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TINA/Spice/LMP8602: LMP8602 - SPICE Model

Part Number: LMP8602
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hello,

 I´m using the LMP8602 SPICE model in LT-SPICE. The model works but there is an issue.

I have made a check-simulation (see attachment):

When all current sense inputs at GND or the same voltage potential the current sense Output has an offset of 74,6mV.

 What is wrong?

Thanks for your help. Regard Chris

  • Hey Christian,

    Thanks for you interest and reaching out on the forum. I don't think anything is wrong actually with this model. The output simulating to 74.6mV is possible and reasonable. The output values only decreased to their low-output swing (VOL) voltage values when I applied a large negative voltage at the inputs.

    The inputs to the first stage (gain1 = 10) have a maximum specified offset voltage (Vos) of 1mV. Additionally, the output swing to GND (VOL1) for first stage (measured at A1 pin) is 2mV. Thus, the voltage at A1/A2 can be calculated with:

    A1 = Vos1*gain1 + VOL1

    A1 =  1mV*10 + 2mV     = 12mV   --> Matches simulation

    A1/A2 drives the second amplifier stage (gain2 = 5) which has Vos2 = 2mV maximum. Additionally, the second stage VOL2 = 10mV typically and this was measured by over driving IN+ and IN- (see note 8 on page 6 of datahseet) so you do not need to include VOL1 if you want to calculate Vout. So the possible maximum Vout is calculated with:

    A2 = Vos1*gain1 = 1mV*10 = 10mV

    Vos2 = 2mV

    VOL2 = 10mV

    Vout = VOL2 + (A2+Vos2)*gain2

    Vout = 10mV + (10mV+2mV)*5

    Vout = 70mV                                 ---> Pretty close to simulation

    You can see the 10mV output offset go away if you provide a mid-supply reference to the part. See sim below where Vout2 = 1.713V = 1.65V +62.8mV.

    More importantly among all of this is the VOL specifications are usually done by driving the input with a negative voltage. If you do this with the model you will see the output swing to their respective VOL values.

    Hope this helps.

    Sincerely,

    Peter Iliya

    Current Sensing Applications

  • Hallo Peter,

    with your answer, I see the issue. In the model is a mix of typ. and max. values.

    Normally I would like to simulate with typ. values. For worst-case simulation, some models have a pin to set the max. values.

    In the data sheet screenshot you can see the marked values your calculation.

    Either the model use always max. values or typ. values but not a mix.

    A1 = Vos1*gain1 + VOL1

    A1_typ =  0.15mV*10 + 2mV     =    2mV        --- typ

    A1_MAX= 1mV*10.05 +10mV    = 21.05mV --- max

    A1_sim=  1mV*10    + 2mV     = 12mV        --> simulation

     

     

    Vout        = VOL2 + (A1     +  Vos2) * gain2

    Vout_typ = 10mV + (2mV + 0.5mV) *5.000         = 22.5mV     --- typ

    Vout_MAX= 40mV + (21.05mV + 2mV ) *5.025 = 155.8mV   --- max

    Vout_ sim= 10mV + (10mV + 2mV ) *5             =70mV         --> simulation

     Could you adjust the model to typ. or max. values?

  • Hey Christian,

    The model is definitely using the typical VOL values. The maximum VOL values (boldfaced) are specified over temperature. You can see this more clearly in the current datasheet version here :

    From the simulations we have done it would seem that the model is using the maximum Vos values. Note the maximum Vos values are not specified over temperature. Unless you running simulations with temperature included, then you should not have to worry about max VOL values.

    I will also say that if you want to calculate the typical and maximum Vout values, you do not include the VOL of first stage (preamplifier stage) since it is already included in the "A2 Output Voltage Swing" spec. This is noted in datasheet. So the typical and maximum Vout values are:

    Vout = VOL2 + ( (A1-VOL1) + Vos2)*gain2

    Vout = VOL2 + ( (Vos1*gain1) + Vos2)*gain2

    Vout_typ = 10mV + (0.15mV*10 + 0.5mV)*5                     = 20 mV, typ at room temperature

    Vout_max = 10mV + (1mV*10.05     +2mV ) *5.025         = 70.55 mV, max at room temperature

    Anyway, I will note down that this model is acting peculiar. If you want, you can try messing around with the internal parameters of the model. Currently, it looks likes like it has the VOL2 set to 4mV instead of 10mV (as seen in previous simulation) and with screen shot of model netlists below. The model also looks like it is using typical Vos values already.

    Sincerely,

    Peter Iliya

    Current Sensing Applications