AN732

AN732 Vishay Siliconix Designing A High-Voltage Non-Isolated Buck-Boost Converter with the Si9121DY Nitin Kalje INTRODUCTION The Si9121DY is a non-isolated buck-boost converter IC, operating from a wide input voltage range of –10 to –60 V with minimal external components. This polarity inverter converts –48 V to +5 V or +3.3 V, making it suitable for applications including digital phones and ISDN power supplies. A non-isolated buck-boost design with the Si9121 eliminates the need for an expensive transformer, while its integrated low on-resistance MOSFET driver, floating feedback error amplifier, fixed-frequency oscillator, output voltage sensing resistor divider, and depletion mode MOSFET for start up/VCC regulation reduce the external component count to less than 10. Current-mode control technology achieves good line transient response, making the Si9121DY suitable for the high source impedance environment. Other features include the under-voltage lock out, programmable soft start, pulse by pulse current limit, hiccup mode with negligible power delivery and dissipation during continuous short circuit conditions, automatic recovery from hiccup after fault removal, and over temperature shutdown. The Si9121 is available in a narrow body SO-8 package and allows power dissipation of up to 1.25 W. while the output capacitor supports the load current. Since the current in an inductor cannot change instantaneously, the voltage across it reverses to maintain a constant current whe...