LTC3225-1_15 Datasheet, PDF (7/14 Pages) Linear Technology – 150mA Supercapacitor Charger

LTC3225/LTC3225-1
OPERATION
Once the output voltage is charged to the preset volt-
age threshold, the part shuts down the internal charge
pump and enters into a low current state. In this state,
the LTC3225/LTC3225-1 consume only about 20μA
from the input supply. The current drawn from C_OUT is
approximately 2μA.
Automatic Cell Balancing
Due to manufacturing tolerances, capacitance and leakage
current can vary from supercapacitor to supercapacitor.
Without the automatic cell balancing scheme used in the
LTC3225/LTC3225-1, the voltages across the supercapaci-
tors could differ from each other and potentially overvoltage
a cell. This can affect the performance and lifetime of a
supercapacitor.
The LTC3225/LTC3225-1 constantly monitor the volt-
age 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 voltage across the su-
percapacitors. 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 superca-
pacitors are mismatched enough that varying the charge
current is not sufficient to balance their voltages, the
LTC3225/LTC3225-1 stop charging the capacitor with the
higher voltage until they are again balanced. This feature
protects either capacitor from experiencing an overvoltage
condition. Attempting to equalize the voltages using parallel
resistors wastes power, discharges the supercapacitors,
and takes time to equalize the voltages. A 30% capacitance
mismatch leads to a 30% initial voltage difference after
charging. It takes hours to equalize the voltages across
1F supercapacitors using 10k resistors.
Shutdown Mode
Asserting SHDN low causes the LTC3225/LTC3225-1 to
enter shutdown mode. With the SHDN pin connected to
V_IN and the input supply removed or grounded, less than
1μA is consumed from the output, allowing the superca-
pacitors to remain charged.
If the input supply is present at V_IN and the SHDN pin is
grounded, the LTC3225/LTC3225-1 draw approximately
1μA of supply current. With the voltage at the C_OUT pin
discharged to 0V, this current drops to less than 1μA. Since
the SHDN pin is a high impedance CMOS input, it should
never be allowed to float.
Output Voltage Programming
The LTC3225/LTC3225-1 have a V_SEL input pin that allows
the user to set the output threshold voltage to either 4.8V
or 5.3V for the LTC3225 and 4V or 4.5V for the LTC3225-1
by forcing a low or high at the V_SEL pin respectively.
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/LTC3225-1 have built-in current limit as well
as overtemperature 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-heat-
ing 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/LTC3225-1 are 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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