LP3923_15 Datasheet, PDF (25/37 Pages) Texas Instruments – Cellular Phone Power Management Unit

LP3923
www.ti.com
SNVS567B – APRIL 2010 – REVISED MAY 2013
Important: Tantalum capacitors can suffer catastrophic failures due to surge current 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 ensured by the manufacturer to have a surge current rating sufficient for the application.
There are no requirements for the ESR (Equivalent Series Resistance) on the input capacitor, but tolerance and
temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain
within its operational range over the entire operating temperature range and conditions.
OUTPUT CAPACITOR
Correct selection of the output capacitor is critical to ensure stable operation in the intended application.
The output capacitor must meet all the requirements specified in the recommended capacitor table over all
conditions in the application. These conditions include DC-bias, frequency and temperature. Unstable operation
will result if the capacitance drops below the minimum specified value.
The LP3923 is designed specifically to work with very small ceramic output capacitors. The LDOs on the LP3923
are specifically designed to be used with X7R and X5R type capacitors. With these capacitors, selection of the
capacitor for the application is dependant on the range of operating conditions and temperature range for that
application. (See section on CAPACITOR CHARACTERISTICS).
It is also recommended that the output capacitor be placed within 1 cm from the output pin and returned to a
clean ground line.
CAPACITOR CHARACTERISTICS
The LDOs on the LP3923 are designed to work with ceramic capacitors on the input and output to take
advantage of the benefits they offer. For capacitance values around 1µF, ceramic capacitors give the circuit
designer the best design options in terms of low cost and minimal area.
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 dependant on the conditions of operation and
capacitor type.
In particular, to ensure stability, the output capacitor selection should take account of all the capacitor parameters
to ensure that the specification is met within the application. Capacitance value 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 dependant on the particular case size with smaller sizes giving
poorer performance figures in general.
As an example Figure 6 shows a typical graph showing a comparison of capacitor case sizes in a Capacitance
vs. DC Bias plot. As shown in the graph, as a result of the DC Bias condition the capacitance value may drop
below the minimum capacitance value given in the recommended capacitor table (0.7 µF in this case). 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.
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