Application Hints

1.0 LM56 TRIP POINT ACCURACY SPECIFICATION

For simplicity the following is an analysis of the trip point

accuracy using the single output configuration show in Fig-

ure 2 with a set point of 82˚C.

Trip Point Error Voltage = V

Comparator Offset Error for V

T1E

Temperature Sensor Error = V

TSE

Reference Output Error = V

RE

1. V

Where:

2. V

3. V

4. V

Using Equations from page 1 of the datasheet.

V

+395 mV

Solving for R2/(R1 + R2) = 0.7227

then,

5. V

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

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.

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 k

Ω 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). There-

fore, the first trip point will be affected by 150 nA of bias

current. The leakage current is very small when the com-

parator input transistor of the different pair is off (see Figure

3).

The effect of the bias current on the first trip point can be

defined by the following equations:

where I

The effect of the bias current on the second trip point can be

defined by the following equations:

where I

The closer the two trip points are to each other the more

significant the error is. Worst case would be when V

=V

01289317

FIGURE 2. Single Output Configuration

www.national.com

8