Selection of series or shunt voltage reference

This paper introduces the selection of different types of voltage reference chips and provides several indicators that need to be considered when selecting series and shunt voltage references.

Series Voltage Reference

A series voltage reference has three terminals: VIN, VOUT, and GND, similar to a linear regulator, but with lower output current and very high accuracy. A series voltage reference is constructed in series with the load (Figure 1) and can be treated as a voltage-controlled resistor between VIN and VOUT . Its internal resistance is adjusted to maintain a constant difference between the VIN value and the voltage drop across the internal resistance (equal to the reference voltage at VOUT). Because current is required to produce the voltage drop, the device draws a small amount of quiescent current to ensure regulation when no load is applied. Series voltage references have the following characteristics:

Power Supply The voltage (VCC) must be high enough to ensure sufficient voltage drop across the internal resistors, but too high a voltage will damage the device.

The device and its package must be able to dissipate the power of the series pass transistor.

At no load, the only power consumption is the quiescent current of the voltage reference.

Series voltage references generally have better initial error and temperature coefficient than shunt voltage references.

Figure 1. Block diagram of a three-terminal series voltage reference.

Series Reference Design

The design of a series voltage reference is quite simple. You only need to ensure that the input voltage and power dissipation are within the maximum specified by the IC: P_SER = (VSUP - VREF)IL + (VSUP x IQ)

For series voltage references, the maximum power dissipation occurs at the highest input voltage and heaviest load: WC_P_SER = (VMAX - VREF)ILMAX + (VMAX x IQ)

Where: P_SER = Power dissipated by the series reference

VSUP = Supply voltage

VREF = Reference Voltage Output

IL = Load current

IQ = Quiescent current of the voltage reference

WC_P_SER = Maximum power consumption

VMAX = Maximum supply voltage

ILMAX = Maximum load current

Shunt Voltage Reference

A shunt voltage reference has two terminals: OUT and GND. It is similar in principle to a Zener diode, but has better voltage regulation characteristics. Similar to a Zener diode, it requires an external resistor and works in parallel with the load (Figure 2). A shunt voltage reference can be thought of as a voltage-controlled current source connected between OUT and GND. By adjusting the internal current, the difference between the power supply voltage and the voltage drop across resistor R1 (equal to the reference voltage at OUT) remains stable. In other words, a shunt voltage reference maintains a constant voltage at OUT by keeping the sum of the load current and the current flowing through the voltage reference constant. A shunt reference has the following characteristics:

Selecting the appropriate R1 ensures that the power requirements are met. The shunt voltage reference has no limit on the maximum supply voltage.

The maximum current provided by the power supply has nothing to do with the load. The power supply current flowing through the load and the reference needs to produce an appropriate voltage drop across resistor R1 to keep the OUT voltage constant.

As a simple 2-terminal device, the shunt voltage reference can be configured into some novel circuits , such as negative voltage regulators, floating regulators, clipping circuits , and limiter circuits.

Shunt voltage references typically have lower operating current than series voltage references.

Figure 2. Block diagram of a 2-terminal shunt voltage reference.

Shunt Reference Design

The design of a shunt voltage reference is slightly more challenging because the value of the external resistor must be calculated. This value (R1) is required to ensure that the voltage drop caused by the voltage reference and the load current is equal to the difference between the supply voltage and the reference voltage. R1 is calculated using the minimum input supply voltage and maximum load current to ensure that the circuit will operate properly under the worst-case conditions. The following equations are used to calculate the value and power dissipation of R1, as well as the power dissipation of the shunt voltage reference (Figure 3).

R1 = (VMIN - VREF) / (IMO + ILMAX)

The current and power dissipation in R1 depends only on the supply voltage. The load current has no effect on this because the sum of the load current and the voltage reference current is a fixed value:

I_R1 = (VSUP - VREF) / R1

P_R1 = (VSUP - VREF)2 / R1

P_SHNT = VREF(IMO + I_R1 - IL)

The worst operating condition occurs when the input voltage is maximum and the output is unloaded:

WC_I_R1 = (VMAX - VREF) / R1

WC_P_R1 = (VMAX - VREF)2 / R1

WC_P_SHNT = VREF(IMO + WC_I_R1) 或 WC_P_SHNT = VREF(IMO + (VMAX - VREF) / R1)

in:

R1 = External resistor

I_R1 = Current of R1

P_R1 = power dissipated by R1

P_SHNT = Power dissipated by the voltage reference

VMIN = Minimum supply voltage

VMAX = Maximum supply voltage

VREF = Reference Output

IMO = Voltage reference minimum operating current

ILMAX = Maximum load current

WC_I_ R1 = Worst case current in R1

WC_P_R1 = Worst case power dissipation in R1

WC_P_SHNT = Worst-case shunt voltage reference power dissipation

Figure 3. A shunt voltage reference adjusts the current (IMO) to produce a stable VREF.

