The REF32xx family of references features an extremely low dropout voltage. With the exception of the REF3212, which has a minimum supply requirement of 1.8V, these references can be operated with a supply of only 5mV above the output voltage in an unloaded condition. For loaded conditions, a typical dropout voltage versus load is shown in Section 5.4.

The REF32xx also features a low quiescent current of 110μA, with a maximum quiescent current over temperature of just 145μA. The quiescent current typically changes less than 2μA over the entire supply range, as shown in Figure 6-2.

Figure 6-2 Supply Current vs Supply Voltage

Supply voltages below the specified levels can cause the REF32xx to momentarily draw currents greater than the typical quiescent current. This momentary current draw can be prevented by using a power supply with a fast rising edge and low output impedance.

For optimal startup when the IN pin and ENABLE pin are tied together, keep the input voltage rise time less than or equal to 2ms. For rise times greater than 2ms, the ENABLE pin must be kept below 0.7V until the voltage at the IN pin has reached the minimum operating voltage. One way to control the voltage at the ENABLE pin is with an additional RC filter, such as that shown in Figure 6-3. The RC filter must hold the voltage at the ENABLE pin below the threshold voltage until the voltage at the input pin has reached the minimum operating voltage.

Figure 6-3 Application Circuit to Control the REF32xx ENABLE Pin

The RC filter in Figure 6-3 can be used as a starting point for the REF3240. The values for R₁ and C₁ have been calculated so that the voltage at the ENABLE pin reaches 0.7V after the input voltage has reached 4.15V; Table 6-1 lists these values. For output voltage options other than 4.096V, the RC filter can be made faster.

In this document, rise time is defined as the time until an exponential input signal reaches 90% of its final voltage. For example, the 2ms value shown in Table 6-1 is valid for an end value of 5V.

If the input voltage has a different shape or the end value is not 5V, then the time until the minimum dropout voltage has been reached must be used to decide if the IN and ENABLE pins can be tied together. Table 6-2 lists these times.