This design idea shows how to make an adjustable voltage regulator provide a regulated output voltage that is less than its internal voltage reference and adjustable to nearly zero. This can be a useful feature when powering core voltages that are already below the bandgap voltage and trending downward.

Many popular voltage regulators with bandgap voltage references can be described by the block diagram shown in Figure 1. At first glance it appears that the minimum output voltage allowed by this regulator is 1.23V. Eq1 shows that when setting Rf2 equal to zero or Rf1 equal to infinity one obtains Vout=Vref1.

Vout = Vref1* (Rf2/Rf1+1)
(Eq1) Figure 1. Regulator with bandgap voltage references

Close inspection of the circuits in Figures 1 and 2 reveals that Figure 1 is a subset of Figure 2. Notice the only change in Figure 2 from Figure 1 is the grounded side of Rf1 has been connected to another reference, Vref2. The equation governing Vout has changed slightly to its more general form.

Vout = Vref1 + (Vref1-Vref2)*Rf2/Rf1
(Eq2) Figure 2. Regulator with a second reference

By inspection one sees that if Vref2 is greater than 1.23V then Vout is less than 1.23V. So, all that one needs to adjust this regulator's Vout below its internal Vref1 of 1.23V is an external voltage higher than Vref1.

Figure 3 shows Rf1 tied to Vin. This connection is most useful when Vin is a tightly regulated supply. Changes in Vin are multiplied by the ratio Rf2/Rf1, a number less than unity, thus reducing those variations at Vout. Rearranging Eq2 provides a solution for Rf1.

Rf1 = (Vref1-Vref2)*Rf2 / (Vout-Vref1
(Eq3) Figure 3. Rf1 tied to Vin

R1 was added to Figure 3 to allow regulated operation without a load. It prevents Vout from rising to Vin when the load is removed if the regulator is unable to sink current. R1 can be omitted if the regulator has bipolar drive capability, as is often the case with synchronous switchers.

R1 (max) = Vout*Rf2 / (Vref1-Vout)
(Eq4) Figure 4. 2.5V reference and Rf1 omitted

The DC error seen at Vout is a combination of the tolerances of Vref1, Vref2, and the resistors used. If we ignore resistor tolerances, the uncertainty in Vout is given by Eq5.

ΔVout = ΔVref2*Rf2/Rf1+ΔVref1*(1+Rf2/Rf1)
(Eq5)

Eq5 shows that errors in either reference voltage are multiplied by the ratio Rf2/Rf1 and added to the tolerance of Vref1. Inspection of Eq5 shows one will minimize Vout error by choosing Vref1 and Vref2 with tight tolerance and picking a value for Vref2 as high as practical. The circuit shown in Figure 4 improves accuracy and temperature stability by using a 2.5v reference for Vref2 rather than Vin. The LM431CIM3 shown is a low cost 2.5v reference with 0.5% tolerance in a sot23 package. Rbias is chosen to provide at least 1ma to the reference. Use this circuit when the tolerance needed for Vout precludes using Vin for Vref2.

The following examples show how one might power an FPGA using adjustable low dropout regulators (LDOs). Parts with initial tolerance specified at 1.5% are used to provide a low voltage output with good accuracy.

Example 1; Vin = 5V, ±5%; Vout1 = 3.3V at 200mA max, and Vout2 desired is 1.0V at 600mA max. Figure 5 shows the circuit implemented with the LP8340CLD used for both outputs. It is rated at 1 amp, stable with ceramic caps, and housed in a tiny LLP making the total solution size very small. A fixed voltage 8340 is chosen for the first LDO to maintain its 1.5% tolerance. For the second 8340 a convenient value for Rf2 is chosen first, then Rf1 and R1 values are calculated from Eq3 and Eq4. Eq5 can be used to show Vout will be within 2.7% of 1.0V. Figure 5. Example of powering an FPGA with an adjustable LDO

Example 2; Vin = 5V, +/-5%; Vout1=3.3V at 0.2amp max, Vout2=0.8V at 1.1A max. Figure 6 shows the circuit implemented with the LP3855ES used for both outputs. It is rated at 1.5A, stable with ceramic caps, and housed in a TO263 package. A fixed voltage 3855 is chosen for the first LDO to maintain its 1.5% tolerance. For the second 3855 a convenient value for Rf2 is chosen first, then Rf1 and R1 values are calculated from Eq3 and Eq4. The feed-forward cap, Cff, was added to improve stability. Eq5 can be used to show Vout will be within 4% of 0.8V. Note that the accuracy of Vout changed from 2.7% to 4% due to its reduction from 1.0V to 0.8V. Figure 6

In summary, a method was shown that allow an adjustable voltage regulator to produce a voltage below its internal reference voltage. This technique is viable in systems where Vin is a stable source or there is one available elsewhere in the system. Although linear regulators are used in the examples shown, one substitute switching regulators for higher load currents and improved efficiency.