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Input Capacitor (C

I

C,RMS

u 1.2(

T

) I

T

+

V

OUT

V

IN

{buck regulator}, and

T

+

|V

(|V

{buck−boost regulator}

Output Capacitor (C

Catch Diode

Inductor

LM2575

1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATOR

SLVS569D – JANUARY 2005 – REVISED JULY 2005

APPLICATION INFORMATION (continued)

possible to the regulator. For operating temperatures below –25

°C, C

addition, since most electrolytic capacitors have decreasing capacitances and increasing ESR as temperature

drops, adding a ceramic or solid tantalum capacitor in parallel increases the stability in cold temperatures.

To extend the capacitor operating lifetime, the capacitor RMS ripple current rating should be:

For both loop stability and filtering of ripple voltage, an output capacitor also is required, again in close proximity

to the regulator. For best performance, low-ESR aluminum electrolytics are recommended, although standard

aluminum electrolytics may be adequate for some applications. Based on the following equation:

Output ripple of 50 mV to 150 mV typically can be achieved with capacitor values of 220

µF to 680 µF. Larger

standard electrolytic capacitors may be used. Alternatively, higher-grade capacitors such as “high frequency”,

“low inductance”, or “low ESR” can be used.

•

At cold temperatures, the ESR of the electrolytic capacitors can rise dramatically (typically 3

× nominal value

at –25

°C). Because solid tantalum capacitors have significantly better ESR specifications at cold

temperatures, they should be used at operating temperature lower than –25

°C. As an alternative, tantalums

also can be paralleled to aluminum electrolytics and should contribute 10% to 20% to the total capacitance.

•

susceptible to this occurrence.

•

The capacitor’s ripple current rating of 52 kHz should be at least 50% higher than the peak-to-peak inductor

ripple current.

As with other external components, the catch diode should be placed close to the output to minimize unwanted

noise. Schottky diodes have fast switching speeds and low forward voltage drops and, thus, offer the best

fast-recovery, or ultra-fast-recovery diode is used in place of a Schottky, it should have a soft recovery (versus

abrupt turn-off characteristics) to avoid the chance of causing instability and EMI. Standard 50-/60-Hz diodes,

such as the 1N4001 or 1N5400 series, are NOT suitable.

Proper inductor selection is key to the performance-switching power-supply designs. One important factor to

consider is whether the regulator will be used in continuous (inductor current flows continuously and never drops

to zero) or in discontinuous mode (inductor current goes to zero during the normal switching cycle). Each mode

has distinctively different operating characteristics and, therefore, can affect the regulator performance and

requirements. In many applications, the continuous mode is the preferred mode of operation, since it offers

greater output power with lower peak currents, and also can result in lower output ripple voltage. The advantages

of continuous mode of operation come at the expense of a larger inductor required to keep inductor current

continuous, especially at low output currents and/or high input voltages.

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