LTC3225EDDB#PBF Linear Technology, LTC3225EDDB#PBF Datasheet - Page 9

• Manufacturer Part Number: LTC3225EDDB#PBF
• Manufacturer: Linear Technology

Specifications of LTC3225EDDB#PBF

• Lead Free Status / Rohs Status: Compliant

APPLICATIONS INFORMATION
V
The type and value of C
present at the input pin (V
it is recommended that low equivalent series resistance
(ESR) multilayer ceramic chip capacitors (MLCCs) be
used for C
recommended because of their high ESR.
The input current to the LTC3225/LTC3225-1 is relatively
constant during both the input charging phase and the
output charging phase but drops to zero during the clock
non-overlap times. Since the non-overlap time is small
(~40ns) these missing “notches” result in only a small
perturbation on the input power supply line. Note that a
higher ESR capacitor, such as a tantalum, results in higher
input noise. Therefore, ceramic capacitors are recom-
mended for their exceptional ESR performance. Further
input noise reduction can be achieved by powering the
LTC3225/LTC3225-1 through a very small series inductor
as shown in Figure 2.
A 10nH inductor will reject the fast current notches,
thereby presenting a nearly constant current load to the
input power supply. For economy, the 10nH inductor can
be fabricated on the PC board with about 1cm (0.4") of
PC board trace.
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or alumi-
num should never be used for the fl ying capacitor since
IN
Capacitor Selection
IN
. Tantalum and aluminum capacitors are not
IN
IN
controls the amount of ripple
). To reduce noise and ripple,
Figure 2. 10nH Inductor Used for Input Noise Reduction
V
IN
0.1μF
10nH
2.2μF
its voltage can reverse upon start-up of the LTC3225/
LTC3225-1. Low ESR ceramic capacitors should always
be used for the fl ying capacitor.
The fl ying capacitor controls the strength of the charge
pump. In order to achieve the rated output current, it is
necessary to use at least 0.6μF of capacitance for the
fl ying capacitor.
The effective capacitance of a ceramic capacitor varies with
temperature and voltage in a manner primarily determined
by its formulation. For example, a capacitor made of X5R
or X7R material retains most of its capacitance from
–40°C to 85°C whereas a Z5U or Y5V type capacitor loses
considerable capacitance over that range. X5R, Z5U and
Y5V capacitors may also have a poor voltage coeffi cient
causing them to lose 60% or more of their capacitance
when the rated voltage is applied. Therefore, when com-
paring different capacitors, it is often more appropriate to
compare the amount of achievable capacitance for a given
case size rather than comparing the specifi ed capacitance
value. For example, over rated voltage and temperature
conditions, a 4.7μF 10V Y5V ceramic capacitor in a 0805
case may not provide any more capacitance than a 1μF 10V
X5R or X7R capacitor available in the same 0805 case. In
fact, over bias and temperature range, the 1μF 10V X5R
or X7R provides more capacitance than the 4.7μF 10V
Y5V capacitor. 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.
8, 11
9
V
LTC3225-1
GND
LTC3225
IN
3225 F02
LTC3225/LTC3225-1
3225fb
9