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UCC21520: application questions

Brian Wang0928
Genius 14020 points
Part Number: UCC21520
Other Parts Discussed in Thread: UCC21521CEVM-286, UCC21530EVM-286,

1. Is DT pin able to left open? Or is it better to connect to VCC or GND if it's no used. is RDT or CDT necessary? 

If it can not left open, what's the reason?

2. How to design the RIN resistor value for INx? Is this RIN resistor able to affect the sinking/souring current of the output x rail?

3. How to well design the RON resistor value?

4. In the application circuitry of the d/s, there is a zener diode of the 5.1V. what's this purpose? Is it necessary?

  • +1
    TI__Expert 3575 points

    Hi Brian, 

    Thank you very much for reaching out. 

    1. Is DT pin able to left open? Or is it better to connect to VCC or GND if it's no used. is RDT or CDT necessary? If it can not left open, what's the reason?

    - It is not recommended to leave DT pin floating as noise could couple into this pin and changing the behavior of the device. RDT is used when the dead time of the device is being adjusted. CDT is recommended to achieve higher noise immunity. 

    2. How to design the RIN resistor value for INx? Is this RIN resistor able to affect the sinking/souring current of the output x rail?

    - You should use an Rin value between 0-100ohms and Cin between 10pf-100pf. 

    3. How to well design the RON resistor value?

    - For the design of Ron take a look at page 29 of the user's guide. The design of these value is very dependent upon application. You must ensure that you are designing for the required drive strength for your system. Section 9.2.2.3 provides examples with the equations for this value selection. 

    4. In the application circuitry of the d/s, there is a zener diode of the 5.1V. what's this purpose? Is it necessary?

    - Could you provide more information about what specific zener are you referring. If you assumption is correct, you are referring to the negative bias voltage application. Some power switches have a Vth very low and application with high di/dt, dv/dt could create a ringing that would go above Vth. As a result, the power switches could be turned On unintentionally. Using a negative Bias would ensure that the voltage is kept below Vth. 

    Let us know if this answers all your questions. 

    Sincerely, 

    Francisco Lauzurique 

  • 0 in reply to Francisco Lauzurique
    TI__Genius 14020 points

    Two more questions here.

    1. Is it ok that DT pin connected to VCCI pin without Resistor? Or it can be connected together directly>

    2.  For the INA/INB filter design, customer tried to test the RIN=499-Ohm and Cin=1000pF-Ohm in solar inverter system. The spike voltage at INA/INB pin could be better and having much voltage rating. Is this design ok or not? Any side effects with this RC combination?

  • +1 in reply to Brian Wang0928
    TI__Genius 13445 points

    Brian,

    1. DT may be tied to VCCI to allow outputs overlap according pin function section in d/s.

    2. THis is not recommended according to Section 9.2.2.1 of the datasheet. You're significantly delaying the dv/dt of the input signal which the input stage is expecting fast slew rate due to TTL type structure.

    My colleague Francisco is reviewing your latest files and will get back to you promptly.

    Regards,

    -Mamadou 

  • 0 in reply to Mamadou Diallo
    TI__Genius 14020 points

    As d/s recommendation, the RIN is recommended connected to INA/INB pin in series. However, in the below ref. design, i see there is a resistor connected to INA/INB pin and GND pin. I'm confused with both. Could you kindly comment on this! thanks. 

    User guide UCC21520EVM-286, UCC20520EVM- 286, UCC21521CEVM-286, and UCC21530EVM-286 Guide (Rev. B) Page 13

    https://www.ti.com/lit/ug/sluubg8b/sluubg8b.pdf?ts=1620610067613&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FUCC21520EVM-286
  • 0 in reply to Brian Wang0928
    TI__Expert 3575 points

    Hi Brian, 

    R4 and R5 resistors can be use as a voltage divider or a pull down resistor. For this design as you can notice on the BOM, these two resistors are not populated. This is to provide more flexibility to the EVM user on what to do with the input voltage and hot to drive the device. If interested on a pull down, I will not recommend using a low value but more on the order of 10Kohms. 

    Thank you very much for reaching out and I hope this answers your questions. 

    Sincerely, 

    Francisco Lauzurique