Design Ideas: March 17, 1994

The feedback arrangement of typical switching regulators doesn't allow the regulated outputs to go lower than the reference voltage. If you try to lower the output by modifying the feedback network, the compensation components the manufacturer recommends may no longer stabilize the regulator's error amplifier. An external reference voltage (Fig 1) helps overcome this problem.

IC₁ regulates by keeping the voltage at its FB pin equal to the internal V_REF, which normally sets a lower limit of 2.21V for V_OUT. The FB voltage usually results from a resistive divider that connects between V_OUT and ground. However, this circuit connects the divider between V_OUT and the higher-voltage shunt-regulator output of D₂. As you adjust R₅, the resulting output voltage ranges from 2.21 to approximately 1.2V, according to the following equation, where V_FB=V_REF=2.21V, and V_Z=zener voltage=7.5V:

V_OUT=V_FB(R₁+R₂)/R₂-V_Z(R₁/R₂).

Because IC₁'s error amplifier is inherently stable, the simple compensation components R₁ and C₁ ensure that the circuit is stable. You can set V_OUT lower than 1.2V if you also modify the compensation network. And, the feedback modification shown in this circuit can let other regulators produce outputs lower than V_REF if you can stabilize their error amplifiers.

IC₁'s highest allowable input voltage is 40V. If V_IN differs significantly from 40V, adjust R₂ as necessary to return the zener current to approximately 1.5 mA. R₃ is an optional load resistor that prevents the otherwise unloaded output from approaching the zener voltage.

The circuit can supply 5A and offers 0.75%/V line regulation for inputs between 30 and 40V. Load regulation for output currents between 0.1 and 5A is 0.4%/A. Losses occur in D₁, which drops about 0.2V, and in the inductor, whose series resistance is approximately 0.06V. Together, these components consume about 2W at 5A. C₂ and the internal, power Darlington transistor also consume power.

When supplying 1A, Fig 1's efficiency for V_OUT=1.2V is approximately 50%-and 60% for V_OUT=2V. Efficiency degrades at light loads because of relatively high supply current. The levels at dc-8.5 mA in the IC and 1.5 mA in the zener diodeÑincrease somewhat with the switching frequency. IC₁'s internal Darlington switch drops about 1.8V. Other regulators that have lower voltage drops across the switch will have higher efficiencies at lower load currents.