lm26484sqx National Semiconductor Corporation, lm26484sqx Datasheet - Page 14

• Manufacturer Part Number: lm26484sqx
• Description: Power Management Unit
• Manufacturer: National Semiconductor Corporation

www.national.com
As shown in , increasing the DC Bias condition can result in
a capacitance value that falls below the minimum value given
in the recommended capacitor specifications table. Note that
the graph shows the capacitance out of spec for the 0402
case size capacitor at higher bias voltages. It is therefore rec-
ommended that the capacitor manufacturers’ specifications
for the nominal value capacitor are consulted for all condi-
tions, 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 44 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.
FIGURE 4. Typical Variation in Capacitance vs. DC Bias
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OUTPUT INDUCTORS & CAPACITORS FOR SW1 AND
SW2
There are several design considerations related to the selec-
tion of output inductors and capacitors:
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The LM26484 has been optimized for use with nominal values
0.5 µH and 22 µF. If other values are needed for the design,
please contact National Semiconductor sales with any con-
cerns.
INDUCTOR SELECTION FOR SW1 AND SW2
A nominal inductor value of 0.5 µH is recommended. It is im-
portant to guarantee the inductor core does not saturate
during any foreseeable operational situation.
Care should be taken when reviewing the different saturation
current ratings that are specified by different manufacturers.
Saturation current ratings are typically specified at 25ºC, so
ratings at maximum ambient temperature of the application
should be requested from the manufacturer.
There are two methods to choose the inductor saturation cur-
rent rating:
Recommended method:
The best way to guarantee the inductor does not saturate is
to choose an inductor that has saturation current rating
greater than the maximum LM26484 current limit of 3.0A. In
this case the device will prevent inductor saturation.
Alternate method:
If the recommended approach cannot be used, care must be
taken to guarantee that the saturation current is greater than
the peak inductor current:
Load transient response;
Stability;
Efficiency;
Output ripple voltage; and
Over-current ruggedness.
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