Linear Technology offers a line of high performance multiple dual output synchronous boost converters. However, most of the datasheet contains examples of these controllers using the same input voltage source for both outputs. In some circumstances there may be a need to derive two different high output voltages from two separate input voltage sources. This is important in designs where the power and correspondingly current of a single input line is limited and both input lines need to be employed for reaching the desired output power levels. These types of requirements are often found in oil and gas exploration equipment where long lines limit power or in military and industrial applications.
The example circuit below and associated LTspice simulation may be useful for customers interesting in this subject and is based on the LTC3788-1, a constant frequency current mode boost controller, which provides a high efficient and relatively simple solutions. The example circuit below highlights two separate input voltages IN1 and IN2 connected to matching input filters. Pairs of switching MOSFETs Q1, Q2 and Q3, Q4 generate output voltages OUT1 and OUT2. The circuit operated from 12V to 24V on IN1 and provides 10A of output current at 24V output, OUT1. The second 12V output, OUT2, delivers 5A from a 5V to 12V input on IN2. The LTC3788-1 provides fixed current limit and one PGOOD output, without phase modulation or a clock output. Alternately, the LTC3788 with adjustable current limit, clock out, phase modulation and two PGOOD outputs could also be used. The 350kHz constant frequency operation results in small and efficient circuit. The converter provides high output voltage accuracy (typically ±3%) over wide load range with no minimum load requirement. The circuit operated from 12V to 24V on IN1 and provides 10A of output current at 24V output, OUT1. The second 12V output, OUT2, delivers 5A from a 5V to 12V input on IN2.
Note: the controller biasing voltage is supplied by the 12V output, OUT2. However care should be taken in biasing the converter in case one of the input voltages is not present. The bias OR function based on diodes or independent auxiliary power supply can also be used.