Datasheet

0402, 6.3V, X5R
0603, 10V, X5R
0 1.0 2.0 3.0 4.0 5.0
DC BIAS (V)
20%
40%
60%
80%
100%
CAP VALUE (% of NOMINAL 1 PF)
LM26484
SNVS573F –SEPTEMBER 2008–REVISED MARCH 2013 www.ti.com
The Buck control loop will force the voltage on V
FB
to be 0.50V.
Table 4 shows ideal resistor values to establish buck voltages from 0.8V to 3.5V along with common resistor
values to establish these voltages. Common resistors do not always produce the target value, error is given in
the delta column.
In addition to the resistor feedback, feedback capacitors are also required. (Table 4 — C3/4/6/8) When choosing
the output voltage for the two bucks, please take into account the fact that, the factory has optimized the
accuracy of Buck1 at the top end of the V
OUT
range and Buck2 for the bottom end of the V
OUT
range.
Figure 19. Typical Variation in Capacitance vs. DC Bias
As shown in Table 4, 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 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 temperature. The capacitor type X7R, which operates over a
temperature range of −55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R
has a similar tolerance 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 temperature varies from 25°C to 85°C. Therefore X7R is recommended 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 equivalent size ceramics. This
means that while it may be possible to find a tantalum capacitor with an ESR value within the stable 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.
OUTPUT INDUCTORS & CAPACITORS FOR SW1 AND SW2
There are several design considerations related to the selection of output inductors and capacitors:
• Load transient response;
• Stability;
• Efficiency;
• Output ripple voltage; and
• Over-current ruggedness.
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 TI sales with any concerns.
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