Design Considerations and Component Selection

APPLICATION NOTE	AN50

Short Circuit Protection

A current sense methodology is implemented to disable the output drive signal to the MOSFET(s) when an over-current condition is detected. The voltage drop created by the output current ﬂowing across a sense resistor is presented to an internal comparator. When the voltage developed across the sense resistor exceeds the comparator threshold voltage, the chip reduces the output drive signal to the MOSFET(s).

The DC-DC converter returns to normal operation after the fault has been removed, for either an over-voltage or a short circuit condition.

In general, a lower operating frequency decreases the peak ripple current ﬂowing in the output inductor, thus allowing the use of a smaller inductor value. Unfortunately, operation at lower frequencies increases the amount of energy storage that must be provided by the bulk output capacitors during load transients due to slower loop response of the controller.

In addition, the efﬁciency losses due to switching of the MOSFETs increase as the operating frequency is increased. Thus, efﬁciency is optimized at lower operating frequencies. An operating frequency of 300 kHz was chosen to optimize efﬁciency while maintaining excellent regulation and tran- sient performance under all operating conditions.

Oscillator

The RC5050 and RC5051 oscillator section uses a ﬁxed cur- rent capacitor charging conﬁguration. An external capacitor (CEXT) is used to preset the oscillator frequency between 200KHz and 1MHz. This scheme allows maximum ﬂexibil- ity in setting the switching frequency and in choosing exter- nal components.