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LME49724 Enable pin and quiescent current

Prodigy 50 points

Replies: 7

Views: 1257

Below part of an email trail between myself and TI support before they suggest I post on E2E which is not what I wanted but here goes:-
"I understand RS provide support but recently I find it useless - they simply attempt to interpret data sheets and have no real knowledge of the products they offer and more important are not aware of limitations of some parts - this is not helped when companies like Vishay and TI buy lines from others and then 're-badge' the data as I think is the case with the problem I have

A colleague placed the LME49724 in circuit making layout changes to replace an AD part that oscillated with usual precautions applied - We did not expect the EN input to draw enough current to pull down a 3.3V 0.5A supply line - I then built the circuit x2 as a simple unbalanced to balanced x1 gain amplifier and with a single 9V supply current limited at 100mA the EN pin draws over 55mA in this arrangement !  

The circuit modified with 10kR resistors in place of 1kR all round works as expected but takes more than 25mA with the EN pin left open - when EN voltage is increased the current rises sharply above 1V

So apart from the numerous meaningless graphs relating to noise and distortion the data sheet shows the wrong quiescent current and operation of the EN line - Do you think RS can help ??"


The answer was that RS are not a distributor for TI ???
The demo board circuit shows a link to connect EN to VEE only - The data sheet voltages would suggest EN can be driven by most logic outputs ref VEE but this does not appear to be the case unless using an open collector/drain etc. but then the data sheet also says EN must not be left floating?
At about 1V above VEE the EN input current  becomes excessive - connecting EN to VCC as shown in many of the sample layouts and demo circuits for modelling would appear to be a mistake - Also the Quiescent current in all the circuits we have built (not modelled) is about 25mA not 10mA typical 15mA max as stated on the data sheet
Has anyone else experienced the same problem ? 

7 Replies

  • Hi Keith,

    I think we haven't had an issue like this.
    Let me take a look at this. I'll come back soon.

    Best regards,
    -Ivan Salazar
    Texas Instruments
  • Hi Keith,

    This part LME49724 is being EOL'd. You can see it listed in this wiki entry.
    I would suggest you to take a look at its replacement part.

    Best regards,
    -Ivan Salazar
    Texas Instruments

  • In reply to Ivan Salazar:

    Ivan

    Not the answer anyone would expect to receive after dozens of emails to TI support and distributors

    Perhaps as member of TI you could explain why the part is being discontinued and why I had to come this far to be informed of such ?

    The OPA1632, which appears to be a suitable replacement albeit with unnecessary high GBW, data sheet states that the EN pin should or could be left open when not require but what about if we wanted to use this device in our configuration as previously described ?  we would of course not be using the low values of FB resistors the data suggests as we don't want the excess dissipation when using a single supply

    Can we rely on this data sheet or do we have to build and test an amplifier or 2 using the part to discover that it too may not work with a single supply with the EN pin driven from a logic gate ref. 0V

     

  • In reply to Keith Snook:

    OPA1632 has a pull up resistor to keep the device operating when the shutdown (pin 7) is not used. The description of the shutdown function is as follows:
    The shutdown (enable) function of the OPA1632 is referenced to the negative supply of the operational amplifier. A valid logic low (< 0.8 V above negative supply) applied to the enable pin (pin 7) disables the amplifier output. Voltages applied to pin 7 that are greater than 2 V above the negative supply place the amplifier output in an active state, and the device is enabled.
    The negative supply for a single supply application will be GND (0V). So a reference to 0V should be fine as long as the high state is above 2V.

    Best regards,
    -Ivan Salazar
    Texas Instruments
  • In reply to Ivan Salazar:

    Ivan

    I don't wish to appear rude but has anyone actually built a single supply amplifier using OPA1632 and connected say a 3.3V logic gate to control EN ref. to 0V ?

    The data sheet for the LME49724 reads similar to your last reply but in practice performs as I described in my early posts on the subject - I am surprised no one has tested this even if the part is to be discontinued - or maybe TI have and hence the OPA1632 to replace it

    Regards Keith

     

  • In reply to Keith Snook:

    Hi Keith,

    I'm not aware of any application driving OPA1632 EN pin with 3.3V, but according to D/S it should work as it is 2V above V-. Although EN pin is usually driven by Vcc/V+.
    Is this testing done with the device alone? or is it in a complete system? Can you share your schematic?

    Best regards,
    -Ivan Salazar
    Texas Instruments
  • In reply to Ivan Salazar:

    Ivan

    This is the general form of my single ended to push-push convertor used in a larger system - single supply Vcc 9V - DAC run from linear 3V3 regulator - EN pin would be controlled by 3V3 logic gate if this worked as the data suggests it should - open collector gate will drive EN okay or a logic gate with a limit resistor and parallel diode works - resistor values increased to reduce unnecessary dissipation.

    Regards Keith

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