LM74502H-Q1: Turn on requirements

Tom Rust

Part Number: LM74502H-Q1

I'm using a 74502H in a back to back mosfet switching application to connect inverters to batteries. VS is typically 50-56V.

I have the EN at 3V, so it should be enabled, but I am not seeing the 12V differential voltage across VCAP, and the src and gate pins are both low.

Pins 2 and 7 are grounded - no OV is used.

How do I get the charge pump to turn on?

7 months ago

0 Shiven Dhir 7 months ago

TI__Mastermind 21190 points

Hi Tom,

Welcome to E2E.

Can you share the FET part number or its CIss and the charge pump cap being used?

Can you share schematic and waveforms during EN transition?

Regards,

Shiven Dhir

0 Tom Rust 7 months ago in reply to Shiven Dhir

Prodigy 30 points

I think I found the issue. cap inserted in wrong polarity. + should be towards VCAP, correct?

See schematic.

https://photos.app.goo.gl/6g8P8reoFWUNyXPm8

0 Shiven Dhir 7 months ago in reply to Tom Rust

TI__Mastermind 21190 points

Hi Tom,

Usually, we use non-polarized capacitor for charge pump. I think even the schematic has a non-polarized capacitor.

Do you have any waveforms of the startup sequence?

Regards,

Shiven Dhir

0 Tom Rust 7 months ago in reply to Shiven Dhir

Prodigy 30 points

Im using a fairly large value, so its polarized. There are 8 fairly high capacitance devices attached. I assume from the datasheet that the waveform when generating is positive towards the VCAP pin - 12V.

The first test device had the polarity reversed, and I saw some black around the device - it appears to have blown out from reverse polarity. Please let me know which is the correct polarity.

0 Shiven Dhir 7 months ago in reply to Tom Rust

TI__Mastermind 21190 points

Hi Tom,

Charge pump is driven with respect to VS pin, VCAP will have ~ 12V over VS pin during normal operation. So positive side of cap should be kept at VCAP.

Regards,

Shiven Dhir

0 Tom Rust 7 months ago in reply to Shiven Dhir

Prodigy 30 points

had the caps replaced with corrected polarity. Now seems to work! Switching properly so far in testing!

0 Shiven Dhir 7 months ago in reply to Tom Rust

TI__Mastermind 21190 points

Hi Tom,

Good to hear that it works now.

Regards,

Shiven Dhir

0 Tom Rust 6 months ago in reply to Shiven Dhir

Prodigy 30 points

We ran several tests with the first working prototype and all seemed good. We were able to send current in or out & switch on and off successfully. We then switched to another test BMS, which is an upgraded version of our BMS. But when we plugged in the ground connection, there was a bang and the 74502H part was popped. Nothing else seems damaged. How can I change design to prevent this? I'm assuming there was a transient or too high current on Vs. Will adding a 10k series resistor and a zener prevent damage on power up? See attached from datasheet.

photos.app.goo.gl/Ds6bL9rFNjvWnBtF7

0 Shiven Dhir 6 months ago in reply to Tom Rust

TI__Mastermind 21190 points

Hi Tom,

Does the damage happen only when you hot plug? During hot plug, due to transients abs-max might be violated. Adding zener and resistor can help.

Regards,

Shiven Dhir

0 Tom Rust 6 months ago in reply to Shiven Dhir

Prodigy 30 points

Yes. The first time installed it went fine. But connection to battery + was done last. This time the - was connected last. Not sure why this would have made a difference, but seems like we need to protect the 74502H. Im going ahead with adding the 10k R and 62V zener - any other suggestions?

0 Shiven Dhir 6 months ago in reply to Tom Rust

TI__Mastermind 21190 points

Hi Tom,

I am unsure why + and - side will have difference. Go ahead with R and Zener and lets test. If it does not resolve, we can dive deeper.

Regards,

Shiven Dhir

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LM74502H-Q1: Turn on requirements