Design Considerations and Component Selection, Oscillator

APPLICATION NOTE	AN42

Over-Voltage Protection

The RC5040 and RC5042 constantly monitor the output voltage for protection against over voltage. If the voltage at the VFB pin exceeds 20% of the selected program voltage, an over-voltage condition is assumed, and the controller dis- ables the output drive signal to the external MOSFET(s).

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 controller disables 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.

Oscillator

The RC5040 oscillator section is implemented using a ﬁxed current capacitor charging conﬁguration. An external capacitor (CEXT) is used to preset the oscillator frequency between 200KHz and 1MHz. This allows maximum ﬂexibil- ity in setting the switching frequency and in choosing exter- nal components.

In general, a lower operating frequency increases the peak ripple current ﬂowing through the output inductor, allowing the use of a larger inductor value. Operation at lower fre- quencies increases the amount of energy storage that the bulk output capacitors must provide during load transients that occur due to the slower loop response of the controller.

In addition, note that the efﬁciency losses due to switching are relatively ﬁxed per switching cycle. Therefore, as the switching frequency increases, the contribution toward efﬁ- ciency due to switching losses also increases.

RC5040 has an optimal operating frequency of 650KHz. This frequency allows the use of smaller inductive and capacitive components while optimizing peak efﬁciency under all operating conditions.