LTC3625/LTC3625-1
11
3625f
applicaTions inForMaTion
Programming Charge Current/Maximum Input Current
The C
BOT
charge current is programmed with a single
resistor connecting the PROG pin to ground. The program
resistor and buck output current are calculated using the
following equation:
R h
V
I
PROG PROG
BUCK
= •
.1 2
where h
PROG
= 118,000 (typical). Excluding quiescent cur-
rent, I
BUCK
is always greater than the average buck input
current. An R
PROG
resistor value of less than 53.6k will
cause the LTC3625/LTC3625-1 to enter overcurrent protec-
tion mode and proceed to charge at 2.65A (typical).
The effective buck input current can be calculated as:
I
I
V
V
VIN
BUCK
BUCK
MID
IN
=
ε
•
where ε
BUCK
is the buck converter efficiency (see the
Typical Performance Characteristics graph Buck Efficiency
vs V
MID
).
Output Voltage Programming
The LTC3625/LTC3625-1 have a V
SEL
input pin that
allows the user to set the output threshold voltage to
either 4.8V/4.0V or 5.3V/4.5V by forcing a low or high at
the V
SEL
pin respectively. In the single inductor application
the chip will balance the supercapacitors to within 50mV
(typical) of each other, resulting in a possible 25mV of
over/undercharge per cell. In the dual inductor application
the chip will balance the supercapacitors to within 100mV
(typical) of each other, resulting in a possible 50mV of
over/undercharge per cell.
Thermal Management
If the junction temperature increases above approximately
150°C, the thermal shutdown circuitry automatically de-
activates the output. To reduce the maximum junction
temperature, a good thermal connection to the PC board
is recommended. Connecting the exposed pad (Pin 13) of
the DFN package to a ground plane under the device on two
layers of the PC board, will reduce the thermal resistance
of the package and PC board considerably.
V
IN
Capacitor Selection
The style and value of capacitors used with the LTC3625/
LTC3625-1 determine input voltage ripple. Because the
LTC3625/LTC3625-1 use a step-down switching power sup-
ply from V
IN
to V
MID
, its input current waveform contains
high frequency components. It is strongly recommended
that a low equivalent series resistance (ESR) multilayer
ceramic capacitor be used to bypass V
IN
.
Tantalum and aluminum capacitors are not recommended
because of their high ESR. The value of the capacitor on
V
IN
directly controls the amount of input ripple for a given
I
BUCK
. Increasing the size of this capacitor will reduce the
input ripple.
Multilayer ceramic chip capacitors typically have excep-
tional ESR performance. MLCCs combined with a tight
board layout and an unbroken ground plane will yield
very good performance and low EMI emissions. There are
several types of ceramic capacitors available, each having
considerably different characteristics. For example, X7R
ceramic capacitors have the best voltage and temperature
stability. X5R ceramic capacitors have higher packing
density but poorer performance over their rated voltage
and temperature ranges. Y5V ceramic capacitors have
the highest packing density, but must be used with cau-
tion because of their extreme non-linear characteristic of
capacitance verse voltage.
The actual in-circuit capacitance of a ceramic capacitor
should be measured with a small AC signal as is expected
in-circuit. Many vendors specify the capacitance versus
voltage with a 1V
RMS
AC test signal and as a result,
overstate the capacitance that the capacitor will present
in the application. Using similar operating conditions as
the application, the user must measure or request from
the vendor the actual capacitance to determine if the
selected capacitor meets the minimum capacitance that
the application requires.
Inductor Selection
Many different sizes and shapes of inductors are avail-
able from numerous manufacturers. Choosing the right
inductor from such a large selection of devices can be
overwhelming, but following a few basic guidelines will
make the selection process much simpler.
