LM4040_05 Datasheet, PDF (25/31 Pages) National Semiconductor (TI) – Precision Micropower Shunt Voltage Reference

Applications Information
The LM4040 is a precision micro-power curvature-corrected
bandgap shunt voltage reference. For space critical applica-
tions, the LM4040 is available in the sub-miniature SOT-23
and SC70 surface-mount package. The LM4040 has been
designed for stable operation without the need of an external
capacitor connected between the “+” pin and the “−” pin. If,
however, a bypass capacitor is used, the LM4040 remains
stable. Reducing design effort is the availability of several
fixed reverse breakdown voltages: 2.048V, 2.500V, 3.000V,
4.096V, 5.000V, 6.000, 8.192V, and 10.000V. The minimum
operating current increases from 60 µA for the LM4040-
2.048 and LM4040-2.5 to 100 µA for the LM4040-10.0. All
versions have a maximum operating current of 15 mA.
LM4040s in the SOT-23 packages have a parasitic Schottky
diode between pin 2 (−) and pin 3 (Die attach interface
contact). Therefore, pin 3 of the SOT-23 package must be
left floating or connected to pin 2.
LM4040s in the SC70 have a parasitic Schottky diode be-
tween pin 1 (−) and pin 2 (Die attach interface contact).
Therefore, pin 2 must be left floating or connected to pin1.
The 4.096V version allows single +5V 12-bit ADCs or DACs
to operate with an LSB equal to 1 mV. For 12-bit ADCs or
DACs that operate on supplies of 10V or greater, the 8.192V
version gives 2 mV per LSB.
The typical thermal hysteresis specification is defined as the
change in +25˚C voltage measured after thermal cycling.
Typical Applications
The device is thermal cycled to temperature -40˚C and then
measured at 25˚C. Next the device is thermal cycled to
temperature +125˚C and again measured at 25˚C. The re-
sulting V_OUT delta shift between the 25˚C measurements is
thermal hysteresis. Thermal hysteresis is common in preci-
sion references and is induced by thermal-mechanical pack-
age stress. Changes in environmental storage temperature,
operating temperature and board mounting temperature are
all factors that can contribute to thermal hysteresis.
In a conventional shunt regulator application (Figure 1) , an
external series resistor (R_S) is connected between the sup-
ply voltage and the LM4040. R_S determines the current that
flows through the load (I_L) and the LM4040 (I_Q). Since load
current and supply voltage may vary, R_S should be small
enough to supply at least the minimum acceptable I_Q to the
LM4040 even when the supply voltage is at its minimum and
the load current is at its maximum value. When the supply
voltage is at its maximum and I_L is at its minimum, R_S should
be large enough so that the current flowing through the
LM4040 is less than 15 mA.
R_S is determined by the supply voltage, (V_S), the load and
operating current, (I_L and I_Q), and the LM4040’s reverse
breakdown voltage, V_R.
01132315
FIGURE 1. Shunt Regulator
25
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