LP3921 Datasheet, PDF (24/34 Pages) National Semiconductor (TI) – Battery Charger Management and Regulator Unit with Integrated Boomer® Audio Amplifier

TABLE 18. LDO Output Programming
Register
Add
(hex)
Name
Data Range Output Voltage
(hex)
01
LDO1PGM
03 - 0F 1.5V to 3.3V
O/P
(def. 1.8V)
02
LDO2PGM
00 - 0F 2.5V to 3.3V
O/P
(def 3.0V)
03
LDO3PGM
05 - 0C 2.7V to 3.05V
O/P
(def 3.0V)
04
LDO4PGM
00 - 0F 1.5V to 3.3V
O/P
(def 3.0V)
05
LDO5PGM
05 - 0C 2.7V to 3.05V
O/P
(def 3.0V)
06
LDO6PGM
05 - 0C 2.7V to 3.05V
O/P
(def 3.0V)
07
LDO7PGM
00 - 0F 1.5V to 3.3V
O/P
(def 3.0V)
See Table 4 for full programmable range of values.
EXTERNAL CAPACITORS
The Low Drop Out Linear Voltage regulators on the LP3921
require external capacitors to ensure stable outputs. The
LDO's on the LP3921 are specifically designed to use small
surface mount ceramic capacitors which require minimum
board space. These capacitors must be correctly selected for
good performance
INPUT CAPACITOR
Input capacitors are required for correct operation. It is rec-
ommended that a 10 µF capacitor be connected between
each of the voltage input pins 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 analogue ground. A ceramic capacitor
is recommended although a good quality tantalum or film ca-
pacitor may be used at the input.
Warning:
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 guaranteed
by the manufacturer to have surge current rating sufficient for
the application. There are no requirements for the ESR (Equiv-
alent 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 ca-
pacitor must meet all the requirements specified in the rec-
ommended capacitor table over all conditions in the applica-
tion. These conditions include DC-bias, frequency and
temperature. Unstable operation will result if the capacitance
drops below the minimum specified value.
The LP3921 is designed specifically to work with very small
ceramic output capacitors. The LDO's on the LP3921 are
specifically designed to be used with X7R and X5R type ca-
pacitors. With these capacitors selection of the capacitor for
the application is dependant on the range of operating con-
ditions and temperature range for that application. (See sec-
tion 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 LDO's on the LP3921 are designed to work with ceramic
capacitors on the input and output to take advantage of the
benefits they offer. For capacitance values around 1 µF, ce-
ramic capacitors give the circuit designer the best design
options in terms of low cost and minimal area.
Generally speaking, input and output capacitors require care-
ful understanding of the capacitor specification to ensure
stable and correct device operation. Capacitance value can
vary with DC bias conditions as well as temperature and fre-
quency 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 perfor-
mance figures in general.
FIGURE 8. DC Bias (V)
30069812
As an example, Figure 8 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 DC Bias con-
dition the capacitance value may drop below minimum ca-
pacitance value given in the recommended capacitor table
(0.7 µF in this case). Note that the graph shows the capaci-
tance out of spec for 0402 case size capacitor at higher bias
voltages. It is therefore recommended that the capacitor man-
ufacturers 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. Ceramic
capacitors have the lowest ESR values, thus making them
best for eliminating high frequency noise. The ESR of a typical
1 µF ceramic capacitor is in the range of 20 mΩ to 40 mΩ,
and also meets the ESR requirements for stability. The tem-
perature performance of ceramic capacitors varies by type.
Capacitor type X7R is specified with a tolerance of ±15% over
temperature range -55ºC to +125ºC. The X5R has similar tol-
erance over the reduced temperature range -55ºC to +85ºC.
Most large value ceramic capacitors (<2.2 µF) are manufac-
tured with Z5U or Y5V temperature characteristics, which
results in the capacitance dropping by more than 50% as the
temperature goes from 25ºC to 85ºC. Therefore X7R is rec-
ommended over these other capacitor types in applications
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