“What could be simpler than a basic voltage constant reference voltage source? As with all design themes, there are trade-offs. This article discusses the different types of voltage references, their key specifications, and design tradeoffs, including accuracy, temperature independence, current drive capability, power dissipation, stability, noise, and cost.
What could be simpler than a basic voltage constant reference voltage source? As with all design themes, there are trade-offs. This article discusses the different types of voltage references, their key specifications, and design tradeoffs, including accuracy, temperature independence, current drive capability, power dissipation, stability, noise, and cost.
An ideal voltage reference has a perfect initial accuracy and maintains its voltage over load current, temperature, and time. In the real world, designers must weigh the following factors: initial voltage accuracy, voltage temperature drift and hysteresis, current source and sink capability, quiescent current (or power dissipation), long-term stability, noise, and cost.
The two most common types of references are zener and bandgap. Zener diodes are typically used in parallel-connected topologies. Bandgap references are typically used in three-terminal series topologies.
Zener Diodes and Parallel Topologies
Zener diodes are diodes optimized for operation in the reverse biased breakdown region. Since breakdown is relatively constant, a stable reference voltage can be generated by back-driving a known current.
A big advantage of Zener diodes is the wide voltage range, from 2V to 200V. They also have a wide range of power handling capabilities, from a few milliwatts to a few watts.
The main disadvantage of Zener diodes is that they are not accurate enough for high precision applications, and their power dissipation makes them ideal for low power applications. An example is the BZX84C2V7LT1G, which has a breakdown voltage or nominal reference voltage of 2.5V with a 2.3V to 2.7V variation, or ±8% accuracy. This is only suitable for applications where accuracy is not critical.
Another problem with Zener references is output impedance. The example above has an internal impedance of 100Ω at 5mA and 600Ω at 1mA. Non-zero impedance causes additional variation in the reference voltage, depending on the load current variation. This effect can be minimized by choosing a low output impedance Zener diode.
Buried Zener diodes are a special type of Zener diodes that are more stable than regular Zener diodes due to their structure, which allows them to be placed below the silicon surface.
An alternative to an actual Zener diode is an active circuit that simulates a Zener diode. The circuit allows this device to greatly improve the classical limitations of Zener diodes. The MAX6330 is one such device. It has a tight initial accuracy of 1.5% (max) over a load variation range of 100µA to 50mA. A typical implementation of such an IC is shown in Figure 1.
Choose the right shunt resistor
All shunt configuration references require a current limiting resistor in series with the reference element. It can be calculated from the following formula:
RS = (VIN(max)C VSHUNT(min))/(ISHUNT(max) + ILOAD(min))≤RS≤(VIN(min)C VSHUNT(max))/(ISHUNT(min) ) + ILOAD(max) )
VIN is the input voltage range
VSHUNT is the regulated voltage
ILOAD is the output current range
ISHUNT is the minimum parallel operating current
Note that a parallel circuit always consumes ILOAD(max) + ISHUNT regardless of whether there is a load or not.
By appropriately sizing RS, the same shunt can be used for 10VIN or 100VIN. Choose the largest nominal resistor value for RS to achieve the lowest current consumption. Remember to provide a safety margin to incorporate the worst-case tolerance of the resistors used. You should also use one of the following two general power equations to ensure that the resistors are rated for adequate power:
PR = IIN(VIN(max) C VSHUNT)
= (VIN(max)C VSHUNT)²/RS
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