ATL431: Voltage comparator output lags input voltage near threshold

Part Number: ATL431

Hello

 My design is using an ATL431B in open loop, as voltage comparator, similar to Fig 30 of the datasheet. As noted in both the datasheet and TI application note slva987a, the output voltage has a small deviation from Vsup as Vin approaches the reference voltage (ie. 2.495). This behavior is shown on Figure 2 of the application report, this image:

Later in the application note, pg7, it is stated that 'roll off' of Vka begins as much as 1V below comparator threshold. This is shown in more detail on figure 8 of the application note.

I am seeing this behavior in my circuit, and would like to know more about how to estimate & reduce the magnitude of this effect by adjusting the circuit components if possible. The circuit is using the ATL431B.

Thank you

  • To be clear, what I am observing is Vout does not track Vin,

    For example, measurements from the circuit above:

    Vin = 4V, Vcomp=0.75V, and Vout = 3.73 (Vout-Vin = 0.3V)

    Vin = 8V, Vcomp=1.5V, and Vout = 7.41 (Vout-Vin = 0.6V)

    Vin = 12V, Vcomp=2.25V, Vouut = 10.97 (Vout-Vin=1V)

    What can be done to reduce  (Vout-Vin) ?

  • Hello Bradley,

    You might try using a smaller resistor value for R4. ATL431 has the sharpest switching gain but as the device starts to conduct, even the small 10-20uA change in current draw through R4=33k can cause a 0.33-0.66V voltage drop. Can you also try removing R5 and R6? They are causing more current draw and voltage drop across R4.

    Thanks,

    Walter

  • Hi Walter,

    After I collected the Vin & Vout measurements, I did notice the linear falloff, which is not like the datasheet graph. The results I measured closely follow expectation according to the divider network (R5+R6), especially well below threshold. For example at 8V input, Voutput would be 7.495, and 7.41 was measured. I will adjust the network and re-measure later today -- I expect this will bring Vout much closer to Vin, which will be suitable for this circuit.