LP3910SQ-AN Datasheet, PDF (54/60 Pages) National Semiconductor (TI) – Power Management IC for Hard Drive Based Portable Media Players

Application Notes
COMPONENT SELECTION
Inductors for BUCK1, BUCK2 and BUCK-BOOST
There are two main considerations when choosing an induc-
tor; the inductor should not saturate and the inductor current
ripple is small enough to achieve the desired output voltage
ripple. Care should be taken when reviewing the different sat-
uration current ratings that are specified by different manu-
facturers.
Saturation current ratings are typically specified at 25°C, 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:
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 follows:
30101233
The followings need to be considered when using the buck-
boost in boost mode:
30101234
I_RIPPLE:
I_OUTMAX:
V_IN:
L:
f:
V_OUT:
Average to peak inductor current
Maximum load current
Maximum input voltage to the buck
Min inductor value including worse case
tolerances (30% drop can be considered for
method 1)
Minimum switching frequency (1.6 mHz)
Buck Output voltage
Method 2:A more conservative and recommended approach
is to choose an inductor that has saturation current rating
greater than the maximum current limit of TBA.
Inductor
L_SW1,2
L_BB
Value Unit Description
Notes
2.2 µH BUCK1,2 Inductor D.C.R.
70 mΩ
2.2 µH Buck-Boost
D.C.R.
Inductor
70 mΩ
External Capacitors
The regulators on the LP3910SQ-AN require external capac-
itors for regulator stability. These are specifically designed for
portable applications requiring minimum board space and
smallest components. These capacitors must be correctly se-
lected for good performance.
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.
Important: Tantalum capacitors can suffer catastrophic fail-
ures due to surge currents when connected to a low
impedance source of power (like a battery or a very large ca-
pacitor). If a tantalum capacitor is used at the input, it must be
guaranteed by the manufacturer to have a surge current rat-
ing sufficient for the application.
There are no requirements for the ESR (Equivalent Series
Resistance) on the input capacitor, but tolerance and tem-
perature coefficient must be considered when selecting the
capacitor to ensure the capacitance will remain approximately
1.0 µF over the entire operating temperature range.
Output Capacitor
The LDOs on the LP3910SQ-AN are designed specifically to
work with very small ceramic output capacitors. A 1.0 μF ce-
ramic capacitor (temperature types Z5U, Y5V or X7R) with
ESR between 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, C_OUT (or V_OUT), but these are not as attractive
for reasons of size and cost.
The output capacitor must meet the requirement for the min-
imum 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 LDO’s.
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
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