LM3674_15 Datasheet, PDF (14/26 Pages) Texas Instruments – LM3674 2-MHz, 600-mA Step-Down DC-DC Converter in SOT-23

LM3674
SNVS405G – DECEMBER 2005 – REVISED APRIL 2015
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8.2.1.2.2 Input Capacitor Selection
A ceramic input capacitor of 4.7 µF, 6.3 V is sufficient for most applications. Place the input capacitor as close as
possible to the V_IN pin of the device. A larger value may be used for 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 minimum input capacitance to ensure good performance is 2.2 µF at 3-V DC
bias; 1.5 µF at 5-V DC bias including tolerances and over ambient temperature range. The input filter capacitor
supplies current to the PFET switch of the LM3674 in the first half of each cycle and reduces voltage ripple
imposed on the input power source. The low equivalent series resistance (ESR) of a ceramic capacitor provides
the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a capacitor with
sufficient ripple current rating. The input current ripple can be calculated as:
I _RMS = I _OUTMAX x
V_OUT
V_IN
x (1 - ^VOUT
V_IN
+
r²
12
)
r
=
(V_IN - V_OUT ) x V _OUT
L x f x I_OUTMAX x V_IN
The worst case is when
V_IN = 2 x V_OUT
(6)
8.2.1.2.3 Output Capacitor Selection
A ceramic output capacitor of 10 µF, 6.3 V 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 ensure good performance is 5.75 µF at 1.8 V 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:
I _ripple
V_PP-C = _{f x 4 x C}
(7)
Voltage peak-to-peak ripple due to ESR:
V_OUT = VPP-ESR = IPP * RESR
(8)
Because these two components are out of phase, the root mean squared (rms) value can be used to get an
approximate value of peak-to-peak ripple.
Voltage peak-to-peak ripple, rms:
V_PP-RMS = V_PP-C2 + V_PP-ESR2
(9)
Note that the output ripple is dependent on the 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.
14
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