Author Topic: Couple of questions about UCC FET drivers  (Read 1118 times)

Offline LoOdaK

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Couple of questions about UCC FET drivers
« on: December 09, 2022, 10:19:10 PM »
Hi all, I have a couple of questions about UCC MOSFET drivers, mainly about UCC27425.

I was reading through the datasheet of the following driver and I'm a little bit confused about certain things.

First of all, these chips have enabled pins. The datasheet states that those pins are internally pulled up with 100k resistors. While looking at some SSTC schematics that use 27425 as a driver, they tie interrupter output to those enable pins. If we do that, we can see from the datasheet that the enable pins are enabled by default if interrupter does not send any signal. Does that mean that 27425 will be running continuously ? If that is the case, how we can prevent this ?

The "solution" that came to my mind is that I could use a pull-down resistor from the enable pins to the ground. Looking at the block diagram, that would create a voltage divider, Am I right ? If so, would that actually work at all ?  Based on the voltage divider calculator online, we could drop-in something like a 500-ohm resistor as a pull-down and we could create 0.06V (Assuming 12V UCC voltage). Does that make any sense ?

Datasheet link: https://www.ti.com/lit/ds/symlink/ucc27423.pdf?ts=1670586039168&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC27423

Offline davekni

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Re: Couple of questions about UCC FET drivers
« Reply #1 on: December 11, 2022, 12:57:07 AM »
Quote
First of all, these chips have enabled pins. The datasheet states that those pins are internally pulled up with 100k resistors. While looking at some SSTC schematics that use 27425 as a driver, they tie interrupter output to those enable pins. If we do that, we can see from the datasheet that the enable pins are enabled by default if interrupter does not send any signal. Does that mean that 27425 will be running continuously ? If that is the case, how we can prevent this ?
If you make no connection to UCC27425 enable pins, yes, it will run continuously.  However, most interrupter outputs send a logic high and low, never high-impedance (tristate).  This avoids continuous operation.
David Knierim

Offline LoOdaK

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Re: Couple of questions about UCC FET drivers
« Reply #2 on: December 11, 2022, 05:15:26 PM »
If you make no connection to UCC27425 enable pins, yes, it will run continuously.  However, most interrupter outputs send a logic high and low, never high-impedance (tristate).  This avoids continuous operation.
Thanks for the answer,
but if we assume that the interrupter doesn't output logic low for whatever reason, are pull-down resistors a good solution ? Does it matter if pull-down resistors remain in the circuit ?

Offline davekni

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Re: Couple of questions about UCC FET drivers
« Reply #3 on: December 11, 2022, 10:31:35 PM »
Quote
but if we assume that the interrupter doesn't output logic low for whatever reason, are pull-down resistors a good solution ? Does it matter if pull-down resistors remain in the circuit ?
Yes, a pull-down resistor will work in case of interrupter outputting high-Z or disconnected.  Can be left in the circuit as long as the interrupter can drive that resistance to a logic high.
David Knierim

Offline LoOdaK

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Re: Couple of questions about UCC FET drivers
« Reply #4 on: December 15, 2022, 10:19:27 AM »
Thanks a lot for the answer

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Re: Couple of questions about UCC FET drivers
« Reply #4 on: December 15, 2022, 10:19:27 AM »

 


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