Here is the circuit diagram of a 12V boost converter using IC LM2698 from National Semiconductors. The LM2698 is a general purpose boost converter with an 18V, 1.35A. 0.2 Ohm internal switch. This results in high efficiency power conversion to outputs ranging from 2.2V to 17V DC. The input supply voltage range is 2.2V to 17V DC. The IC has excellent line and load regulation and frequency compensation over the entire input voltage range.
In the circuit diagram given here, the IC is wired to produce an output voltage of 12V from 5V input. The working of this circuit is as follows. Input voltage is fed to the analog power input (pin6) of the IC. During the first phase the internal switch is closed and the diode D1 is reverse biased. Energy will be stored in the inductor L1 and the load current is supplied by the capacitor C3. During second phase the internal switch will be closed and the diode D1 will be forward biased. Energy stored in the inductor L1 is transferred to the output capacitor C3 and the load. The switching frequency of the IC can be selected using the FSLCT (pin 7) of the IC. If pin 3 is connected to the supply voltage, the switching frequency will be 1.25 Mhz and if pin 3 is connected to the ground, switching frequency will be 600Khz. Resistors R2 and R3 forms the feedback network while R1 and C1 forms the compensation network. The input capacitor C2 prevents the impedance interactions of the circuit with the supply voltage source.
- Assemble the circuit on a good quality PCB.
- The input supply can be anything between 4.5 to 5.5V DC.
- Do not connect loads that consume more than 400mA.
- At 12V output , the IC can deliver only up to 400mA.
- C2 and C3 must be low ESR multi-layer ceramic capacitors.
- D1 can be a low forward drop Schottky diode (like 0MQ040N from Motorola).
- Pin 3 (SHDN) connected to the ground will shutdown the circuit.