LTC3225-1_15 Datasheet, PDF (8/14 Pages) Linear Technology – 150mA Supercapacitor Charger

LTC3225/LTC3225-1
APPLICATIONS INFORMATION
Programming Charge Current
The charge 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/LTC3225-1 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/LTC3225-1 is
similar to that of a linear regulator with an effective input
voltage of twice the actual input voltage. In an ideal regulat-
ing voltage doubler the power efficiency is given by:
η_{2 x IDEAL}
=
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/LTC3225-1 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 agree-
ment 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.
Charging Time Estimation
The estimated charging time with equal initial voltages
across the two supercapacitors 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 charge
current given by:
I_OUT
=
1800V
R_PROG
When the charging process starts with unequal initial volt-
ages across the supercapacitors, only the capacitor 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/
LTC3225-1. If the junction temperature increases above
approximately 150°C, the thermal shutdown circuitry auto-
matically deactivates the output. To reduce the maximum
junction temperature, a good thermal connection to the PC
board is recommended. Connecting the GND pin (Pin 8)
and the Exposed Pad (Pin 11) of the DFN package to a
ground plane under the device on two layers of the PC
board can reduce the thermal resistance of the package
and PC board considerably.
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