power consumption of comparator, LMV7239

Hello,

Our customer want to know the power consumption of LMV7239 with input signal frequency has variation.

They think that it has lower power consumption when lower frequency signal inputs.

If the thought  is right, do you have some data or tendency for LMV7239?

(And they are examining with the input frequency from 100Hz to 1MHz)

Thanks,

H.Sato

  • Hello Hiroshi,

    Yes. The supply current will increase with the toggle frequency.

    Comparators draw peak currents during the time the output transitions. This is caused by the driving of the internal capacitance of the output stage gates.

    Remember:  i = C * dv / dt

    These peaks can be as high as 1000% or more of the quiescent current and lasts the duration of the output slew time. This is commonly called comparator "shoot-through current".

    These current peaks will be integrated by the supply bypass capacitor and will result in a rise in the average DC supply current.

    (This is why even a moderate speed comparator must be treated as a high speed device with proper supply bypassing)

    The LMV7239 has a 1.7ns risetime - so the output will transition in about 2ns. This means there will be 2ns peak current pulses on the supplies for every output transition (high to low and low to high). The rise in supply current will be proportional to the duty cycle of the shoot-through peak currents to the input signal rep-rate.

    However, The output capacitive load will have the greatest effect on the supply current. Slewing 5V in 2ns into several pF requires milliamps of peak current - which the output must supply. The more "C", the more current required.

    I already had a 7239 in the bench from a previous case...so I was able to do a quick measurement:

    At Vs=5V, Vin=0-5V 50% square wave, 2.5V Vref:

    Freq (Hz) Is (uA)
    No Load
    Is (uA)
    w/20pF Load
    100 71.5 71.5
    1K 71.7 71.7
    10K 72.3 73.6
    100K 78.3 96.0
    1MEG 139 167

    The shoot-through current followed the output slewing period, as expected, and was about 2ns wide.

    The un-loaded shoot-through current spike was about 11.3mA. Loaded with 20pF it was 31mA peak over about 5ns.

     Again, this was a very crude test on a ring-gy protoboard setup - so I am not expecting these numbers to agree perfectly with theory - but they are close.

    Regards,

    Paul Grohe

    SVA Precision Applications

  • In reply to Paul Grohe:

    Hello Paul-san,

    Thnak you very much for your comment and taking data.

    These are very useful for us, then the customer will be able to decide to evaluate the device.

     

    Thanks and best regards,

    Hiroshi Sato