LTC3255
11
3255f
For more information www.linear.com/LTC3255
APPLICATIONS INFORMATION
V
OUT
Ripple and Capacitor Selection
The type and value of capacitors used with the LTC3255
determine several important parameters such as output
ripple and charge pump strength. The value of C
OUT
directly controls the amount of output ripple for a given
load current. Increasing the size of C
OUT
will reduce the
output ripple.
To reduce output noise and ripple, it is suggested that a
low ESR (equivalent series resistance < 0.1Ω) ceramic
capacitor (10μF or greater) be used for C
OUT
. Ceramic
capacitors typically have exceptionally low ESR which,
combined with a tight board layout, should yield excellent
performance. Tantalum and aluminum capacitors can be
used in parallel with a ceramic capacitor to increase the
total capacitance but are not recommended to be used
alone because of their high ESR.
V
IN
Capacitor Selection
The
total amount and type of capacitance necessary for
input bypassing is very dependent on the impedance of
the input power source as well as existing bypassing al-
ready on the V
IN
node. For optimal input noise and ripple
reduction, it is recommended that a low ESR ceramic
capacitor be used for C
IN
bypassing. Low ESR will reduce
the voltage steps caused by changing input current, while
the absolute capacitor value will determine the level of
ripple. An electrolytic or tantalum capacitor may be used
in parallel with the ceramic capacitor on C
IN
to increase
the total capacitance, but due to the higher ESR, it is not
recommended that an electrolytic or tantalum capacitor be
used alone for input bypassing. The LTC3255 will operate
with capacitors less than 1μF, but depending on the
source
impedance, input noise can feed through to the output
causing degraded performance. For best performance,
1μF or greater total capacitance is suggested for C
IN
.
Flying Capacitor Selection
The flying capacitor should always be a ceramic type.
Polarized capacitors such as tantalum or aluminum electro-
lytics are not recommended. The flying capacitor controls
the strength of the charge pump. In order to achieve the
rated output
current, it is necessary for the flying capaci-
tor to have at least 0.4μF of capacitance over operating
temperature with a bias voltage equal to the programmed
V
OUT
(see Ceramic Capacitor Selection Guidelines). The
voltage rating of the ceramic capacitor should be V
OUT
+
1V or greater.
Ceramic Capacitor Selection Guidelines
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates.
For
example, a ceramic capacitor made of X5R or X7R
material will retain most of its capacitance from –40°C
to 85°C, whereas a Z5U or Y5V style capacitor will lose
considerable capacitance over that range (60% to 80%
loss typical). Z5U and Y5V capacitors may also have a
very strong voltage coefficient, causing them to lose an
additional 60% or more of their capacitance when
the rated
voltage is applied. Therefore, when comparing different
capacitors, it is often more appropriate to compare the
amount of achievable capacitance for a given case size
rather than discussing the specified capacitance value. For
example, over rated voltage and temperature conditions,
a 4.7μF, 10V, Y5V ceramic capacitor in an 0805 case may
not provide any more capacitance than a 1μF, 10V, X5R
or X7R
available in the same 0805 case. In fact, over bias
and temperature range, the 1μF, 10V, X5R or X7R will
provide more capacitance than the 4.7μF, 10V, Y5V. The
capacitor manufacturer’s data sheet should be consulted
to determine what value of capacitor is needed to ensure
minimum capacitance values are met over operating
temperature and bias voltage. Table 1 is a list of ceramic
capacitor
manufacturers in alphabetical order:
