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LM348: Upgrading To Later Chip

Part Number: LM348

Hi,

I am currently working on an old design using a LM348N and I am looking to upgrade this chip. I understand this chip is based on the old UA741 which is not favoured because it does not swing close to zero.

I have a number of questions below:

1) The output of the comparator is to turn an N-FET gate ON/OFF.  Would you recommend i used a push-pull output for this, and would i have to use a pull-up or pull-down resistor?

2) Am i correct in remembering that it is not recommend to use an amplifier like the LM348N as a comparator, its better to use comparators in comparator appliactions?

3) My slightly modified requirements are below:

* Dual Package

* Comparator 

* Supply voltage -12V and +12V

* Output - push-pull i think as i want to turn an N-FET gate ON/OFF

4) I have tried to select a couple of comparators but i am not sure if these are correct as i am weak on op-amps:

* LM393M/NOPB

* LM2903M

Look forward to your replay and any input.

Thanks,

Rocketman46

  • Hello Rocketman,

    We do not have a push-pull comparator that operates on 24V, yet. This is probably why an op-amp was originally used.

    All of our >24V comparators are open collector output, which will require a pull-up resistor. The most common would be the LM393 family or the TLV1702.

    The advantage of the totem-pole (R-R) output is that it can source current to turn the gate on faster. Otherwise, the speed the gate can be driven going high will be at the mercy of the time constant of the gate capacitance and the pull-up resistor value. The going-low speed will be identical between totem-pole and open collector output since both have a transistor driving to V-. The totem pole output would allow symmetrical turn-on and turn-off times for the MOSFET.

    The LM393 family is the comparator version of the LM324 op-amp family. Luckily, the dual comparators and dual op-amps have the same pinout - so that would allow the most flexibility for upgrades/updates.

    At the end of August we will introducing a 40V push-pull output. Samples should be available at the end of July. I can get someone in contact with you if you are interested.

    We generally discourage the use of op-amps as comparators because there are issues with input clamps, especially on the high voltage precision devices.

    See page 8 of this presentation:

     https://training.ti.com/sites/default/files/docs/2104%20-%20Comparator%20Applications%204.pdf

    One particular high voltage op-amp that mentions it can be used as a comparator is the OPA192 (see page 24-25 of the datasheet).

    I would not recommend the op-amp LM358 family, as it will require a pull-down resistor to V- when driving high impedance loads (like a gate), due to it's "unique" output stage that will cause the output to float at 1V above V- if it is not sourcing enough current.

    Regards,

    Paul Grohe

    Low Power Amplifiers and Comparators (LPAC) Applications

  • In reply to Paul Grohe:

    Hi,

    Thanks for your reply.

    Below is what I am trying to achieve.  I think my comparator and op-amp selection is correct but I am not sure if my circuit design is correct.  Could you offer any design advice.

    The function of the circuit is:

    * if(POT Voltage > +0.25V) then ADC 1 is supplied POT voltage.

    * if(POT Voltage < -0.25V) then ADC 2 is supplied POT voltage.

    Thanks,

    Rocketman46

  • In reply to Rocketman46:

    Opps spotted a mistake on U2 wiring.  Here is a revised circuit:

  • In reply to Rocketman46:

    Hi Rocketman,

    I believe your design will work to control the fets to ADC1 and ADC2. Make sure you are not exceeding the Vgs limits of the FET as I see in the bottom implementation for ADC2 you are applying 10V while in the top implementation for ADC1 you are applying 5V. I also recommend creating a logic table based on your input conditions to ensure you are getting the intended output.

    Thanks,
    Jaskaran