LTC3225-1_15 Datasheet, PDF (6/14 Pages) Linear Technology – 150mA Supercapacitor Charger

LTC3225/LTC3225-1
SIMPLIFIED BLOCK DIAGRAM
C_FLY
9
1
2
4
V_IN
C⁺
C^–
SHDN
V_IN
SOFT-START AND
SHUTDOWN CONTROL
UVLO
THERMAL
PROTECTION
PROG
7
R_PROG
1.2V
RUN
i
3000i
CHARGE
PUMP
CLK
RUN/STOP
OSCILLATOR
POR
C_{OUT 10}
CX
3
C_TOP
GND
8
C_BOT
R1
1.2V
V_REF
1.088V (LTC3225)
1.067V (LTC3225-1)
V_SEL
6
C1
V_REF – 2%
V_REF – 6%
V_REF – 7.2%
R2
+
C2
–
POR
PGOOD
5
Figure 1
3225 F01
OPERATION
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.
6
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.
3225fb