Datasheet
I
RMSCIN
=
©
§
2
D x
I
OUT
2
+
©
§
12
I
RIPPLE
I
OUT
x D
C
IN
x F
V
PPIN
=
+ I
OUT
x ESR
CIN
I
RIPPLE
V
COUT
=
8 x F x C
OUT
V
ROUT
= I
RIPPLE
x ESR
COUT
V
PPOUT
=
V
COUT
2
+ V
ROUT
2
LM26480
www.ti.com
SNVS543I –JANUARY 2008–REVISED MAY 2013
• V
COUT
: Estimated reactive output ripple
• V
ROUT
: Estimated real output ripple
• V
PPOUT
: Estimated peak-to-peak output ripple (5)
The output capacitor needs to be mounted as close as possible to the output pin of the device. For better
temperature performance, X7R or X5R types are recommended. 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 by case size. DC bias curves should be
requested from them as part of the capacitor selection process. ESR is typically higher for smaller packages.
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 (ESR
COUT
). ESR
COUT
is frequency dependent as well as temperature
dependent. The R
ESR
should be calculated with the applicable switching frequency and ambient temperature.
Input Capacitor Selection for SW1 AND SW2
It is required to use a ceramic input capacitor of at least 4.7 μF and 6.3V with an ESR of less than 500 mΩ.
The input power source supplies average current continuously. During the PFET switch on-time, however, the
demanded di/dt is higher than can be typically supplied by the input power source. This delta is supplied by the
input capacitor.
A simplified “worst case” assumption is that all of the PFET current is supplied by the input capacitor. This will
result in conservative estimates of input ripple voltage and capacitor RMS current. Input ripple voltage is
estimated as follows:
• V
PPIN
: Estimated peak-to-peak input ripple voltage
• I
OUT
: Output current, Amps
• C
IN:
Input capacitor value, Farads
• ESR
IN:
Input capacitor ESR, Ohms (6)
This capacitor is exposed to significant RMS current, so it is important to select a capacitor with an adequate
RMS current rating. Capacitor RMS current estimated as follows:
• I
RSCIN
: Estimated input capacitor RMS current (7)
Model Type Vendor Voltage Rating Case Size
4.7 µF for CIN
C2012X5R0J475K Ceramic, X5R TDK 6.3V 0805, (2012)
JMK212BJ475K Ceramic, X5R Taiyo-Yuden 6.3V 0805, (2012)
GRM21BR60J475K Ceramic, X5R Murata 6.3V 0805, (2012)
C1608X5R0J475K Ceramic, X5R TDK 6.3V 0603, (1608)
10 µF for COUT
GRM21BR60J106K Ceramic, X5R Murata 6.3V 0805, (2012)
JMK212BJ106K Ceramic, X5R Taiyo-Yuden 6.3V 0805, (2012)
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