RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
important parameter.
Application Information (Continued)
A graph shown in Figure 13 shows the relationship between
an electrolytic capacitor value, its voltage rating, and the
RMS current it is rated for. These curves were obtained from
the Nichicon “PL” series of low ESR, high reliability electro-
lytic capacitors designed for switching regulator applications.
Other capacitor manufacturers offer similar types of capaci-
tors, but always check the capacitor data sheet.
The output capacitor requires an ESR value that has an
upper and lower limit. For low output ripple voltage, a low
ESR value is needed. This value is determined by the maxi-
mum allowable output ripple voltage, typically 1% to 2% of
the output voltage. But if the selected capacitor’s ESR is
extremely low, there is a possibility of an unstable feedback
loop, resulting in an oscillation at the output. Using the
capacitors listed in the tables, or similar types, will provide
design solutions under all conditions.
“Standard” electrolytic capacitors typically have much higher
ESR numbers, lower RMS current ratings and typically have
a shorter operating lifetime.
Because of their small size and excellent performance, sur-
face mount solid tantalum capacitors are often used for input
bypassing, but several precautions must be observed. A
small percentage of solid tantalum capacitors can short if the
inrush current rating is exceeded. This can happen at turn on
when the input voltage is suddenly applied, and of course,
higher input voltages produce higher inrush currents. Sev-
eral capacitor manufacturers do a 100% surge current test-
ing on their products to minimize this potential problem. If
high turn on currents are expected, it may be necessary to
limit this current by adding either some resistance or induc-
tance before the tantalum capacitor, or select a higher volt-
age capacitor. As with aluminum electrolytic capacitors, the
RMS ripple current rating must be sized to the load current.
If very low output ripple voltage (less than 15 mV) is re-
quired, refer to the section on Output Voltage Ripple and
Transients for a post ripple filter.
An aluminum electrolytic capacitor’s ESR value is related to
the capacitance value and its voltage rating. In most cases,
higher voltage electrolytic capacitors have lower ESR values
(see Figure 14 ). Often, capacitors with much higher voltage
ratings may be needed to provide the low ESR values re-
quired for low output ripple voltage.
The output capacitor for many different switcher designs
often can be satisfied with only three or four different capaci-
tor values and several different voltage ratings. See the
quick design component selection tables in Figure 2 and 4
for typical capacitor values, voltage ratings, and manufactur-
ers capacitor types.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
Electrolytic capacitors are not recommended for tempera-
tures below −25˚C. The ESR rises dramatically at cold tem-
CFF —A Feedforward Capacitor CFF, shown across R2 in
Figure 1 is used when the ouput voltage is greater than 10V
or when COUT has a very low ESR. This capacitor adds lead
compensation to the feedback loop and increases the phase
margin for better loop stability. For CFF selection, see the
design procedure section.
@
peratures and typically rises 3X
−25˚C and as much as
10X at −40˚C. See curve shown in Figure 15.
Solid tantalum capacitors have a much better ESR spec for
cold temperatures and are recommended for temperatures
below −25˚C.
01258329
01258328
FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
FIGURE 13. RMS Current Ratings for Low ESR
Electrolytic Capacitors (Typical)
CATCH DIODE
Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
a fast diode and must be located close to the LM2596 using
short leads and short printed circuit traces.
OUTPUT CAPACITOR
COUT —An output capacitor is required to filter the output
and provide regulator loop stability. Low impedance or low
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When select-
ing an output capacitor, the important capacitor parameters
are; the 100 kHz Equivalent Series Resistance (ESR), the
Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
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