LM56
Application Hints
1.0 LM56 TRIP POINT ACCURACY SPECIFICATION
For simplicity the following is an analysis of the trip point ac-
curacy using the single output configuration show in
with a set point of 82°C.
Trip Point Error Voltage = V
TPE
,
Comparator Offset Error for V
T1E
Temperature Sensor Error = V
TSE
Reference Output Error = V
RE
1289317
FIGURE 2. Single Output Configuration
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 page 1 of the datasheet.
V
T1
=1.25VxR2/(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 Electrical Characteristic 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 vari-
ation.
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.
2.0 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
.
This bias current of one comparator input will not flow if the
temperature is well below the trip point level. As the temper-
ature 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
.
The effect of the bias current on the first trip point can be de-
fined by the following equations:
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:
where I
B
= 300 nA (the maximum specified error).
The closer the two trip points are to each other the more sig-
nificant the error is. Worst case would be when V
T1
= V
T2
=
V
REF
/2.
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