www.ti.com

Catch Diode

Inductor

LM2575

1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATOR

SLVS569E – JANUARY 2005 – REVISED JANUARY 2006

APPLICATION INFORMATION (continued)

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.

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.

The type of inductor chosen can have advantages and disadvantages. If high performance/quality is a concern,

then more-expensive toroid core inductors are the best choice, as the magnetic flux is contained completely

within the core, resulting in less EMI and noise in nearby sensitive circuits. Inexpensive bobbin core inductors,

however, generate more EMI as the open core does confine the flux within the core. Multiple switching regulators

located in proximity to each other are particularly susceptible to mutual coupling of magnetic fluxes from each

other’s open cores. In these situations, closed magnetic structures (such as a toroid, pot core, or E-core) are

more appropriate.

Regardless of the type and value of inductor used, the inductor never should carry more than its rated current.

Doing so may cause the inductor to saturate, in which case the inductance quickly drops, and the inductor looks

like a low-value resistor (from the dc resistance of the windings). As a result, switching current rises dramatically

(until limited by the current-by-current limiting feature of the LM2575) and can result in overheating of the

inductor and the IC itself. Note that different types of inductors have different saturation characteristics.

6