There are two basic ways to limit the input-voltage range of a dc/dc converter. The first is to limit the maximum duty cycle of the pulse-width modulator. The second is to use a current-limited source.

Most dc/dc converters today do not operate in applications that have such a high source resistance. Hence, manufacturers of these converters don’t go to great expense to set an exact value for the dropout point. Most manufacturers actually design the converters for wider-than-specified input-voltage ranges to ensure that all converters can easily meet the voltage-range specification. Thus, a typical converter might operate with an input voltage 3 to 30% lower than the data sheet’s listed minimum.

Also, many dc/dc-converter-control chips lack tightly controlled dropout voltages. Switching-regulator ICs use two common methods to limit the duty cycle and, hence, the drop-out voltage. The first method, which the Texas Instruments TL494 (Figure A) and Motorola MC34060 use, involves feeding an analog voltage to a comparator input (the dead-time-adjust pin) to set the maximum duty cycle. Using such a chip allows for a duty-cycle setpoint of about 5% when you account for worst-case conditions.

The second common method, which the Siliconix Si9110 (Figure B) implements, is to use a flip-flop to set the maximum duty cycle to 50% or less. This digital method is more accurate because it allows a duty-cycle repeatability between converters of around 2 to 3%, considering worst-case conditions. Other “all-in-one” ICs, such as the Linear Technology LT1070, that include both the control circuitry and power switch operate at a duty cycle exceeding 90% with no convenient way of limiting the duty cycle.

Other less common methods of limiting the input range include adjusting the RC oscillator components to increase the reset time and clamping the error amplifier’s feedback voltage. Most manufacturers’ application literature contains circuit ideas for accomplishing duty-cycle limiting for a particular IC.

The second way to limit the input range, limiting the source current, can also prevent operation on the right side of the maximum-power point. Limit the current to a value less than the calculated maximum-power current to provide a safety margin for component and circuit tolerances. When using a current-limited source, ensure that the initial voltage ramp to the converter is not so slow that start-up problems occur. The limited charging current of the source and a large value of input capacitance at the input of the dc/dc converter can cause a slow ramp-up. One way to handle this situation is to use a smaller value of capacitance at the dc/dc converter’s input.