This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC27211: What is the minimum turn on time(bottom switch to ensure charge bootstrap capacitor ) and dead time of UCC27211 ?

Part Number: UCC27211

Hi  Everyone , 

I have observed and used UCC27211DDA high-low side gate driver for my 4 switch buck boost application. I will ask a basic and simple question for you. For my application minimum bottom switch turn on time very important for bootstrap capacitor charging time(minimum charging time) . Using minimum turn on time information , I will determine the duty cycle limitations .(like buck max duty does not exceed the 0.9, and boost min duty does not lower than 0.07 etc.) 

But in datasheet , there is no any information about the this time. How can i get this information ? Can you explain briefly please ? 

Another basic question that,  Dead band time is typically 2nsec, isn't it ?  This time was a little ? I expect greater than 100nsec .  

I'm looking for your responses and helps .

Manythanks. 

Burak 

  • Hi, Burak, Today is a holiday in the US. My colleague will get back with you tomorrow.
  • Hello Burak,

    Thanks for your interest in our drivers.

    In terms of the minimum on time for low-side, you want to make sure the boot cap has sufficient time to replenish so that during the high-side on-time, the HB_HS voltage does not fall below the UVLO threshold.

    As you know, the bootstrap will charge from the VDD capacitor through the boot diode and resistor to the low-side Rds,on of the low-side FET. The charging time of the boot capacitor can be approximated by the time constant t = Cboot * [ Rboot + Rds,on ]. 

    You will size the minimum low-side on-time to be slightly than this calculated value.

    As for the dead time, 2ns is too small depending on your application, and you're correct you want to have sufficient margin against shoot-through and choose your dead time according to your application requirements taking into account acceptable dead-time losses.

    Regards,

    -Mamadou  

  • Hi Mamadou ,

    Thank you for brief explanation .  In addition I want to ask one more think about Rboot  resistance .

    In ucc27211dda  does not include Rboot resistance . Isn't it? I have not seen  ın ucc27211dda configuration and electrical parameters .



    what is the Rboot resistance of UCC27211DDA ?  If I take  0 ?  

    The formula which you given  t= Cboot*(Rboot( taken 0) +Rds,on(mosfet)+diode on resistance(dynamic resistance typ=5 ohm , max=0.85 ohm)   This time it will be more accurate ?

    Manythanks for your helps.

    Burak

  • Hello Burak,

    Most gate drivers do not include boot resistance in the IC. This component is implemented externally and is generally optional but highly recommended. The boot diode on the other hand will commonly be included in the driver IC for board space and cost saving.

    As for the 2nd question, yes you will also take into account low-side FET Rd,son and diode dynamic characteristics.

    The link below might also be helpful.

    https://www.ti.com/lit/an/slua887/slua887.pdf

    Regards,

    -Mamadou