1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATOR
SLVS569E – JANUARY 2005 – REVISED JANUARY 2006
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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
C
OUT
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 C
OUT
:
•
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.
•
Low ESR for C
OUT
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 C
OUT
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.
Catch Diode
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
performance, especially for switching regulators with low output voltages (V
OUT
< 5 V). If a high-efficiency,
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.
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, 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
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