Regulator Loss Cutter Circuit

Description: Large input-to-output voltage differentials, caused by wide input voltage variations, reduce a linear regulator's efficiency and increase its power dissipation. A switching preregulator can reduce this power dissipation by minimizing the voltage drop across an adjustable linear regulator to a constant 1.5-V value. The circuit operates the LT1084 at slightly above its dropout voltage. To minimize power dissipation, a low-dropout linear regulator was chosen. The LT1084 functions as a conventional adjustable linear regulator with an output voltage that can be varied from 1.25 to 30 V. Without the preregulator (for a 40-V input and a 5-V output at 5 A), it would be virtually impossible to find a heatsink large enough to dissipate enough energy to keep the linear-regulator junction temperature below its maximum value. With the preregulator technique, however, the linear regulator will dissipate only 7.5 W under worst-case loading conditions for the entire input-voltage range of 15 to 40 V. Even under a short-circuit fault condition, the 1.5-V drop across the LT1084 is maintained.

The described circuit employs a switching preregulator to enhance the efficiency of a linear voltage regulator, specifically the LT1084. The primary challenge addressed by this configuration is the substantial voltage differential between the input and output, which can lead to significant power losses and increased thermal management requirements in linear regulators. By utilizing a switching preregulator, the input voltage is effectively reduced before it reaches the LT1084, thereby limiting the voltage drop across the linear regulator to a fixed value of 1.5 V. This strategic reduction in voltage drop is crucial for maintaining operational efficiency and minimizing heat generation.

The LT1084 is a low-dropout adjustable linear regulator that can provide an output voltage adjustable from 1.25 V to 30 V. Its design allows it to operate efficiently even when the input voltage is only slightly above the output voltage, making it suitable for applications where space and thermal management are critical. In scenarios where the input voltage is as high as 40 V and the desired output is 5 V at a load current of 5 A, the power dissipation of the linear regulator would be unmanageable without the preregulator. The preregulator ensures that the maximum power dissipation of the LT1084 is limited to 7.5 W, even under worst-case conditions across the input voltage range of 15 V to 40 V.

This configuration is particularly advantageous during fault conditions, such as short circuits, where the linear regulator must maintain its operational parameters. The design guarantees that the 1.5 V drop across the LT1084 remains constant, which is essential for protecting the device from thermal overload. Overall, this circuit design exemplifies an effective approach for managing high input voltages while ensuring efficient power regulation and thermal stability. Large input-to-output voltage differentials, caused by wide input voltage variations, reduce a linear regulator`s efficiency and increase its power dissipation. A switching preregulator can reduce this power dissipation by minimizing the voltage drop across an adjustable linear regulator to a constant 1.5-V value.

The circuit operates the LT1084 at slightly above its dropout voltage. To minimize power dissipation, a low-dropout linear regulator was chosen. The LT1084 functions as a conventional adjustable linear regulator with an output voltage that can be varied from 1.25 to 30 V. Without the preregulator (for a 40-V input and a 5-V output at 5 A), it would be virtually impossible to find a heatsink large enough to dissipate enough energy to keep the linear-regulator junction temperature below its maximum value. With the preregulator technique, however, the linear regulator will dissipate only 7.5 W under worst-case loading conditions for the entire input- voltage range of 15 to 40 V.

Even under a short-circuit fault condition, the 1.5-V drop across the LT1084 is maintained.