Datasheet

www.ti.com
APPLICATION INFORMATION
Input Capacitor (C
IN
)
Output Capacitor (C
OUT
)
Catch Diode
TL2575 , TL2575HV
1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS
SLVS638B – MAY 2006 – REVISED JANUARY 2007
For stability concerns, an input bypass capacitor (electrolytic, C
IN
≥ 47 µ F) needs to be located as close as
possible to the regulator. For operating temperatures below –25 ° C, C
IN
may need to be larger in value. In
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:
I
C,RMS
> 1.2(t
on
/T)I
LOAD
where
t
on
/T = V
OUT
/V
IN
{buck regulator} and
t
on
/T = |V
OUT
|/(|V
OUT
| + V
IN
) {buck-boost regulator}
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 voltage = (ESR of C
OUT
) × (inductor ripple current)
Output ripple of 50 mV to 150 mV typically can be achieved with capacitor values of 220 µ F to 680 µ F. 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.
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.
14
Submit Documentation Feedback