UCC5880-Q1: Any limitation on nFLT pin cap?

Scarlett Cao

Part Number: UCC5880-Q1

Hi team

In customer design, they have multiple caps at nFLT pin up to 5nF, much higher than our recommendation. During fault condition, the nFLT is quickly pull low which cause larger discharge current. What is the I/O internal MOSFET structure and tr spec? Is there any risk they have many caps at the port and what's the limitation for cap value?

Thank you

Scarlett

over 1 year ago

0 Akshat Murali over 1 year ago

TI__Expert 3735 points

Hi Scarlett,

Too much capacitance will add a delay, since each cap will need to charge up before reaching the pin. We recommend a 100Ohm and 100pF cap on the nFLT pins. I would also recommend to add 10KOhm resistor pull up to VCC1. Looks like they have a much smaller pull up resistor.

Can you share what the large discharge current value is? Also what is the purpose of adding so many capacitors?

Regards,

Akshat

0 guozhu Lyu over 1 year ago

Prodigy 100 points

hello，Murali;

Why we add so many capacitors is the drivers are far from the MCU and on another board, to avoid the drivers be affected.

Your suggestion are100R in series from MCU to nFLT pins, and 100pF near the nFLT pins, and the pull up resistor is 10K. In our design, besides the delay, any other disadvantages about these three values?

Our concern is that, the large capacitors need to discharge with the internal MOSFET in nFLT pins. We not sure it will damage the MOSFET or not, pls help to check above, many thanks.

0 Akshat Murali over 1 year ago in reply to guozhu Lyu

TI__Expert 3735 points

Hello Guozhu,

Understood, thank you for the explanation. It is important in that case to have higher capacitance, since the board is far away from the MCU. Since the capacitance is still pretty small (5nF), it should not have any problems and damage any internal MOSFETs.

Regards,

Akshat

0 guozhu Lyu over 1 year ago in reply to Akshat Murali

Prodigy 100 points

Hello, Akshat;

how about the value of R0808 and R7214 in our design, they are different from your suggestions, if the value is not accepet, and why?

0 Akshat Murali over 1 year ago in reply to guozhu Lyu

TI__Expert 3735 points

Hello Guozhu,

3.16K is okay for the pullup resistor, but 10K is recommended, since it is a weaker pullup. The nFLT pins also have an internal pullup so It gives a good balance of timing and power.

R7214 is for the RC filter of the nFLT1 pin to reduce the noise. By changing the value to something like 4.64K, you are changing the cutoff frequency. If any thing, I would suggest to check this FAQ out for input filter design: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/944837/faq-ucc21520-what-should-i-consider-for-input-filtering-design-when-using-the-ucc2122x-ucc2152x-ucc2154x-dual-channel-gate-drivers

Regards,

Akshat

0 guozhu Lyu over 1 year ago in reply to Akshat Murali

Prodigy 100 points

Understood, thanks, Akshat.

Guozhu

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UCC5880-Q1: Any limitation on nFLT pin cap?