Many systems require the fast transient response, load regulation, and low noise afforded by linear regulators. However, linear regulations are inefficient. 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.
Looking more closely at the setup (see the figure), the circuit operates the LT1084 at slightly above its dropout voltage (a linear regulator's dropout voltage is the minimum voltage drop required to support regulation). 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 by resistor R7. Output voltage can be determined by:
Vout =1.25 V (1 + R7/R6)
The switching preregulator is used to step down the input voltage until the voltage drop across the LT1084 is 1.5 V. The preregulator consists of an LT1074 switching-regulator IC and an LT1006 op amp. The LT1006 is connected in a noninverting configuration and uses the resistor network of R4 and R5 to set its gain.
The amplifier senses the voltage drop across the LT1084 regulator and feeds the amplified voltage into the LT1074 feedback pin. the LT1074 compares the voltage on the feedback pin to an internal 2.21-V reference voltage and then controls the LT1074's duty cycle until the two values are equal.
The RC network (R1 and C2) connected to the VC pin along with R2 and C3 provides sufficient compensation to stabilize this control loop. To calculate the voltage drop (VREF) across the linear regulator, use the following equation:
VREF = 2.21 V × R5 /(R4 + R5)
The LT1074 is a step-down (buck) switching-regulator IC that contains a power switch in addition to its control circuitry. The power switch uses a composite pnp as the "high-side switch." Consequently, when the switch turns on, the LT1074's VSW pin is connected to the input voltage. While the switch is turned on, current flows from the input voltage source, through the switch, the inductor, the LT1084 regulator, and into the load.
When the switch turns off, the current flowing through the inductor (L1) forces the VSW node voltage to drop until diode D1 becomes forward biased, providing a path for the inductor current. When the switch turns on again, the entire cycle repeats itself. For the LT1074 regulator to function properly, the input voltage must be at least 8 V above the output voltage level.
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.