WT2575M
INPUT CAPACITOR CIN —A low ESR aluminum or tantalum bypass capacitor is needed between the
input pin and ground pin. It must be located near the regulator using short leads. This capacitor prevents
large voltage transients from appearing at the input, and provides the instantaneous current needed
each time the switch turns on. Selecting an input capacitor requires consulting the manufacturers data
sheet for maximum allowable RMS ripple current. For a maximum ambient temperature of 40°C, a
general guideline would be to select a capacitor with a ripple current rating of approximately 50% of the
DC load current. For ambient temperatures up to 70°C, a current rating of 75% of the DC load current
would be a good choice for a conservative design. The capacitor voltage rating must be at least 1.25
times greater than the maximum input voltage, and often a much higher voltage capacitor is needed to
satisfy the RMS current requirements.
OUTPUT CAPACITOR COUT —-An output capacitor is required to filter the output and provide regulator
loop stability. Low impedance or low ESR Electrolytic or solid tantalum capacitors designed for switching
regulator applications must be used. When selecting an output capacitor, the important capacitor
parameters are; the 100 kHz Equivalent Series resistance (ESR), the RMS ripples current rating, voltage
rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter.
The output capacitor requires an ESR value that has an upper and lower limit. For low output ripple
voltage, a low ESR value is needed. This value is determined by the maximum allow able output ripple
voltage, typically 1% to 2% of the output voltage. But if the selected capacitor’s ESR is extremely low,
there is a possibility of an unstable feedback loop, resulting in an oscillation at the output.
CATCH DIODE D —-Buck regulators require a diode to provide a return path for the inductor current
when the switch turns off. This must be a fast diode and must be located close to the WT2575 using
short leads and short printed circuit traces. Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance, especially in low output voltage applications
(5V and lower). Ultra fast recovery, or High-Efficiency rectifiers are also a good choice, but some types
with an abrupt turnoff characteristic may cause instability or EMI problems.
INDUCTOR SELECTION L —-All switching regulators have two basic modes of operation; continuous
and discontinuous. The difference between the two types relates to the inductor current, whether it is
flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. Each mode
has distinctively different operating characteristics, which can affect the regulators performance and
requirements. Most switcher designs will operate in the discontinuous mode when the load current is low.
The WT2575 (or any of the Simple Switcher family) can be used for both continuous and discontinuous
modes of operation. There is a formula for general applications:
(Note The unit of voltage is V, the unit of current is A)
WEITRON
http://www.weitron.com.tw
28-Aug-06
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