LM334SM datasheet(8/26 Pages) TI

8 / 26 page

LM134, LM234, LM334

SNVS746E – MARCH 2000 – REVISED MAY 2013

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Application as a Zero Temperature Coefficent Current Source

Adding a diode and a resistor to the standard LM134 configuration can cancel the temperature-dependent

characteristic of the LM134. The circuit shown in Figure 15 balances the positive tempco of the LM134 (about

+0.23 mV/°C) with the negative tempco of a forward-biased silicon diode (about

−2.5 mV/°C).

Figure 15. Zero Tempco Current Source

The set current (ISET) is the sum of I1 and I2, each contributing approximately 50% of the set current, and IBIAS.

IBIAS is usually included in the I1 term by increasing the VR value used for calculations by 5.9%. (See

CALCULATING RSET.)

(5)

The first step is to minimize the tempco of the circuit, using the following equations. An example is given using a

value of +227

μV/°C as the tempco of the LM134 (which includes the IBIAS component), and −2.5 mV/°C as the

tempco of the diode (for best results, this value should be directly measured or obtained from the manufacturer

of the diode).

(6)

(7)

With the R1 to R2 ratio determined, values for R1 and R2 should be determined to give the desired set current.

The formula for calculating the set current at T = 25°C is shown below, followed by an example that assumes the

forward voltage drop across the diode (VD) is 0.6V, the voltage across R1 is 67.7mV (64 mV + 5.9% to account

for IBIAS), and R2/R1 = 10 (from the previous calculations).

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