9.1 Application Information

The LM4051-N is a precision micro-power curvature-corrected bandgap shunt voltage reference. For space critical applications, the LM4051-N is available in the sub-miniature SOT-23 surface-mount package. The LM4051-N 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 LM4051-N remains stable. Design effort is further reduced with the choice of either a fixed 1.2-V or an adjustable reverse breakdown voltage. The minimum operating current is 60 μA for the LM4051-1.2 and the LM4051-ADJ. Both versions have a maximum operating current of 12 mA.

LM4051-N's using the SOT-23 package have pin 3 connected as the (–) output through the package’s die attach interface. Therefore, the LM4051-1.2’s pin 3 must be left floating or connected to pin 2 and the LM4051-ADJ’s pin 3 is the (–) output.

The typical thermal hysteresis specification is defined as the change in +25 ̊C voltage measured after thermal cycling. 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 resulting V_OUT delta shift between the 25 ̊C measurements is thermal hysteresis. Thermal hysteresis is common in precision references and is induced by thermal-mechanical package 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 18), an external series resistor (R_S) is connected between the supply voltage and the LM4051-N. R_S determines the current that flows through the load (I_L) and the LM4051-N (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 LM4051-N 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 LM4051-N is less than 12 mA.

R_S should be selected based on the supply voltage, (V_S), the desired load and operating current, (I_L and I_Q), and the LM4051-N's reverse breakdown voltage, V_R.

Equation 1.

The LM4051-ADJ’s output voltage can be adjusted to any value in the range of 1.24 V through 10 V. It is a function of the internal reference voltage (V_REF) and the ratio of the external feedback resistors as shown in Figure 20. The output voltage is found using Equation 2:

Equation 2.

where

• V_O is the output voltage

Equation 3.

The actual value of the internal V_REF is a function of V_O. The corrected V_REF is determined by Equation 4:

Equation 4.

where

• V_Y = 1.22 V

∆V_REF/∆V_O is found in the LM4051-ADJ Electrical Characteristics and is typically −1.55 mV/V. You can get a more accurate indication of the output voltage by replacing the value of V_REF in Equation 2 with the value found using Equation 4.