.2+12 to 65+12)V. In order to do this, you can follow the kind of work-around with discrete devices given in the below application note, in which the GND voltage is boosted up by a zener diode to a higher voltage-

    /cfs-file/__key/communityserver-discussions-components-files/196/8475.slua486.pdf

    But nevertheless, the entire range of voltages cannot be accommodated unless the range(highest-lowest system voltage) is below (65-3.2=)61.8V.

    I hope this provides clarity. Kindly let me know if further support is needed.

    Thanks and best regards,

    Divyanshu 

  • Hi Phillip,

    We have an arrangement with other devices (LM74800, B2B FET controller) which can support use cases in which protection against higher voltages is desired. Can you please tell us if the 100V requirement is due to system transients or is the continuous system voltage requirement? Is there protection required against the higher voltages, in case the downstream loads are rated for lower voltages? We can recommend a solution based on the answers.

    If possible, also please let us know about the overall use-case for which these requirements are arising.

    Thanks and best regards,

    Divyanshu

  • Hi Divyanshu,

    Thank you for the responses. 100V is the maximum working voltage (not a transient). I've read through slua486 and I have been considering how to implement it.

    The 100V is fed to a Buck DCDC converter, which converts it to ~15V for charging a battery and powering a load. 

    It is possible for the user to connect the 100V source backwards (they are individual wires from a solar array), so we need reverse polarity protection.

    Due to the body diodes of the FETs in the DCDC converter, there is a power path between the battery and the 100V input connectors, so the solution also needs reverse current protection incase the input is shorted or the input voltage is below the battery voltage.

    Is this enough information?

    Thanks,

    Phillip

  • Hi Phillip,

    Thanks for the detailed explanation of the requirements. I have a few solutions in mind-

    - one option is to use wide-bandgap devices which can sustain higher voltages. But I will need to discuss this with the GaN ICs team. It might be possible to use GaN drivers with comparators and come up with a solution.

    - LM74611 is another device in ideal diode product portfolio which can sustain up to 100V of input voltage. Let me confirm about it's operation and applicability to your use-case with the support for it. Till then, kindly go through the datasheet and let me know in case you see some spec violation for your use-case:

    https://www.ti.com/lit/gpn/sm74611

    Thanks and best regards,

    Divyanshu

  • Hi Divyanshu,

    It does not look like the SM74611 would work for reverse polarity protection, as it's max DC reverse voltage is 30V. This final solution needs to be able to withstand ~ -115V (15V +100V).

    Thanks,

    Phillip

  • Hi Philip,

    I have raised a query with the wide-bandgap team. Let us take up further discussion at that forum. Here is the link: 

    (+) Power protection using wide-bandgap devices - Power management forum - Power management - TI E2E support forums

    Thanks and best regards,

    Divyanshu