APPLICATION NOTE 4003

Abstract: This article describes some simple steps in choosing between types of voltage references. Explanations of key parameters to use in the selection process between series- and shunt-type voltage references are presented.

- The power-supply voltage (V
_{CC}) must be high enough to allow a voltage drop across the internal resistance, but not high enough to damage the reference IC. - The IC and its package must handle power dissipation in the series pass element.
- With no load current, the only source of power dissipation is the reference IC's quiescent current.
- Series references generally have a better initial tolerance and temperature coefficient than do shunt references.

P_SER = (V

For a series reference, the worst-case power dissipation occurs for maximum power-supply voltage and maximum load:

WC_P_SER = (V

where:

P_SER = power in series reference

V

V

IL = load current

I

WC_P_SER = worst-case power in series reference

V

IL

- Given an R1 appropriately sized for power dissipation, the shunt reference imposes no limit on the maximum power-supply voltage.
- The power supply delivers the same maximum current regardless of load. Supply current flows through load and reference, dropping just the right voltage across R1 to maintain the OUT reference voltage.
- As a simple 2-terminal device, the shunt regulator can be used in novel circuit configurations such as negative regulators, floating regulators, clipping circuits, and limiting circuits.
- Shunt references generally have lower operating currents than do series references.

R1 = (V

The current and power dissipation in R1 depend only on the power-supply voltage. Load current has no effect, because the sum of currents through load and reference is constant:

I_R1 = (V

P_R1 = (V

P_SHNT = V

The worst-case conditions are maximum power-supply voltage and no load:

WC_I_R1 = (V

WC_P_R1 = (V

WC_P_SHNT = V

or

WC_P_SHNT = V

where:

R1 = external resistor

I_R1 = current flowing through R1

P_R1 = power dissipation in R1

P_SHNT = power dissipation in shunt reference

V

V

V

I

IL

WC_I_R1 = worst-case current through R1

WC_P_R1 = worst-case power dissipation in R1

WC_P_SHNT = worst case power dissipation in shunt reference

- If you need better than 0.1% initial accuracy and a 25ppm temperature coefficient, you should probably select a series reference.
- If you want the lowest operating current, consider a shunt reference.
- Be careful when combining a shunt reference with a widely varying power supply or load. Be sure to calculate the expected power dissipation, which can be much higher than that of an equivalent series reference. (See examples, which follow.)
- For power-supply voltages higher than 40V, a shunt reference may be your only choice.
- Consider shunt references when building a negative reference, floating reference, clipping circuit, or limiting circuit.

V

V

V

IL

We have narrowed the search to two parts, as follows:

WC_P_SER = (V

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

The series reference is the only part that dissipates power in this circuit, so the total worst-case power dissipation is 21.8µW.

R1 = (V

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

WC_I_R1 = (V

WC_I_R1 = (3.6V - 2.5V)/250kΩ = 4.4µA

WC_P_R1 = (V

WC_P_R1 = (3.6V - 2.5V)

WC_P_SHNT = V

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

Total worst-case power dissipation for this circuit is the sum of the worst-case power dissipations in R1 (WC_P_R1) plus the worst-case power in the shunt reference (WC_P_SHNT), which equals 18.3µW.

The preferred part for this application is the MAX6008 shunt reference, because its power dissipation is 18.3µW (vs. 21.8µW for the MAX6029 series reference). This example illustrates the large effect of power-supply variation on design. Initially it appeared that the shunt reference had a huge advantage with its 1µA minimum operating current. However, to guarantee operation under worst-case conditions, the operating current had to be increased to 4.4µA. Any variation in the power-supply voltage greater than specified in this case (3.0V to 3.6V) would call for the use of a series reference.

V

V

V

IL

IL

We consider the same two parts:

WC_P_SER = (V

WC_P_SER (1mA IL) = (3.6V - 2.5V)1mA + (3.6V × 5.75µA)

= 1.12mW (1% of time)

WC_P_SER (1µA IL) = (3.6V - 2.5V)1µA + (3.6V × 5.75µA)

= 21.8µW (99% of time)

Average power dissipation = 1.12mW × 1% + 21.8µW × 99% = 32.78µW

R1 = (V

R1 = (3.0V - 2.5V)/(1µA + 1mA) = 499Ω

For I

WC_P_R1 = (V

WC_P_R1 = (3.6V - 2.5V)

P_SHNT = V

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

For I

WC_P_R1 = (V

WC_P_R1 = (3.6V - 2.5V)

P_SHNT = V

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

Average power dissipation is 2.42mW × 1% + 2.5µW × 1% + 2.42mW × 99% + 2.5mW × 99% = 4.895mW.

As you can see, the average power dissipation in the shunt reference is over 100 times higher than in the series reference. For applications in which the load current varies widely, a series reference is usually the better choice.

A similar article appeared on the