CM2596

2005/03/04

Champion Microelectronic Corporation

Page 8

APPLICATION INFORMATION

External component selector

Input Capacitors (Cin)

It is required that VIN must be bypassed with at least a 100uF electrolytic capacitor for stability. Also, it is strongly recommended

the capacitor’s leads must be short, wide and located near the regulator as possible. The important parameters for the capacitor

are the voltage rating and the RMS current rating. For a maximum ambient temperature of 40

℃, a general guideline would be to

select a capacitor with a ripple current rating of approximately 50% of the DC load current. The capacitor voltage rating must be

at least 1.25 times greater than the maximum input voltage.

For low operating temperature range, for example, below -25

℃, the input capacitor value may need to be larger. This is due to

the reason that the capacitance value of electrolytic capacitors decreases and the ESR increases with lower temperatures and

age. Paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures.

An output capacitor is also required to filter the output voltage and is needed for loop stability. The capacitor should be located

near the CM2596 using short PC board traces. Low ESR types capacitors are recommended for low output ripple voltage and

good stability. Generally, low value or low voltage (less than 12V) electrolytic capacitors usually have higher ESR numbers. For

example, the lower capacitor values (220uF – 1000uF) will yield typically 50mV to 150mV of output ripple voltage, while

larger-value capacitors will reduce the ripple to approximately 20mV to 50mV.

The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the output capacitor and

the amplitude of the inductor ripple current (

Output Ripple Voltage = (

Some capacitors called “high-frequency”, “low-inductance”, or “low-ESR” are recommended to use to further reduce the output

ripple voltage to 10mV or 20mV. However, very low ESR capacitors, such as Tantalum capacitors, should be carefully evaluated.

Catch Diode

This diode is required to provide a return path for the inductor current when the switch is off. It should be located close to the

CM2596 using short leads and short printed circuit traces as possible.

To satisfy the need of fast switching speed and low forward voltage drop, Schottky diodes are widely used to provide the best

efficiency, especially in low output voltage switching regulators (less than 5V). Besides, fast-Recovery, high-efficiency, or

ultra-fast recovery diodes are also suitable. But some types with an abrupt turn-off characteristic may cause instability and EMI

problems. A fast-recovery diode with soft recovery characteristics is better choice.