LM4040C82ILPR datasheet(27/61 Pages) TI2

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8.2.2 Detailed Design Procedure

When using LM4040 as a comparator with reference, determine the following:

• Input voltage range

• Reference voltage accuracy

• Output logic input high and low level thresholds

• Current source resistance

8.2.2.1 LM4040 Voltage and Accuracy Choice

When using LM4040 as a reference for an ADC, the ADC's FSR (Full Scale Range), Resolution and LSB must

be determined. LSB can be determined by:

LSB=FSR/(2N-1)

With N being the resolution or Number of Bits. FSR and Resolution can be determined by the ADC's data sheet.

Vref can be determined by:

Vref=FSR+LSB

Though modern data converters use calibration techniques to compensate for any error introduced by a Vref's

inaccuracy, use the highest accuracy available. This is due to errors in the calibration method that can allow

some non-linearity introduced by the Vref's initial accuracy.

A good example is the LM4040x-41 that is designed to be a cost-effective voltage reference as required in 12-bit

data-acquisition systems. For 12-bit systems operating from 5V supplies (see Figure 8-1), the LM4040A-41

(4.096V, 0.01%) only introduces 4 LSBs (4mV) of possible error in a system that consists of 4096 LSBs.

8.2.2.2 Cathode and Load Currents

In a typical shunt-regulator configuration (see Figure 8-2), an external resistor, RS, is connected between the

supply and the cathode of the LM4040. RS must be set properly, as RS sets the total current available to supply

the load (IL) and bias the LM4040 (IZ). In all cases, IZ must stay within a specified range for proper operation of

the reference. Taking into consideration one extreme in the variation of the load and supply voltage (maximum IL

and minimum VS), RS must be small enough to supply the minimum IZ required for operation of the regulator, as

given by data-sheet parameters. At the other extreme, maximum VS and minimum IL, RS must be large enough

to limit IZ to less than the maximum-rated value of 15mA.

RS is calculated according to Equation 1:

(

)

(

)

(1)

LM4040

IZ + IL

Figure 8-2. Shunt Regulator

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LM4040A, LM4040B, LM4040C, LM4040D

SLOS456P – JANUARY 2005 – REVISED MARCH 2025

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