LP3905SD-30EV National Semiconductor, LP3905SD-30EV Datasheet - Page 12

• Manufacturer Part Number: LP3905SD-30EV
• Description: BOARD EVALUATION LP3905SD-30
• Manufacturer: National Semiconductor

Specifications of LP3905SD-30EV

• Main Purpose: DC/DC, Step Down with LDO
• Outputs And Type: 4, Non-Isolated
• Voltage - Output: 1.2V, 1.875, 2.8V, 2.8V
• Current - Output: 600mA, 600mA, 100mA, 100mA
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LP3905
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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Application Information
Buck DC/DC Convertor Input Capacitor Selection
A ceramic input capacitor of 10µF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible
to the V
improved input voltage filtering. Use X7R or X5R types, do
not use Y5V. DC bias characteristics of ceramic capacitors
must be considered when selecting case sizes like 0805 and
0603. The input filter capacitor supplies current to the PFET
switch of the LP3905 in the first half of each cycle and
reduces voltage ripple imposed on the input power source. A
ceramic capacitor’s low ESR provides the best noise filtering
of the input voltage spikes due to this rapidly changing
current. Select a capacitor with sufficient ripple current rat-
ing. The input current ripple can be calculated as:
DC/DC CONVERTOR OUTPUT CAPACITOR SELECTION
Use a 10µF, 6.3V ceramic capacitor. Use X7R or X5R types,
do not use Y5V. DC bias characteristics of ceramic capaci-
tors must be considered when selecting case sizes like 0805
and 0603. DC bias characteristics vary from manufacturer to
manufacturer and dc bias curves should be requested from
them as part of the capacitor selection process.
The output filter capacitor smoothes out current flow from the
inductor to the load, helps maintain a steady output voltage
during transient load changes and reduces output voltage
ripple. These capacitors must be selected with sufficient
capacitance and sufficiently low ESR to perform these func-
tions.
The output voltage ripple is caused by the charging and
discharging of the output capacitor and also due to its R
and can be calculated as:
Voltage peak-to-peak ripple due to capacitance can be ex-
pressed as follows
Voltage peak-to-peak ripple due to ESR can be expressed
as follows
V
Because these two components are out of phase the rms
value can be used to get an approximate value of peak-to-
peak ripple.
Voltage peak-to-peak ripple, root mean squared can be ex-
pressed as follows
PP-ESR
IN
= (2 * I
pin of the device. A larger value may be used for
RIPPLE
) * R
ESR
(Continued)
ESR
12
Note that the output voltage ripple is dependent on the
inductor current ripple and the equivalent series resistance
of the output capacitor (R
The R
dependent); make sure the value used for calculations is at
the switching frequency of the part.
LINEAR REGULATORS
Capacitor Selection
The LP3955 is designed to work with ceramic capacitors on
the output to take advantage of the benefits they offer: for
capacitance values in the range of 0.47µF to 10µF range,
ceramic capacitors are the smallest, least expensive and
have the lowest ESR values (which makes them best for
eliminating high frequency noise). The ESR of a typical
1µFceramic capacitor is in the range of 20mW to 40mW,
which easily meets the ESR requirement for stability by the
LP3955. For both input and output capacitors careful inter-
pretation of the capacitor specification is required to ensure
correct device operation. The capacitor value can change
greatly dependant on the conditions of operation and capaci-
tor 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. 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 pa-
rameters are also dependant on the particular case size with
smaller sizes giving poorer performance figures in general.
As an example Figure 11 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 mini-
mum capacitance value given in the recommended capacitor
table (0.7µF 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 applica-
tion.
FIGURE 11. Capacitor Performance (DC Bias)
ESR
is frequency dependent (as well as temperature
ESR
).
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