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LM74700-Q1: Appropriate for charging port protection?

Part Number: LM74700-Q1
Other Parts Discussed in Thread: LM74700DDFEVM


A client contacted us regarding the protection of the charging port for their e-scooters and other PEV’s. It seems that some customers are measuring the pack voltage via the port’s two pins and short-circuiting the battery pack, damaging the port pins.

They have a 10A fuse that blows when the port is shorted but it doesn’t do enough to prevent damage to the pins and the PEV needs to be brought in for fuse replacement which is quite inconvenient.

The packs are 10S and 12S li-ion, 8Ah to 32Ah. There is no BMS between the charging port and the battery pack, just the standard fuse. Some of their customers also wire up their own higher current chargers to speed up charging but accidentally reverse the polarity.

We were considering using the LM74700-Q1 to provide input short-circuit and reversed charger polarity protection.

The anode side of the LM74700-Q1 would be connected to the charging port connector. The cathode side would be connected to a the battery pack. The standard fuse would still be used between the LM74700-Q1 and the battery pack. Is the LM74700-Q1 appropriate for this application?

We also have a low profile requirement. Is the large 220uF electrolytic cap on the cathode side of the LM74700DDFEVM circuit recommended for an application like this? The wiring to the battery pack will typically be short, a few centimeters or less. Maximum length will be about 25cm. Can we just use ceramic caps? If yes, what would your recommendation be for the cathode-side cap value? We have no holdup requirement, just cathode-side spike suppression. The datasheet lists 1uF min/47uF typical so we were considering laying out pads for two paralleled caps, 10-22uF each, and testing their effectiveness.

Some chargers must see the pack voltage before charging to begin. Unfortunately this means that the LM74700-Q1 would need to be enabled at all times. Our client is grudgingly willing to accept the additional quiescent current but we were wondering if it is acceptable to connect the EN pin to the CATHODE pin, using the battery pack’s voltage to keep it enabled?

We were considering just using a high value resistor across the MOSFET (with EN connected to ANODE) to provide pack voltage for the charger to detect but weren’t sure if that would work for all chargers or interfere with the LM74700-Q1’s operation.

Regards, John M.

  • Hi John,

    Thanks for reaching out to us. We will get back to you by tomorrow.

  • Thank you.

    I think there might be a problem using the LM74700-Q1 for charging port protection. If I have read the datasheet correctly the charge pump will not be active, so the MOSFET will be off, if the ANODE pin is not above the undervoltage lockout threshold. Since some chargers do not output a voltage until they detect a connected battery then the LM74700-Q1 will not keep the MOSFET turned on even if we connect EN to the CATHODE pin (where the battery pack is connected). There will be no voltage at the ANODE pin from the charger or from the battery (since the MOSFET will be off).

    If this is the case, is there a TI product that can provide protection from external short circuits and reversed charger polarity for a charge port and battery pack? At least a +65V/-65V rating is required.

  • Hi John,

    Please see my response below for your questions,

    Regrading Enable connection:

    In the case where the Enable is connected to Anode and charger does not provide any voltage until reading the battery voltage, 

    • Your idea of connecting a high value resistance from Cathode to Anode to bias the Anode is good. The down sides I see with this approach is  continuous  Quiescent current (as the device is enabled and has voltage on Anode) and continuous reverse current flow  when the battery polarity is connected in reverse orientation. During input reverse polarity the LM74700 will turn OFF the FET but reverse current can flow through the resistance. You can select the resistor based on the acceptable reverse current in your application.  
    • How about connecting a resistor from Cathode to point X as shown below. Point X needs to be connected to Anode only when the charger is plugged in. Can we design the connector in such a way that the point X is connected to Anode through mechanical connection of the charger. If this is possible, we can get rid of the continuous Quiescent current as the device Anode is floating until the charger is connected at input. 


    Regrading the Output Capacitance Requirement: 

    The output cap on LM74700 is required on the LM74700 only to keep the voltage on the CATHODE pin stable during transient events on the load. 

    The 47 uF mentioned in the datasheet is part of a design example and is not a definitive requirement. You need to select a output capacitance based on your testing. You need to check that the voltage on CATHODE pin is stable during all load conditions. As you mentioned, testing with 10 uF and 22 uF can be a good starting point. 

  • Hello Praveen,

    Thank you for your reply! I will contact the client regarding the use of a connector that can connect the resistor to the anode only when plugged in and we will test the chosen capacitance with appropriate load transients.