LTC3225 Datasheet, PDF (8/12 Pages) Linear Technology – 150mA Supercapacitor Charger

LTC3225
APPLICATIONS INFORMATION
Programming Charge Current
The charging current is programmed with a single resistor
connecting the PROG pin to ground. The program resistor
and the input/output charge currents are calculated using
the following equations:
I_VIN
=
3600V
R_PROG
I_OUT
=
I_VIN
2
(with matched output capacitors)
An R_PROG resistor value of 2k or less (i.e., short circuit)
causes the LTC3225 to enter overcurrent shutdown mode.
This mode prevents damage to the part by shutting down
the internal charge pump.
Power Efficiency
The power efficiency (η) of the LTC3225 is similar to that
of a linear regulator with an effective input voltage of twice
the actual input voltage. In an ideal regulating voltage
doubler the power efficiency is given by:
η_2xIDEAL
=
P_OUT
P_IN
=
V_OUT • I_OUT
V_IN • 2I_OUT
=
V_OUT
2V_IN
At moderate to high output power the switching losses
and quiescent current of the LTC3225 are negligible and
the above expression is valid. For example, with V_IN = 3.6V,
I_OUT = 100mA and V_OUT regulated to 5.3V, the measured
efficiency is 71.2% which is in close agreement with the
theoretical 73.6% calculation.
Effective Open-Loop Output Resistance (R_OL)
The effective open-loop output resistance (R_OL) of a charge
pump is an important parameter that describes the strength
of the charge pump. The value of this parameter depends
on many factors including the oscillator frequency (f_OSC),
value of the flying capacitor (C_FLY), the non-overlap time,
the internal switch resistances (R_S) and the ESR of the
external capacitors.
Output Voltage Programming
The LTC3225 has a V_SEL input pin that allows the user to
set the output threshold voltage to either 4.8V or 5.3V by
forcing a low or high at the V_SEL pin respectively.
Charging Time Estimation
The estimated charging time when the initial voltage
across the two output supercapacitors is equal is given
by the equation:
( ) _{tCHRG =} ^C_OUT ^•
V_COUT – V_INI
I_OUT
where C_OUT is the series output capacitance, V_COUT is the
voltage threshold set by the V_SEL pin, V_INI is the initial
voltage at the C_OUT pin and I_OUT is the output charging
current given by:
I_OUT
=
1800V
R_PROG
When the charging process starts with unequal initial
voltages across the output supercapacitors, only the ca-
pacitor with the lower voltage level is charged; the other
capacitor is not charged until the voltages equalize. This
extends the charging time slightly. Under the worst-case
condition, whereby one capacitor is fully depleted while
the other remains fully charged due to significant leakage
current mismatch, the charging time is about 1.5 times
longer than normal.
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3225.
If the junction temperature increases above approximately
3225f
8