LP3971 Datasheet, PDF (43/57 Pages) National Semiconductor (TI) – POWER MANAGEMENT UNIT FOR ADVANCED APPLICATION PROCESSORS

LP3971
www.ti.com
SNVS432U – JANUARY 2006 – REVISED OCTOBER 2008
in the capacitance value falling 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.
100%
0603, 10V, X5M
80%
60%
40%_
20%
_₀
0402, 6.3V, X5R
2.0
3.0
4.0
5.0
1.0
_
_
_
_
DC BIAS (V)
Figure 10. Graph Showing a Typical Variation in Capacitance vs. DC Bias
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 4.7 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.
BUCK CONSIDERATIONS
Inductor Selection
There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor
current ripple is small enough to achieve the desired output voltage ripple. Different saturation current rating
specs are followed by different manufacturers so attention must be given to details. Saturation current ratings are
typically specified at 25°C so ratings at max ambient temperature of application should be requested from
manufacturer.
There are two methods to choose the inductor saturation current rating.
Method 1
The saturation current is greater than the sum of the maximum load current and the worst case average to peak
inductor current. This can be written as
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