Selecting a Voltage Reference

Understanding the differences between series and shunt voltage references will allow you to choose the most appropriate device for your application. To find the best device, it is best to consider both series and shunt references. After calculating the parameters of both types, you can determine the device type. Here are some empirical methods:

If you need better than 0.1% initial accuracy and 25ppm temperature coefficient, you should generally choose a series voltage reference.

If the lowest operating current is required, choose a shunt voltage reference.

Shunt voltage references must be used with caution when used over a wide supply voltage range or under large dynamic load conditions. Be sure to calculate the expected value of the dissipated power, which can be much higher than a series voltage reference with the same performance (see the example below).

For applications with supply voltages above 40V, a shunt voltage reference may be the only choice.

When building a negative voltage regulator, floating regulator, clipping circuit, or limiter circuit, a shunt voltage reference is generally considered.

Example 1: Low voltage, fixed load

In this portable application, the most critical parameter is power consumption. The following are the corresponding technical indicators:

VMAX = 3.6V

VMIN = 3.0V

VREF = 2.5V

ILMAX = 1μA

We narrowed it down to two devices: the MAX6029 series voltage reference

IQ = 5.75μA

WC_P_SER = (VMAX - VREF)ILMAX + (VMAX x IQ)

WC_P_SER = (3.6V - 2.5V)1μA + (3.6V x 5.75μA) = 21.8μW

The series reference is the only device consuming power in the circuit, so the total power dissipation under worst-case operating conditions is 21.8μW.

Shunt Voltage Reference MAX6008

IMO = 1μA

R1 = (VMIN - VREF) / (IMO + ILMAX)

R1 = (3.0V - 2.5V) / (1μA + 1μA) = 250k

WC_I_R1 = (VMAX - VREF) / R1

WC_I_R1 = (3.6V - 2.5V) / 250k = 4.4μA

WC_P_R1 = (VMAX - VREF)2 / R1

WC_P_R1 = (3.6V - 2.5V)2 / 250k = 4.84μW

WC_P_SHNT = VREF(IMO + (VMAX - VREF) / R1)

WC_P_SHNT = 2.5V(1μA + (3.6V - 2.5V) / 250k) = 13.5μW

The total worst-case power dissipation is the sum of the power dissipated in R1 (WC_P_R1) and the power dissipated in the shunt reference (WC_P_SHNT), so the total power dissipation is 18.3μW. The most suitable device for this application would be the MAX6008 shunt voltage reference, which dissipates 18.3μW (while the MAX6029 dissipates 21.8μW).

This example shows that the power supply voltage variation has a significant impact on the design. Initially, the 1μA minimum operating current of the shunt voltage reference has a great advantage, but in order to ensure operation under the worst operating conditions, its operating current is forced to increase to 4.4μA. If the power supply voltage variation range is wider than the requirement in this example (3.0V to 3.6V), the series voltage reference will be given priority.

Example 2: Low voltage, varying load

This example is similar to Example 1, but with some minor changes to the specifications. Instead of a fixed 1μA load, the load in this example draws current periodically, drawing 1μA for 99ms and 1mA for 1ms:

VMAX = 3.6V

VMIN = 3.0V

VREF = 2.5V

ILMAX = 1mA (1% of the time)

ILMIN = 1μA (99% of the time)

We consider the same two devices:

Series Voltage Reference MAX6029

IQ = 5.75μA

WC_P_SER = (VMAX - VREF)ILMAX + (VMAX x IQ)

WC_P_SER (1mA IL) = (3.6V - 2.5V)1mA + (3.6V x 5.75μA) = 1.12mW (1% of the time)

WC_P_SER (1μA IL) = (3.6V - 2.5V)1μA + (3.6V x 5.75μA) = 21.8μW (99% of the time)

Average power consumption = 1.12mW x 1% + 21.8μW x 99% = 32.78μW

Shunt Voltage Reference MAX6008

IMO = 1μA

R1 = (VMIN - VREF) / (IMO + ILMAX)

R1 = (3.0V - 2.5V) / (1μA + 1mA) = 499Ω

For ILOAD = 1mA:

WC_P_R1 = (VMAX - VREF)2 / R1

WC_P_R1 = (3.6V - 2.5V)2 / 499 = 2.42mW (1% of the time)

P_SHNT = VREF(IMO + I_R1 - IL)

P_SHNT = 2.5V(1μA + 1mA - 1mA) = 2.5μW (1% of the time)

For ILOAD = 1μA:

WC_P_R1 = (VMAX - VREF)2 / R1

WC_P_R1 = (3.6V - 2.5V)2 / 499Ω = 2.42mW (99% of the time)

P_SHNT = VREF(IMO + I_R1 - IL)

P_SHNT = 2.5V(1μA + 1mA - 1μA) = 2.5mW (99% of the time)

Average power consumption = 2.42mW x 1% + 2.5μW x 1% + 2.42mW x 99% + 2.5mW x 99% = 4.895mW.

From the above example, we can see that the power consumption of the shunt voltage reference exceeds 100 times that of the series voltage reference. For applications with a wide range of load current variations, the series voltage reference is a better choice.