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TLV2402: TLV2402 as comparator

Intellectual 980 points

Replies: 20

Views: 420

Part Number: TLV2402

Dear Forum Members,

I started to evaluate a TLV2402 device and I get better results then it was expected referring to the slow rate.

In my configuration initially the 2402 output is at H level and driving a PNP transistor trough a 1KOhm (emitter connected to VDD). When certain event occures its output goes down to L adding base current to the PNP for opening whose collector rise to ~VDD.

Accoring to the datasheet SR=0,0025V/us, thus I would expect about 0,6V/SR=240us delay (PNP base potential should be below its emitter (VDD) pot. by 0,6V) but I measured only 28us by a two-channel scope in single mode

You may have some idea why is this huge difference. Maybe the '2402 has asymetric response time when operated in comparator mode and the L->H transition at its output takes much more time?

Thanks in advance!

BR,

Jospech

  • Hi Joseph,

    a simulation shows about 390µs.

    Can you give us a schematic and your scope plot?

    Kai

  • In reply to kai klaas69:

    Hi Kai,

    thank you for the response! I added pictures. Green waveform is the T2 collector signal. Yellow is the comp input signal: if T2 out gous to L, input signal disappears, it is normal operation of the circuit (over current detection->switching off current path).

    Thanks,

    Joseph

    scope:

  • In reply to Joseph82:

    Hello Joseph,

    Your input signal high is only about 28us which is far short for the op amp to respond. In addition to the slew rate, there is also an effect called "overload recovery time". This is the time for the op amp to leave the non-linear saturated VOL or VOH state that naturally occur when op amp is used as a comparator.

    The second problem is that the input pulses to nearly -1V; this violates the op amp common mode range and might violate the 10mA absolute maximum rating.  On page 13 of the data sheet, this is stated "The TLV2401/2/4 has a negative common-input range that exceeds ground by 100 mV. If the inputs are taken much below this, reduced open loop gain will be observed with the ultimate possibility of phase inversion."

    Is the goal of the circuit just to output low voltage during the pulse high time and ignore the negative after pulse?

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Joseph82:

    Hi Joseph,

    All of the points that Ron mentioned in his reply are discussed in detail in "Using Operational Amplifiers as Comparators".

    Thank you.

    Tim Claycomb

  • In reply to Joseph82:

    Joseph,

    For what it is worth, the TLV3702 micropower comparator is based on the TLV2402 amplifier core.

    So if you want to use the TLV2402 as a comparator...why not use the true comparator version of the TLV2402! It has the same pinout and will drop-in replace the TLV2402.

    The comparator will not have a "slew" time, so it will be faster (1us) than the op-amp and will not have the overload recovery time issue.

    Also note that these are micropower devices, and cannot sink/source much current. The TLV2402 current limit is 200uA!

    From your schematic, when the output is low, the output is sinking almost 6mA (7V - 0.8V) / 1k=6.2mA). That is well into the current limit region (see figure 14 of the TLV2402 datasheet). I'm surprised it's even working! Change R2 to 100k and see if it makes a difference.

    Regards,
    Paul Grohe
    TI Comparators (LAMPS-CMPS) Applications Group

  • In reply to Ron Michallick:

    Hi Ronald,

    I undertand the nature of the overload recovery time: it might be simliar to the phenomenon of the bipolar transistor saturation, insn't?. But I do not know from wich point the SR shall be considered too? Because in my case, it seems, the SR does not play any role (unitil the 2402 output does reach the VDD-0,6V).

    Why can the output reach this VDD-0,6 level so qickly despite the  very small SR value? Does SR has effect only lower output levels?

    The Yellow input signal pulse is high level however there is negative glitch at its falling edge:

    this occures when 2402 out falls to L switching off (large) current ->2402 input (current monitor) signal disappear. I think it is caused by stray inductance: i Will add a schottky to clamp the glitch. Thank you vm for this remark too!

    BR, Jospeh

  • In reply to Tim Claycomb:

    Hi Tim,  thanks vm for the nice article, it is useful for me. Especially the consideration of the integrated clamp diodes in case of bipolar amps.

    BR,

    Joseph

  • In reply to Paul Grohe:

    Hi Paul,

    I see, Becasue I need an operational amplifier too and I would like to use one package. And the device should have -Q1 qualification (this restriction highly reduce the component palette).

    Yes, I know the 2402 can deliver only 200uA and I wanted to activate current limit (to 200uA) to ensure largest base current to the transistor as possible.

    May the resultant 200uA current degrade the device? I Will increase the resistor to ensure only 100uA.

    BR,

    Joseph

  • In reply to Joseph82:

    Hi Joseph,

    I don't think that the TLV2402 is actually switching. But T2 could do. R1 and R3 are extremely high ohmic and could be sensitive to capacitive stray coupling. Or, by other words, other components and signals of your circuit could inject charge into the base of T2 via capacitive stray coupling and can make T2 turn-on. Or, there's some inductive ground current coupling in your circuit which can make T2 turn-on.

    Are you using a breadboard design or a setup using longer wires, poor ground connections or weak supply voltages? Parasitic capacitances and inductances are very common in such designs and result in distorted switching signals with heavy over- and undershots, just what your scope plot shows. Ensure, that you always have a proper power supply decoupling at each chip and switching transistor, that you always have a very proper signal ground (a solid ground plane is the best) and that you keep the circuit sufficiently low ohmic. Keep the individual circuit parts and modules compact but have enough distance between interfering cuircuit parts. Shield the interfering circuits parts against each other by connecting the individual shields to the local grounds. Avoid loop areas and keep the distance between each forward and return conductor minimal. 

    Kai

  • In reply to kai klaas69:

    Hi Kai,

    it is very interesing that you pointed. Yes, I measured the T2 collector not directly the TLV output. I Will check agin.

    I used high resistance values because orignally a timming capacitor was also used at T1 collector (fast charging and slow discharge by T2 circuitry - for delayed reset) however it not not working correctly, and not used at the moment.

    The circuit relaized on a single sided SMD PCB with short traces.

    Joseph

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