Datasheet

LM3668
SNVS449M –JUNE 2007–REVISED MAY 2013 www.ti.com
Input Capacitor Selection
A ceramic input capacitor of at least 10 µF, 6.3V is sufficient for most applications. Place the input capacitor as
close as possible to the PV
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 or 0603. The input filter capacitor supplies current to the PFET switch of the
LM3668 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. For applications where input voltage is 4V or higher, it is best to use a higher voltage rating capacitor to
eliminate the DC bias affect over capacitance.
Output Capacitor Selection
A ceramic output capacitor of 22 µF, 6.3V (use 10V or higher rating for 4.5/5.0V output option) is sufficient for
most applications. Multilayer ceramic capacitors such as X5R or X7R with low ESR is a good choice for this as
well. These capacitors provide an ideal balance between small size, cost, reliability and performance. Do not use
Y5V ceramic capacitors as they have poor dielectric performance over temperature and poor voltage
characteristic for a given value. In other words, ensure the minimum C
OUT
value does not exceed −40% of the
above-suggested value over the entire range of operating temperature and bias conditions.
Extra attention is required if a smaller case size capacitor is used in the application. Smaller case size capacitors
typically have less capacitance for a given bias voltage as compared to a larger case size capacitor with the
same bias voltage. Please contact the capacitor manufacturer for detailed information regarding capacitance
verses case size. Table 4 lists several capacitor suppliers.
The output filter capacitor smooths 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.
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.
Table 4. Suggested Capacitors and Suppliers
Case Size
Model Type Vendor Voltage Rating
Inch (mm)
10 µF for C
IN
(For 4.5/5.0V option, use 10V or higher rating capacitor)
GRM21BR60J106K Ceramic, X5R Murata 6.3V 0805 (2012)
JMK212BJ106K Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012)
C2012X5R0J106K Ceramic, X5R TDK 6.3V 0805 (2012)
LMK212 BJ106MG (±20%) Ceramic, X5R Taiyon-Yuden 10V 0806(2012)
LMK212 BJ106KG (±10%) Ceramic, X5R Taiyon-Yuden 10V 0805(2012)
22 µF for C
OUT
(For 4.5/5.0V option, use 10V or higher rating capacitor)
JMK212BJ226MG Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012)
LMK212BJ226MG Ceramic, X5R Taiyo-Yuden 10V 0805 (2012)
Layout Considerations
As for any high frequency switcher, it is important to place the external components as close as possible to the
IC to maximize device performance. Below are some layout recommendations:
1. Place input filter and output filter capacitors close to the IC to minimize copper trace resistance which will
directly effect the overall ripple voltage.
2. Route noise sensitive trace away from noisy power components. Separate power GND (Noisy GND) and
Signal GND (quiet GND) and star GND them at a single point on the PCB preferably close to device GND.
3. Connect the ground pins and filter capacitors together via a ground plane to prevent switching current
circulating through the ground plane. Additional layout consideration regarding the WSON package can be
found in TI Application Note AN1187.
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