LM56_15 Datasheet, PDF (9/22 Pages) Texas Instruments – LM56 Dual Output Low Power Thermostat

LM56
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SNIS120G – APRIL 2000 – REVISED FEBRUARY 2013
1. V_TPE = ±V_T1E − V_TSE + V_RE
Where:
2. V_T1E = ±8 mV (max)
3. V_TSE = (6.20 mV/°C) x (±3°C) = ±18.6 mV
4. V_RE = 1.250V x (±0.01) R2/(R1 + R2)
Using Equations from Figure 1.
V_T1= 1.25V x R2/(R1 + R2) = 6.20 mV/°C)(82°C) + 395 mV
Solving for R2/(R1 + R2) = 0.7227
then,
5. V_RE = 1.250V x (±0.01) R2/(R1 + R2) = (0.0125) x (0.7227) = ±9.03 mV
The individual errors do not add algebraically because, the odds of all the errors being at their extremes are rare.
This is proven by the fact the specification for the trip point accuracy stated in the LM56 Electrical Characteristics
for the temperature range of −40°C to +125°C, for example, is specified at ±3°C for the LM56BIM. Note this trip
point error specification does not include any error introduced by the tolerance of the actual resistors used, nor
any error introduced by power supply variation.
If the resistors have a ±0.5% tolerance, an additional error of ±0.4°C will be introduced. This error will increase to
±0.8°C when both external resistors have a ±1% tolerance.
BIAS CURRENT EFFECT ON TRIP POINT ACCURACY
Bias current for the comparator inputs is 300 nA (max) each, over the specified temperature range and will not
introduce considerable error if the sum of the resistor values are kept to about 27 kΩ as shown in the typical
application of Figure 1. This bias current of one comparator input will not flow if the temperature is well below the
trip point level. As the temperature approaches trip point level the bias current will start to flow into the resistor
network. When the temperature sensor output is equal to the trip point level the bias current will be 150 nA
(max). Once the temperature is well above the trip point level the bias current will be 300 nA (max). Therefore,
the first trip point will be affected by 150 nA of bias current. The leakage current is very small when the
comparator input transistor of the different pair is off (see Figure 15).
The effect of the bias current on the first trip point can be defined by the following equations:
(1)
where I_B = 300 nA (the maximum specified error).
The effect of the bias current on the second trip point can be defined by the following equations:
(2)
where I_B = 300 nA (the maximum specified error).
The closer the two trip points are to each other the more significant the error is. Worst case would be when V_T1 =
V_T2 = V_REF/2.
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