“Decrypting” the Voltage Reference IC

Voltage reference ICs are accurate, temperature-compensated voltage sources that provide a specific output voltage value, for example, 1.225 V, 4.096 V, or 10.000 V. Voltage reference ICs can be in shunt (two-terminal) or series (three-terminal) topology.

1 How to configure a shunt voltage reference IC?

A shunt voltage reference IC uses an external series resistor, RS, which is used to set the maximum current supplied to the load (see Figure 1a). RS is selected to achieve the desired range of variation of the supply voltage VS, the load current IL, and the reverse voltage VR of the reference source, while ensuring that the reverse current flowing through the reference source remains at an acceptable level. When the load is almost constant and the supply variation is small, a shunt voltage reference IC can be considered.

2 What is a series voltage reference IC?

A typical series voltage reference IC contains a voltage reference and an amplifier that controls the output voltage (see Figure 1b). The reference voltage source is usually a temperature compensated bandgap or buried Zener type. When the load current is variable and the output voltage change is strictly required when the temperature changes, a series reference voltage IC can be considered.

3 What are the main characteristics of a voltage reference IC?

The two most critical characteristics are initial output voltage accuracy and temperature coefficient. The initial accuracy of an ideal voltage reference should be very good and not change with supply voltage, temperature, load current, and time, although this is not achievable. In systems where calibration is not possible or inconvenient, the initial accuracy of the voltage reference IC is important. Usually, it is obtained by calibration of the entire system.

Initial accuracy is defined with fixed input voltage and no load current (for series type) or fixed bias current (for shunt type). The temperature coefficient is the change in output voltage with operating temperature, normalized to the output voltage at 25°C, and its polarity can vary from device to device, with some devices having a positive temperature coefficient and others having a negative temperature coefficient. In many ADC applications, the initial accuracy of the reference source can be calibrated in the microcontroller or DSP. However, the temperature coefficient cannot be corrected without calibrating the system over the entire temperature range, which is very expensive. Therefore, to achieve the best performance, a precision converter should choose a voltage reference with a low temperature coefficient.

4 Does a voltage reference IC need to be able to both sink and source current?

Yes, if you are using a successive approximation register (SAR) converter or other switched capacitor device. These devices have current spikes that require a reference source capable of sinking 100μA or more. Most reference sources cannot sink even 10nA without introducing reference voltage errors. While many reference sources designed in the past five years can sink some amount of current, you should check the data sheet carefully to confirm.

5 What does the voltage dropout of a voltage reference IC mean?

The dropout voltage is the minimum difference between the input voltage and the output voltage that a voltage reference IC can use to ensure normal operation. It is sometimes also called the "margin" of the voltage reference IC.

Figure 1: (a) A two-terminal shunt voltage reference IC operates similarly to a Zener diode with a resistor in series between the voltage reference and the power supply. However, a shunt voltage reference has higher accuracy and better temperature stability. (b) Some series voltage reference ICs use sophisticated reference voltage technologies, such as temperature coefficient curve correction circuits and laser trimmed, thin-film resistors, to improve the temperature coefficient and increase initial accuracy.

6 What is the output voltage temperature hysteresis effect of a voltage reference IC?

The change in output voltage at a reference temperature (usually 25°C) caused by successive but opposite temperature changes (i.e., from cold to hot and hot to cold). The error caused by this effect is proportional to the temperature change of the system in question. This parameter is related to the circuit design used and the package type of the voltage reference IC.

7 How does long-term stability affect the performance of voltage reference ICs?

Long-term stability is the drift of the output voltage of a device sample tested for 1000 hours at 25°C. The output voltage changes gradually over time, with the largest change in the first 500 hours. This parameter is important in high-performance applications or applications where regular calibration is not possible.

8 Can a voltage reference IC be turned on and off instantly?

After turning on, these devices typically settle to within 0.1% of their final value within 200μs, and when the device is operated at minimum dropout and under heavy load, the on-time may increase to 1ms.

Fast turn-on voltage reference ICs typically allow the end user to turn off the power supply when it is not needed and provide a fast response when turning the power on.

9 What are the main characteristics of the voltage reference internal circuit?

Such a circuit must have low power consumption, low noise, and low drift. It is also best to have a linear temperature coefficient, excellent long-term stability, and low thermal hysteresis. An example circuit uses two junction field effect transistors (JFETs), one of which has an additional channel injection to increase its pinch-off voltage. By making the drain current of the two field effect transistors the same, the difference in pinch-off voltage is amplified to form a highly stable voltage reference.