Design Ideas: August 15, 1996
The circuit in 
Figure
1 uses the comparator and voltage reference of a low-battery detector to
preregulate the input voltage. The circuit provides an additional regulated
voltage output at VOUT1 and significantly reduces the linear
regulator's (VOUT2's) power dissipation. The two outputs (12V at 50
mA and 5V at 100 mA) are independent.
The simple switching preregulator consists of a voltage comparator, a voltage reference, an external current switch, an inductor, and a catch diode. The switching frequency is a function of the ripple voltage present on the comparator input (the LBI pin); the comparator switches on and off in response to the ripple voltage at the LBI pin.
R1 and R2 determine the average voltage at VOUT1 according to the following formula:
where VREF=1.225V. You can set VOUT1 to any voltage between VIN-1 to 7V, which is the minimum input voltage for IC1 with a 5V output. The comparator's hysteresis multiplied by the R1/R2 gain determines the peak-to-peak amplitude of the ripple voltage on VOUT1, which is constant. The frequency of the ripple voltage is a function of the load current.
When you turn on the input supply, the MOSFET's gate is at 0V and the transistor is on, charging C1 with a constant current. The voltage across C1 continues to rise until it reaches the high threshold voltage of the LBI detectors, at which point the comparator output turns off. The load current causes the voltage across C1 to drop until this voltage reaches the detector's low threshold voltage, which is approximately 100 mV lower than the high threshold voltage. At this point, the MOSFET turns on again. VOUT1 oscillates between the two thresholds with approximately 100 mV p-p of ripple. You can easily reduce this ripple with an optional RC filter, such as R3 and C2.
The maximum input voltage is limited to the open-collector breakdown voltage of IC1's LBO input. For higher input voltages, you can add an optional zener diode (D1) with the appropriate voltage rating. (DI #1907)