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LM5008MM/NOPB - continuous conduction vs discontinuous conduction

steve correll
Prodigy 30 points
Other Parts Discussed in Thread: LM5008

Good morning, I am trying to get a better understanding of this converter and I am hoping you may be able to provide me with some insight. 

My first question is in regards to the voltage requirements for the chip.  We have a circuit where this part is used, and the output of the step down regulator is sized to provide 12 V out.  Given that the chip is a step down regulator (as opposed to a step up), my interpretation is that the input voltage would need a minimum of 12V at the input.  Is this correct?  ...The reason I ask is that we are seeing the chip function down to 9 volts and I was wondering if powering the chip this low would stress the chip?  If I set my output to be 12V, is there a minimum input voltage that I should provide?

My goal is to get a better understanding of the chip, as the engineer who designed the circuit set the minimum input voltage as 30V, and I have a customer that would like to power it below 30V (possibly down to 12V).  I am trying to understand the chip better to determine why the engineer set the min input voltage at 30V.  I recognize that there is likely a very good reason why the floor was set to 30V, but I need to understand the part better in order to provide a rationale explanation as to why or why not the chip can be powered below 30V.  *The input voltage comes directly from an input power supply and only connects to this chip (not any other parts)

I also have further question in regards to the ripple voltage and the continuous conduction vs. discontinuous conduction modes of operation, but I will hold those questions for now until we get a dialogue started.

Thank you in advance for your help in understanding this part better.

Thanks,

Steve

  • 0
    TI__Intellectual 1435 points

    Hello Steve,

    The minimum input required to maintain constant output of a step-down regulator is determined by Vout and the maximum duty cycle of the regulator.  If the maximum duty cycle is 100%, then the Vin voltage can drop to 12V and still supply 12V at the output.  If the maximum duty cycle is 90%, then the minimum input for 12V output is approximately 12V/ 0.9 = 13.3V.

    The maximum duty cycle of the LM5008 is limited by the minimum off-time  and the programmed switching frequency set by the RT/SD pin resistor.    The minimum off time is specified to be 300ns typical in the LM5008 datasheet.   The worst case (maximum) off time could be approximately 400ns.   The switching frequency can be calculated from the datasheet equations if the you know the value of the RT resistor connected from VIN to RT/SD.    See equation 2 on page 7 of the datasheet.

    After calculating frequency F, the period of the switching frequency is T = 1/F

    The maximum duty cycle is     Dmax  =  (T- Toff)/T    where Toff is the minimum off time of 300ns typ, 400ns max.

    Finally, the minimum input voltage is     Vin-min =   Vout / Dmax

    I hope this helps you understand the step down regulators and the LM5008.

  • 0
    TI__Genius 11490 points

    Hi Steve,

    The part can work down to 9.5V without being stressed. The only caveat is that as the VIN decreases and approaches 12V, the output will start dropping below 12V. You have to decide if this is ok for your downstream circuits.