Datasheet

V
ROUT
= I
RIPPLE
x ESR
COUT
and V
COUT
=
8 x F
S
x C
OUT
I
RIPPLE
V
OUT-RIPPLE-PP
=
V
2
ROUT
+ V
2
COUT
8 x F
S
x C
OUT
üI
RIPPLE
where üI
RIPPLE
=
V
OUT
V
IN
2 x L x F
S
D x (V
IN
- V
OUT
)
and D =
V
OUT-RIPPLE-PP
=
VOUT1/2/3
SW1/2/3
L
COUT
ESR
OUTPUT
CAPACITOR
LM10506
SNVS729E –SEPTEMBER 2011–REVISED MARCH 2013 www.ti.com
The ceramic capacitor’s capacitance can vary with temperature. The capacitor type X7R, which operates over a
temperature range of −55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R
has a similar tolerance over a reduced temperature range of −55°C to +85°C. Many large value ceramic
capacitors, larger than 1µF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance
can drop by more than 50% as the temperature varies from 25°C to 85°C. Therefore X7R is recommended over
Z5U and Y5V in applications where the ambient temperature will change significantly above or below 25°C.
Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more
expensive when comparing equivalent capacitance and voltage ratings in the 0.47 µF to 44 µF range. Another
important consideration is that tantalum capacitors have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a tantalum capacitor with an ESR value within the stable range, it
would have to be larger in capacitance (which means bigger and more costly) than a ceramic capacitor with the
same ESR value. It should also be noted that the ESR of a typical tantalum will increase about 2:1 as the
temperature goes from 25°C down to −40°C, so some guard band must be allowed.
Output Capacitor Selection
The output capacitor of a switching converter absorbs the AC ripple current from the inductor and provides the
initial response to a load transient. The ripple voltage at the output of the converter is the product of the ripple
current flowing through the output capacitor and the impedance of the capacitor. The impedance of the capacitor
can be dominated by capacitive, resistive, or inductive elements within the capacitor, depending on the frequency
of the ripple current. Ceramic capacitors have very low ESR and remain capacitive up to high frequencies. Their
inductive component can usually be neglected at the frequency ranges at which the switcher operates.
The output-filter capacitor smooths out the current flow from the inductor to the load and helps maintain a steady
output voltage during transient load changes. It also reduces output voltage ripple. These capacitors must be
selected with sufficient capacitance and low enough ESR to perform these functions.
Note that the output voltage ripple increases with the inductor current ripple and the Equivalent Series
Resistance of the output capacitor (ESR
COUT
). Also note that the actual value of the capacitor’s ESR
COUT
is
frequency and temperature dependent, as specified by its manufacturer. The ESR should be calculated at the
applicable switching frequency and ambient temperature.
(3)
Output ripple can be estimated from the vector sum of the reactive (capacitance) voltage component and the real
(ESR) voltage component of the output capacitor where:
(4)
where:
• V
OUT-RIPPLE-PP
: estimated output ripple,
• V
ROUT
: estimated real output ripple,
• V
COUT
: estimated reactive output ripple. (5)
30 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM10506