Datasheet

LTC3225/LTC3225-1
6
3225fb
SIMPLIFIED BLOCK DIAGRAM
The LTC3225/LTC3225-1 are dual cell supercapacitor char-
gers. Their unique topology maintains a constant output
voltage with programmable charge current. Their ability
to maintain equal voltages on both cells while charging
protects the supercapacitors from damage that is possible
with other charging methods, without the use of external
balancing resistors. The LTC3225/LTC3225-1 include an
internal switched capacitor charge pump to boost V
IN
to a
regulated output voltage. A unique architecture maintains
relatively constant input current for the lowest possible
input noise. The basic charger circuit requires only three
external components.
Figure 1
OPERATION
Normal Charge Cycle
Operation begins when the SHDN pin is pulled above 1.3V.
The C
OUT
pin voltage is sensed and compared with a preset
voltage threshold using an internal resistor divider and
a comparator. The preset voltage threshold is selectable
with the V
SEL
pin. If the voltage at the C
OUT
pin is lower
than the preset voltage threshold, the oscillator is enabled.
The oscillator operates at a typical frequency of 0.9MHz.
When the oscillator is enabled, the charge pump operates
charging up C
OUT
. Each time the charge pump starts up
from shutdown, the input current drawn by the internal
charge pump ramps up at approximately 20mA/µs until it
reaches a level which is determined by R
PROG
.
–
+
–
+
CHARGE
PUMP
10
9
3
8
C
OUT
C
TOP
C
BOT
V
IN
1.2V
1
3000i
CLK
RUN/STOP
C1
C2
2
C
–
C
+
4
SHDN
CX
5
PGOOD
3225 F01
GND
SOFT-START AND
SHUTDOWN CONTROL
THERMAL
PROTECTION
OSCILLATOR
V
REF
– 2%
V
REF
– 6%
V
REF
– 7.2%
1.088V (LTC3225)
1.067V (LTC3225-1)
1.2V
V
REF
V
SEL
6
PROG
R
PROG
RUN
i
7
R1
R2
POR
UVLO
POR
V
IN
C
FLY