• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 1297 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

LM2940 Discrepancy

Darral Teeples
Prodigy 20 points
Other Parts Discussed in Thread: LM317

I was looking at your Tech. Data Sheet from the Newark electronics site on the LM2940. The PDF # is 1882643.pdf.First, let me state my purpose for this 10v regulator. I am adding Li-Po batteries to a radio control transmitter that is usually powered by 8 nicad batteries- typ 9.6 volts. These 3 cell lipos can easily go over 12.5 v and I'd prefer a steady 10v to my transmitter. I thought I had found my answer with this regulator especially with the dropout voltage of 500mV or 1/2V. But then looking at your section 6.6 and the Input Voltage parameter, I see it has a MIN 11.5v!!  So how can this be when the specs state that the dropout voltage would be 10.5????


I'm not sure I follow the logic of the characteristics table and would appreciate ANY help there. Otherwise, if the iV was 10.5 min, I could live with that. My MAIN concern is, what happens with the differences. In other words, I turn on my transmitter and the voltage of my lipos are at 11.1??? It's below the MIN value. Will the regulator shut down? Will it regulate?


PLEASE inform and advise me here. It's very confusing. I will await your advice before trying to find a substitute OR purchasing this chip.

Sincerely, Darral Teeples

  • 0
    TI__Mastermind 22610 points

    For LM2940-10 the Recommended Operating Input Voltage minimum is 11.5V. From Vin= 11.5V to 26V the conditions with Min/Max limits are ensured.

    As Vin falls closer to Vout+Vdo, dynamic performance (PSRR, Line transient response, Load transient response, perhaps Load regulation) will begin to deteriorate and Quiescent current will increase.

    The regulator does not shutdown.

    As Vin fall lower, the regulator output voltage will follow Vin down as it falls, i.e. Vout= Vin-Vdo.

    The Vdo value is dependent on the output current and device temperature. The 500mV is a typical Vdo value that applies if the output current is at 1A and the device temperature is close to 25C.

    See "Figure 12. Low Voltage Behavior" in the LM2940 datasheet for the how the LM2940-10 output voltage behaves with respect to the input voltage.

  • 0 in reply to Donald Jones
    Prodigy 20 points
    First...if I'm reading you right, if Vin falls to say 10V, the Vout will be 9.5? (Vout = Vin - Vdo) I'm fine with the voltage falling, just as long as the chip doesnt shut down because of it to which you say it doesnt.

    One other question and this is concerning info I read on this chip from online:
    Your specs reads of using a "tantalum" type capacitor for the 22uF and not the "electrolytic" style aluminum cans etc. How critical is this capacitor? I noticed one Youtube clip where it was NOT regulating to spec until the cap was added. My experience is limited when it comes to the ESR's and the critical components. I've dealt with LM317's, LM7805's and dont remember reading these tight specs (cap requirements on those). Since I'm inputting 12.6v to drop to 10, and the circuit can technically go from 12v to 9, I'm not sure the ESR will have a big play here. Just not sure how the capacitors/chip will react. The operating temp of the inside the radio should be anywhere from 30 deg F to 100 max.

    Thank you.
  • 0 in reply to Darral Teeples
    TI__Mastermind 22610 points

    " ... if Vin falls to say 10V, the Vout will be 9.5? ... "

    That is correct.

    " ... as long as the chip doesnt shut down ..."

    It will not shutdown.

    LM2940 is PNP LDO output, the LM317/LM7085 are NPN Voltage regulators. Typical PNP LDO's have marginal stability due to phase shift in the PNP output, and require careful selection of the output capacitor.

    See (Sections 9, 10, and 11): AN-1148 Linear Regulators: Theory of Operation and Compensation

    The 22uF is the minimum output capacitance, you can go higher, you cannot go lower. ESR can be a bit sloppy as you have a 100 milli-ohm to 1 ohm range.

    The 100 milli-ohm minimum can be inversely scaled with the Cout value.

    If you would rather use Ceramic (MLCC), that's possible by simulating the required ESR with a discrete resistor.

    See : Ceramic Capacitors Replace Tantalum Capacitors in LDOs:

    When the input voltage is low enough that the regulator is operating in dropout mode, the ESR of Cout will not matter as the control loop will be at the rail, the PNP pass element will be saturated, and compensation for stability will be irrelevant.

  • 0 in reply to Donald Jones
    TI__Mastermind 22610 points
    The links appear to be sketchy, not always visible. Trying again ...

    AN-1148 Linear Regulators: Theory of Operation and Compensation
    ti.com/lit/an/snva020b/snva020b.pdf

    Ceramic Capacitors Replace Tantalum Capacitors in LDOs
    ti.com/lit/pdf/SLVA214A