8.1.1 Input Capacitors

Although not always required, TI recommends an input capacitor. A supply bypass capacitor on the input assures that the reference is working from a source with low impedance, which improves stability. A bypass capacitor can also improve transient response by providing a reservoir of stored energy that the reference can use in case where the load current demand suddenly increases. The value used for C_IN may be used without limit.

8.1.2 Output Capacitors

The LM4140 requires a 1-µF (nominally) output capacitor for loop stability (compensation) as well as transient response. During the sudden changes in load current demand, the output capacitor must source or sink current during the time it takes the control loop of the LM4140 to respond.

This capacitor must be selected to meet the requirements of minimum capacitance and equivalent series resistance (ESR) range.

In general, the capacitor value must be at least 0.2 µF (over the actual ambient operating temperature), and the ESR must be within the range indicated in Figure 22, Figure 23, and Figure 24.

Figure 22. 0.22-µF ESR Range

Figure 24. 10-µF ESR Range

Figure 23. 1-µF ESR Range

8.1.3 Tantalum Capacitors

Surface-mountable solid tantalum capacitors offer a good combination of small physical size for the capacitance value, and ESR in the range required by the LM4140. The results of testing the LM4140 stability with surface mount solid tantalum capacitors show good stability with values in the range of 0.1 µF. However, optimum performance is achieved with a 1-µF capacitor.

Table 2 shows tantalum capacitors that have been verified as suitable for use with the LM4140.

8.1.4 Aluminum Electrolytic Capacitors

Although probably not a good choice for a production design, because of relatively large physical size, an aluminium electrolytic capacitor can be used in the design prototype for an LM4140 reference. A 1-µF capacitor meeting the ESR conditions can be used. If the operating temperature drops below 0°C, the reference may not remain stable, as the ESR of the aluminium electrolytic capacitor increases, and may exceed the limits indicated in the figures.

8.1.5 Multilayer Ceramic Capacitors

Surface-mountable multilayer ceramic capacitors may be an attractive choice because of their relatively small physical size and excellent RF characteristics.

However, they sometimes have an ESR values lower than the minimum required by the LM4140, and relatively large capacitance change with temperature. The manufacturer's datasheet for the capacitor must be consulted before selecting a value. Test results of LM4140 stability using multilayer ceramic capacitors show that a minimum of 0.2 µF is usually required.

Table 4 shows the multilayer ceramic capacitors that have been verified as suitable for use with the LM4140.

8.1.6 Reverse Current Path

The P-channel Pass transistor used in the LM4140 has an inherent diode connected between the V_IN and V_REF pins (see Figure 25).

Figure 25. Internal P-Channel Pass Transistor

Forcing the output to voltages higher than the input, or pulling V_IN below voltage stored on the output capacitor by more than a V_be, forward biases this diode and current flows from the V_REF terminal to V_IN. No damage to the LM4140 occurs under these conditions as long as the current flowing into the output pin does not exceed 50 mA.

8.1.7 Output Accuracy

Like all references, either series or shunt, the after assembly accuracy is made up of primarily three components: initial accuracy itself, thermal hysteresis, and effects of the PCB assembly stress.

LM4140 provides an excellent output initial accuracy of 0.1% and temperature coefficient of 6ppm/°C (B Grade).

For best accuracy and precision, the LM4140 junction temperature must not exceed 70°C.

The thermal hysteresis curve on this datasheet are performance characteristics of three typical parts selected at random from a sample of 40 parts.

Parts are mounted in a socket to minimize the effect of PCB's mechnical expansion and contraction. Readings are taken at 25°C following multiple temperature cycles to 0°C and 70°C. The labels on the X axis of Figure 26 indicate the device temperature cycle prior to measurement at 25°C.

Figure 26. Typical Thermal Hysteresis

The mechanical stress due to the PCB's mechanical and thermal stress can cause an output voltage shift more than the true thermal coefficient of the device. References in surface mount packages are more susceptible to these stresses because of the small amount of plastic molding which support the leads.

Following the recommendations on Layout can minimize the mechanical stress on the device.

8.2.1 Precision DAC Reference

Figure 27. Precision DAC Reference Schematic

8.2.1.3 Application Curves

Figure 28. Output Voltage vs Load Current

Figure 29. Typical Temperature Coefficient
(Sample of 5 Parts)

8.2.2 Boosted Output Current

Figure 30. Boosted Output Current Schematic

8.2.3 Boosted Output Current With Current Limiter

Figure 31. Boosted Output Current With Current Limiter Schematic

8.2.4 Complimentary Outputs

* Low Noise Op Amp such as OP-27

Figure 32. Complimentary Outputs Schematic

8.2.5 Voltage Reference With Force and Sense Output

Figure 33. Voltage Reference With Force and Sense Output Schematic

8.2.6 Precision Programmable Current Source

Figure 34. Precision Programmable Current Source Schematic

8.2.7 Strain Gauge Conditioner for 350-Ω Bridge

Figure 35. Strain Gauge Conditioner for 350-Ω Bridge Schematic

8.2.8 Bipolar Voltage References for Low Power ADC

Figure 36. Bipolar Voltage References for Low Power ADC Schematic

8.2.9 Self-Biased Low Power ADC Reference With Trim Current Sources

Figure 37. Self-Biased Low Power ADC Reference With Trim Current Sources Schematic