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
LM26484
SNVS573F –SEPTEMBER 2008–REVISED MARCH 2013 www.ti.com
where
• 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.
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 10 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:
where
• 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:
where
• I
RSCIN
: Estimated input capacitor RMS current (7)
Model Type Vendor Voltage Rating Case Size Inch
(mm)
10 µF for CIN or COUT; C9, C2, C1, C5, C7, C10
GRM21BR60J106K Ceramic, X7R Murata 6.3V 0805, (2012)
JMK212BJ106K Ceramic, X5R Taiyo-Yuden 6.3V 0805, (2012)
LMK212C106KG-T Ceramic, X7R Taiyo-Yuden 10V 0805, (2012)
C1608X5R0J106K Ceramic, X5R TDK 6.3V 0603, (1608)
18 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM26484