LM74700-Q1: Enable pin is not disabling the Mosfet Gate

Amir Eshraghi

Part Number: LM74700-Q1
Other Parts Discussed in Thread: LM5060

I am using LM74700 on a 12v battery to protect from reverse polarity and turn of the battery when the battery is low or if power is plugged.

my problem is when enable pin is zero the Gate voltage does not change so the Mosfet still is on and battery does not disconnect.

Below is the schematic

over 5 years ago

0 Karikalan Selvaraj over 5 years ago

TI__Genius 16305 points

Hello Amir,

Are you facing this issue in simulation or testing on the EVM or on your test board?

I am attaching the Enable and disable behavior of LM7400-Q1 showing Gate turning off (Gate-Source = 0V), note gate is not pulled to GND to turn off the MOSFET. Instead Vgs of MOSFET is made 0V by pulling gate to source (anode pin). Also note that, though LM74700-Q1 turns off the MOSFET, body diode would still conduct in forward direction, will block any reverse current.

Regards,

Kari.

0 Amir Eshraghi over 5 years ago in reply to Karikalan Selvaraj

Expert 1260 points

Thanks for the response. This is actual board that I designed you can see below PCB view.

Below you can see the VGS does not go to zero and Mosfet still is on

0 Karikalan Selvaraj over 5 years ago in reply to Amir Eshraghi

TI__Genius 16305 points

When disabled using EN pin (low), MOSFET is turned off by shorting Gate pin to Anode pin (source of MOSFET).

Eventually Vgs of the MOSFET is 0V and the MOSFET remains off.

Gate voltage will be same as the anode voltage when turned off, Vgs =0V.

This is observed in your waveforms, no issues here as it is the expected behavior.

Regards,

Kari.

0 Amir Eshraghi over 5 years ago in reply to Karikalan Selvaraj

Expert 1260 points

Thanks for the quick replay. So I understand the mosfet suppose to be turn off because VGS will be zero but in my case mosfet remain on and does not turn off the battery

0 Karikalan Selvaraj over 5 years ago in reply to Amir Eshraghi

TI__Genius 16305 points

Hello Amir,

MOSFET is OFF, but body diode of the MOSFET will conduct and power path remain ON.

Regards,

Kari.

0 Amir Eshraghi over 5 years ago in reply to Karikalan Selvaraj

Expert 1260 points

Hi Kari,

Thank you a lot for your support, So this is a wrong chip for my purpose since I need something to turn off the battery power when Enable trigger it. So i can turn of the unit when the battery is low. (Also protect from reverse polarity)

Also can you let me know what is purpose of enable in this case.

Thank you

0 Karikalan Selvaraj over 5 years ago in reply to Amir Eshraghi

TI__Genius 16305 points

Can you please have a look at LM76202 device which is an integrated ideal diode that can cut off the power path when disabled.

But this is limited to 2A.

LM5060 also can do the job of reverse polarity protection and cutoff power path, but this can not do reverse current blocking.

Please refer to this device as well if you need support for current >2A..

Regarding EN pin of LM74700, pulling the EN pin puts the device in low Iq mode and turns off the MOSFET.

Let us know if you require further info.

Regards,

Kari.

0 Amir Eshraghi over 5 years ago in reply to Karikalan Selvaraj

Expert 1260 points

Hi,

Unfortunately my circuit is pulling around 3A and can not use LM76202

Thank you

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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LM74700-Q1: Enable pin is not disabling the Mosfet Gate