LTC3225 Datasheet, PDF (7/12 Pages) Linear Technology – 150mA Supercapacitor Charger

LTC3225
OPERATION
The LTC3225 is a dual cell supercapacitor charger. Its
unique topology maintains a constant output voltage with
programmable charging current. Its 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 includes 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.
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 4.8V/5.3V
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. The input current drawn by the
internal charge pump ramps up at approximately 20mA/μs
each time the charge pump starts up from shutdown.
Once the output voltage is charged to the preset voltage
threshold, the part shuts down the internal charge pump and
enters into a low current state. In this state, the LTC3225
consumes only about 20μA from the input supply. The
current drawn from C_OUT is approximately 2μA.
Automatic Cell Balancing
The LTC3225 constantly monitors the voltage across both
supercapacitors while charging. When the voltage across
the supercapacitors is equal, both capacitors are charged
with equal currents. If the voltage across one supercapacitor
is lower than the other, the lower supercapacitor’s charge
current is increased and the higher supercapacitor’s charge
current is decreased. The greater the difference between
the supercapacitor voltages, the greater the difference
in charge current per capacitor. The charge currents can
increase or decrease as much as 50% to balance the volt-
age across the supercapacitors. When the cell voltages
are balanced, the supercapacitors are charged at a rate
of approximately:
I_COUT
=
1
2
• I_VIN
If the leakage currents or capacitances of the two su-
percapacitors are mismatched enough that varying the
charging current is not sufficient to balance their volt-
ages, the LTC3225 stops charging the capacitor with the
higher voltage until they are again balanced. This feature
protects either capacitor from experiencing an overvoltage
condition.
Shutdown Mode
Asserting SHDN low causes the LTC3225 to enter shut-
down mode. When the charge pump is first disabled, the
LTC3225 draws approximately 1μA of supply current from
V_IN and C_OUT. After V_OUT is discharged to 0V, the current
from V_IN drops to less than 1μA. With SHDN connected
to V_IN, the output sinks less than 1μA when the input sup-
ply is removed. Since the SHDN pin is a high impedance
CMOS input, it should never be allowed to float.
Output Status Indicator (PGOOD)
During shutdown, the PGOOD pin is high impedance. When
the charge cycle starts, an internal N-channel MOSFET
pulls the PGOOD pin to ground. When the output voltage,
V_OUT, is within 6% (typical) of its final value, the PGOOD
pin becomes high impedance, but charge current continues
to flow until V_OUT crosses the charge termination voltage.
When V_OUT drops 7% below the charge termination volt-
age, the PGOOD pin again pulls low.
Current Limit/Thermal Protection
The LTC3225 has built-in current limit as well as overtem-
perature protection. If the PROG pin is shorted to ground,
a protection circuit automatically shuts off the internal
charge pump. At higher temperatures, or if the input
voltage is high enough to cause excessive self-heating
of the part, the thermal shutdown circuitry shuts down
the charge pump once the junction temperature exceeds
approximately 150°C. It will enable the charge pump once
the junction temperature drops back to approximately
135°C. The LTC3225 is able to cycle in and out of thermal
shutdown indefinitely without latch-up or damage until the
overcurrent condition is removed.
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