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TPS40304 MOSFET selection

Shareef
Expert 1985 points
Other Parts Discussed in Thread: TPS40304, TPS40075, TPS40303

Hi,

I require a simple clarification, for a design based on TPS40304 it is recommended to use MOSFETs from NexFET. Can I select MOSFETs other than NexFET which are closely matching in features and verify the new design in SwitcherPro?

Also can my minimum duty cycle of TPS40304 go down to 6% as mentioned in the datasheet?

Regards,

Khader Shareef

  • 0
    TI__Genius 17240 points

    Hi Khader,

    Please find my comments below.

    I require a simple clarification, for a design based on TPS40304 it is recommended to use MOSFETs from NexFET. Can I select MOSFETs other than NexFET which are closely matching in features and verify the new design in SwitcherPro?

    Yes, you can.

    Also can my minimum duty cycle of TPS40304 go down to 6% as mentioned in the datasheet?

    Yes, you can.

    Regards,

    Na

  • 0 in reply to Na Kong
    Expert 1985 points

    Hi Na Kong,

    Thank you very much for your response. Your comments are really helpful to me, since I am doing TPS40304 based buck converter design now, One more doubt is that can the MOSFET selected for High-side be used for Low-side also?

    regards,

    Khader Shareef  

     

  • 0 in reply to Shareef
    TI__Genius 17240 points

    Hi Khader,

    For MOSFET selection, you may refer to the guidlines given in TPS40075's datasheet, which is general design considerations for synchronous buck converters.

    For low-side FET, conduction loss dominates and switching loss is minimal due to the conduction of its body diode, which is different from high-side FET. Moreover, for the application with low-duty cycle, low-side FET conducts for much longer time than high-side FET, so RMS current is higher (sqrt(D) vs sqrt(1-D)). Therefore, FETs with smaller Rds,on are more favorable for low-side FET.

    Regards,

    Na

  • 0 in reply to Na Kong
    Expert 1985 points

    Hi Na Kong,

    Thank you for the detailed response. One last question, in my design I am generating two supplies 3.3V and 0.8V. As per the power sequencing requirement, 3.3V must up come first and then 0.8V should be up. Since my 3.3V generating SMPS does not have a Power Good signal output, can I do the following?

    Connect the EN pin of TPS40304 to a resistor (R2) pulled down to Ground, so that my TPS40304 is disabled by default. The same EN pin is connected to R1 whose other leg is pulled to 3.3V forming a Resistor divider circuit. The values of R1/R2 divider circuit is chosen so that when 3.3V is present, VEN is within (0.7V - 1.0V), satisfying the VIH range for the EN pin of TPS40304. Is this fine or do you foresee any issues with this?

    Thanks & Regards,

    Khader Shareef

  • 0 in reply to Shareef
    TI__Genius 17240 points

    Hi Khader,

    The enable functionality and startup sequency can be found on page 10 of the datasheet. After the calibration time, the voltage on EN/SS is pulled down to 0.4V by an internal current source of 140uA. If there is another external source, i.e. 3.3V for your case, trying to pull up this pin, there will be a racing condition and may cause the part not working properly or even damage.

    I would suggest use open-drain configuration as shown in the figure on the first page of the datasheet to control EN/SS pin. Leave EN/SS floating, the part will turn on.

    Regards,

    Na

  • 0 in reply to Na Kong
    Expert 1985 points

    Hi Na,

    Thank you very much for the clarification. I will use the recommended open-drain configuration circuit to control EN/SS pin as suggested, just curious to know what if I reduce the pull up current from 3.3V to say less 0.5 uA by using a very high pull up resistance (of the order of MOhms), then my dominant current will be internal current source of 140 uA?

    Regards,

    Khader Shareef I

     

  • 0 in reply to Shareef
    TI__Genius 17240 points

    Hi Khader,

    If the pull-up current is very small, I think the internal current source will dominant. However, the problem of using very high resistance ( several Mohms) is the sensitivity to noise. Any leakage current may cause large voltage variation on EN/SS. Another concern is that EN/SS is also used as a second non-inverting input to the error amplifier after an 800-mV downward level-shifter. When the soft-start is iniitiated after the calibration, the voltage on EN/SS is supposed to be charged up gradually to control the smooth rise of output voltage. If there is another charging current by external source, it needs to be taken into consideration for the startup timing calculation. Similarly, this will affect the dummy soft-start timeouts in the hiccup mode after overcurrent fault.

    Regards,

    Na

  • 0 in reply to Na Kong
    Prodigy 10 points

    Hi,


    I have a question related to the soft-start on TPS40303: How long is the soft-start timeout?


    Thanks for your help.

    Behzad