LTC3225 Datasheet, PDF (9/12 Pages) Linear Technology – 150mA Supercapacitor Charger

LTC3225
APPLICATIONS INFORMATION
150°C, the thermal shutdown circuitry automatically
deactivates the output. To reduce the maximum junction
temperature, a good thermal connection to the PC board
is recommended. Connecting the GND pin (Pin 8) and the
Exposed Pad (Pin 11) of the DFN package to a ground
plane under the device on two layers of the PC board
can reduce the thermal resistance of the package and PC
board considerably.
V_IN Capacitor Selection
The type and value of C_IN controls the amount of ripple
present at the input pin (V_IN). To reduce noise and ripple,
it is recommended that low equivalent series resistance
(ESR) multilayer ceramic chip capacitors (MLCCs) be
used for C_IN. Tantalum and aluminum capacitors are not
recommended because of their high ESR.
The input current to the LTC3225 is relatively constant dur-
ing 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 recommended for their exceptional
ESR performance. Further input noise reduction can be
achieved by powering the LTC3225 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 flying capacitor since
its voltage can reverse upon start-up of the LTC3225.
Low ESR ceramic capacitors should always be used for
the flying capacitor.
The flying 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
flying 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 coefficient
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 specified 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.
10nH
V_IN
9
V_IN
0.1μF
2.2μF LTC3225
8, 11
GND
3225 F02
Figure 2. 10nH Inductor Used for Input Noise Reduction
3225f
9