lm26480sqx-bf National Semiconductor Corporation, lm26480sqx-bf Datasheet - Page 25

• Manufacturer Part Number: lm26480sqx-bf
• Description: Lm26480 Externally Programmable Dual High-current Step-down Dc/dc And Dual Linear Regulators
• Manufacturer: National Semiconductor Corporation

LDO CAPACITOR SELECTION
Input Capacitor
An input capacitor is required for stability. It is recommended
that a 1.0 μF capacitor be connected between the LDO input
pin and ground (this capacitance value may be increased
without limit). This capacitor must be located a distance of not
more than 1 cm from the input pin and returned to a clean
analog ground. Any good quality ceramic, tantalum, or film
capacitor may be used at the input.
Warning: Important: Tantalum capacitors can suffer catastrophic failures
Output Capacitor
The LDOs on the LM26480 are designed specifically to work
with very small ceramic output capacitors. A 1.0 μF ceramic
capacitor (temperature types Z5U, Y5V or X7R) with ESR be-
tween 5 mΩ to 500 mΩ, are suitable in the application circuit.
It is also possible to use tantalum or film capacitors at the
device output COUT (or VOUT), but these are not as attrac-
tive for reasons of size and cost. The output capacitor must
meet the requirement for the minimum value of capacitance
and also have an ESR value that is within the range 5 mΩ to
500 mΩ for stability.
Capacitor Characteristics
The LDOs are designed to work with ceramic capacitors on
the output to take advantage of the benefits they offer. For
capacitance values in the range of 0.47 μF to 4.7 μF, ceramic
capacitors are the smallest, least expensive and have the
lowest ESR values, thus making them best for eliminating
high frequency noise. The ESR of a typical 1.0 μF ceramic
capacitor is in the range of 20 mΩ to 40 mΩ, which easily
meets the ESR requirement for stability for the LDOs.
For both input and output capacitors, careful interpretation of
the capacitor specification is required to ensure correct device
operation. The capacitor value can change greatly, depend-
ing on the operating conditions and capacitor type.
In particular, the output capacitor selection should take ac-
count of all the capacitor parameters, to ensure that the
specification is met within the application. The capacitance
can vary with DC bias conditions as well as temperature and
frequency of operation. Capacitor values will also show some
decrease over time due to aging. The capacitor parameters
are also dependent on the particular case size, with smaller
sizes giving poorer performance figures in general. As an ex-
ample, the graph below shows a typical graph comparing
Capacitor
CLDO1
CLDO2
CSW1
CSW2
due to surge currents when connected to a low impedance
source of power (like a battery or a very large capacitor). If a
tantalum capacitor is used at the input, it must be guaranteed
by the manufacturer to have a surge current rating sufficient for
the application.
There are no requirements for the ESR on the input capacitor,
but tolerance and temperature coefficient must be considered
when selecting the capacitor to ensure the capacitance will re-
main approximately 1.0 μF over the entire operating tempera-
ture range.
Min Value
0.47
0.47
10
10
Unit
µF
µF
µF
µF
Description
LDO1 output capacitor Ceramic, 6.3V, X5R
LDO2 output capacitor Ceramic, 6.3V, X5R
SW1 output capacitor
SW2 output capacitor
25
different capacitor case sizes in a capacitance vs. DC bias
plot.
As shown in the graph, increasing the DC bias condition can
result in the capacitance value that falls below the minimum
value given in the recommended capacitor specifications ta-
ble. Note that the graph shows the capacitance out of spec
for the 0402 case size capacitor at higher bias voltages. It is
therefore recommended that the capacitor manufacturers’
specifications for the nominal value capacitor are consulted
for all conditions, as some capacitor sizes (e.g. 0402) may not
be suitable in the actual application.
The ceramic capacitor’s capacitance can vary with tempera-
ture. The capacitor type X7R, which operates over a temper-
ature range of −55°C to +125°C, will only vary the capacitance
to within ±15%. The capacitor type X5R has a similar toler-
ance over a reduced temperature range of −55°C to +85°C.
Many large value ceramic capacitors, larger than 1 μF are
manufactured with Z5U or Y5V temperature characteristics.
Their capacitance can drop by more than 50% as the tem-
perature varies from 25°C to 85°C. Therefore X7R is recom-
mended over Z5U and Y5V in applications where the ambient
temperature will change significantly above or below 25°C.
Tantalum capacitors are less desirable than ceramic for use
as output capacitors because they are more expensive when
comparing equivalent capacitance and voltage ratings in the
0.47 μF to 4.7 μF range. Another important consideration is
that tantalum capacitors have higher ESR values than equiv-
alent size ceramics. This means that while it may be possible
to find a tantalum capacitor with an ESR value within the sta-
ble range, it would have to be larger in capacitance (which
means bigger and more costly) than a ceramic capacitor with
the same ESR value. It should also be noted that the ESR of
a typical tantalum will increase about 2:1 as the temperature
goes from 25°C down to −40°C, so some guard band must
be allowed.
Recommended Type
Ceramic, 6.3V, X5R
Ceramic, 6.3V, X5R
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