Understanding Voltage References: Ultra-low dropout - not just for the series reference

In the first post in our Understanding Voltage References series, we discussed the differences between a series and shunt voltage reference.  Today I’ll address ultra-low dropout and how it is not just for the series reference.

Have you ever needed a voltage reference that has to tolerate wide-input voltage ranges yet is still capable of low-dropout operation? For example, most series references with low dropout do not support up to 12V. This is where a shunt reference can be very handy.

Figure 1: Driving an ADC external-reference pin with a shunt reference

In the application shown in Figure 1, the LM4040 shunt reference voltage is 4.096V, which is a common choice for analog to digital converters (ADCs) because 1 mV is equivalent to one least significant bit (LSB) with a 12-bit ADC.

The shunt reference requires an external resistor to set the supply current. The load current for the voltage reference can be determined from the ADC data sheet. For this example, let’s use the ADS8320. In the circuit shown in Figure 1, the current draw of the external reference pin is listed as 40µA in the ADC data sheet. With an external resistor value of 576Ω, the voltage reference will remain in its operational area over an input-voltage range of 4.16V to 12.75V. That’s a dropout voltage of 64mV, with full reference functionality beyond 12V. To compare the shunt reference low dropout to a series reference with the same 4.096V reference voltage, the maximum dropout voltage for the REF5040 is specified as 200mV in the data sheet.

If 64mV isn’t low enough for you, using an external resistor value of 100Ω allows for an even lower minimum input voltage of 4.11V and a dropout of 14mV. The trade-off of this ultra-low dropout is illustrated by the maximum input voltage, which would be limited to 5.59V before the quiescent current exceeds the device’s maximum rating. To compare this to a series reference again, the REF3240 has a low dropout of 5mV but a maximum input voltage of 5.5V. Note that the series dropout improvement is with a different part, while the shunt is the same part with a different resistor.

Table 1 summarizes the voltage and current values for the two resistor values for the LM4040 shunt reference.

Table 1: Voltage and current parameters for different external resistors of the LM4040 shunt reference

These very low dropout voltages are possible because the load current seen by the reference device is very small at 40µA. If the load were 100µA, the same 576Ω resistor would allow for a minimum V_IN of 4.2V with a dropout of 104mV, which is still low. In applications with higher load currents, the dropout will increase as the required resistance increases.

In part three of “Understanding Voltage References,” Marek Lis will explain how to achieve shunt reference flexibility with series reference precision.

In the meantime, visit www.ti.com/vref for more information on TI’s voltage reference products and consider these additional resources that can help you with your design:

• Part 1 of this series discusses the differences between a series and shunt voltage reference “Shunt versus series: Which voltage reference topology is right for you?”
• For a detailed explanation of voltage-reference selection with respect to ADC resolution, download the white paper “Voltage Reference Selection Basics."