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WEBENCH® Tools/LM2941C: LM2941C

SANTHOSH RAJAGOPALAN
Prodigy 80 points
Part Number: LM2941C
Other Parts Discussed in Thread: LM2941

Tool/software: WEBENCH® Design Tools

I have the following question regarding this LM2941CT Voltage Regulator:

 It has been specified in the datasheet of LM2941CT that the maximum dropout voltage is 1V.

Hence it is understood that if the Output has to be maintained at say x volts, the minimum input voltage should be x+1V.

My question is if the input voltage drops below x volts, what will be the output voltage? Since the input voltage is less than x volts in this case, of course output will no longer be x volts. My question here in this case (where input is less than the desired output voltage of "x volts") is what will be the exact value of output voltage? Will the output voltage become 0 volts in this case or will it drop to any other value?

 

Please help to clarify on this ASAP.

We need this information very urgently.

Thanks a lot in advance.

  • 0
    Intellectual 515 points

    No, the LM2941 output voltage does not suddenly drop to zero if the  Vin ≥ 1V+1V requirement is not met.

    The output voltage will generally become Vin-Vdo, where Vdo is primarily dependent on the load current (Iout), and the junction temperature (Tj). See Figures 1 and 2 in the datasheet.

    Be aware that if, or when, Vin falls below the Minimum Operating Voltage (6V) the LM2941 is outside the ensured operating range and the Vdo value will probably be considerably more than 1V.

  • 0 in reply to Daddio
    Prodigy 80 points
    Hi Daddio,
    Thanks for the explanation.
    I do have a follow-up question.
    Let us take some numerical example. My nominal voltage in the circuit is +15V. The desired output voltage is set through the R1 and R2 resistors (refer to datasheet for the resistors in the schematic) to be at 9V. Since Vref is typically 1.275V, R1 and R2 are accordingly designed so that the output voltage,Vo is 9V.

    Let us assume load is consuming maximum current and the junction temperture is high too. Based on the figure 1 and figure 2 in the datasheet, let us assume we are having the maximum dropout voltage of 0.7V.

    Hence, as long as input voltage,Vi is equal or greater than 9+0.7=9.7V, the output voltage can maintain 9V.

    If Vi drops below 9.7V, the output will also drop accordingly.

    Say if Vi is 8V;load is consuming maximum current and junction temperature is very high, Vo will be 8-0.7 = 7.3V?
    Is this correct?
    If yes, then i have another question. When Vi is greater than Vo+Vdo, Vo value followed our design on R1 and R2. Hence the desired Vo was maintained. But when Vi dropped below Vo+Vdo, Vo will simply equal Vin-Vdo? In this case still Vref will be 1.275V?

    Please clarify and double-confirm on this.
    Thanks a lot in advance.
  • 0 in reply to SANTHOSH RAJAGOPALAN
    Intellectual 515 points

    Note that the ensured maximum dropout voltage at 1A, is 1.0V. 

    If Vin =15V, Vout = 9V, and Iout = 1A, then Pdiss= ((15V - 9V) x 1A)= 6W. Then, if Rθja= 41C/W (i.e. TO-263) and Ta= 25C, the junction temperature wants to be 271C (i.e. Tj = (Pdiss x Rθja) + Ta) ).

    Of course, the LM2941 junction temperature will never get that hot as the thermal shutdown circuitry will activate, the output will be turned off, and Vout will go to zero until the junction temperature cools.

    "... Say if Vi is 8V;load is consuming maximum current and junction temperature is very high, Vo will be 8-0.7 = 7.3V? Is this correct? ..."

    In general, yes.

    "... But when Vi dropped below Vo+Vdo, Vo will simply equal Vin-Vdo? ..."

    If Vin= 8V, and Iout= 1A, then this is outside the ensured operating range, and the general rule is that Vout= Vin-Vdo.

    "... In this case still Vref will be 1.275V? ..."

    The voltage developed by R1/R2, seen at the ADJ pin, will fall accordingly with Vout (Ohms Law), but the internal band-gap reference voltage will not be affected until Vin is less than 6V. See the Functional Block Diagram to see how the sensed ADJ pin voltage and the band-gap reference voltage establish Vout with the control loop.