I

C,RMS

u 1.2(

ton

T

) I

LOAD, where:

ton

T

+

V

OUT

V

IN

{buck regulator}, and

ton

T

+

|V

OUT|

(|V

OUT| ) VIN)

{buck−boost regulator}

LM2575

SLVS569F – JANUARY 2005 – REVISED AUGUST 2015

www.ti.com

Typical Application (continued)

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

(2)

8.2.2.2 Output Capacitor (COUT)

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::

Output Ripple Voltage = (ESR of COUT) × (inductor ripple current)

(3)

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

μF to 680 μF. Larger

COUT can reduce the ripple 20 mV to 50 mV peak-to-peak. To improve further on output ripple, paralleling of

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

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

The following should be taken into account when selecting COUT:

•

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 must be used at operating temperature lower than –25°C. As an alternative, tantalums

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

•

Low ESR for COUT is desirable for low output ripple. However, the ESR should be greater than 0.05 Ω to

avoid the possibility of regulator instability. Hence, a sole tantalum capacitor used for COUT is most

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.

8.2.2.3 Catch Diode

As with other external components, the catch diode must 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

performance, especially for switching regulators with low output voltages (VOUT < 5 V). If a high-efficiency, fast-

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

turnoff 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.

8.2.2.4 Inductor

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, because 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.

The LM2575 can operate in either continuous or discontinuous mode. With heavy load currents, the inductor

current flows continuously and the regulator operates in continuous mode. Under light load, the inductor fully

discharges and the regulator is forced into the discontinuous mode of operation. For light loads (approximately

200 mA or less), this discontinuous mode of operation is perfectly acceptable and may be desirable solely to

keep the inductor value and size small. Any buck regulator eventually operates in discontinuous mode when the

load current is light enough.

10

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