LM3687 Datasheet, PDF (24/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
SNVS473A – DECEMBER 2007 – REVISED JULY 2008
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The ESR (Equivalent Series Resistance) of this input capacitor should be in the range of 3mΩ to 300mΩ. The
tolerance and temperature coefficient must be considered when selecting the capacitor to ensure the
capacitance will remain ≥ 470nF over the entire operating temperature range.
OUTPUT CAPACITOR
V_{OUT_DCDC}
A ceramic output capacitor of 10 µF, 6.3V is sufficient for most applications. 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. 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 minimum output capacitance to guarantee good performance is 5.75µF at 1.8V DC bias including
tolerances and over ambient temperature range. 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 functions.
The output voltage ripple is caused by the charging and discharging of the output capacitor and by the R_ESR and
can be calculated as:
Voltage peak-to-peak ripple due to capacitance can be expressed as follow:
V_PP-C =
I_RIPPLE
4xfxC
(3)
Voltage peak-to-peak ripple due to ESR can be expressed as follow:
V_PP-ESR = (2*I_RIPPLE) * R_ESR
Because these two components are out of phase, the rms (root mean squared) value can be used to get an
approximate value of peak-to-peak ripple. The peak-to-peak ripple voltage, rms value can be expressed as
follow:
V_PP-RMS = V_PP-C2 + V_PP-ESR2
(4)
Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series
resistance of the output capacitor (R_ESR). The R_ESR is frequency dependent (as well as temperature dependent);
make sure the value used for calculations is at the switching frequency of the part.
V_{OUT_LIN}
The linear regulator is designed specifically to work with very small ceramic output capacitors. A ceramic
capacitor (dielectric types X7R, Z5U, or Y5V) in the 2.2µF range (up to 10µF) and with an ESR between 3mΩ to
300mΩ is suitable as C_{OUT_LIN} in the LM3687 application circuit.
This capacitor must be located a distance of not more than 1cm from the V_{OUT_LIN} pin and returned to a clean
analogue ground. It is also possible to use tantalum or film capacitors at the device output, V_{OUT_LIN}, but these
are not as attractive for reasons of size and cost (see the section Capacitor Characteristics).
CAPACITOR CHARACTERISTICS
The LM3687 is designed to work with ceramic capacitors on the outputs to take advantage of the benefits they
offer. For capacitance values in the range of 1µF to 4.7µF, 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 1µF ceramic capacitor is in the range of 3mΩ to 40mΩ, which easily meets the ESR requirement for
stability for the LM3687.
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, depending on the operating conditions and
capacitor type.
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