LTC3226_12 Datasheet, PDF (11/16 Pages) Linear Technology – 2-Cell Supercapacitor Charger

LTC3226
APPLICATIONS INFORMATION
Programming the Supercapacitor
Termination Voltage (CPO)
The termination voltage of the supercapacitor stack on
the CPO pin can be programmed for any voltage between
2.5V to 5.3V by using a resistor divider from the CPO pin
to GND via the CPO_FB pin such that:
V_CPO
=
V_{CPO _ FB}
•
⎛
⎝⎜
1+
R_CP1
R_CP2
⎞
⎠⎟
where V_{CPO_FB} is 1.2V. See the block diagram in Figure 1.
Typical values for R_CP1 and R_CP2 are in the range of 40k
to 5M.
Programming the Input Current Limit for the Charger
The input current limit for the LTC3226 charge pump is
programmed by using a single resistor from the PROG
pin to ground. The input current limit is typically 10,500
times the current out of the PROG pin. The PROG pin
voltage always servos to 1V as long as the part is not in
sleep mode. The program resistor and the input current
limit are calculated using the following equations:
R_PROG
=
10,500
•
1V
I_VIN(ILIM)
,
I_VIN(ILIM)
=
10,500
•
1V
R_PROG
where I_VIN(ILIM) is the input current limit for the charge
pump charger. The maximum allowable input current limit
of 315mA can be achieved by using a PROG resistor of
33.2k. To maximize the charge transfer rate, the charge
pump operates in 1x mode when the supercapacitor voltage
is less than the input voltage and the charge current out of
CPO pin is only limited by the programmed input current
limit. However, in 2x mode, the average charge current is
approximately half the input current limit.
Programming the Input Voltage Threshold for the
Power-Fail Comparator
The input voltage threshold below which the power-fail
status pin PFO indicates a power-fail condition and the
LTC3226 switches the internal LDO on can be programmed
by using a resistor divider from the V_IN pin to GND via the
PFI pin such that:
V_IN(PFO
_
HI
_
LO)
=
V_PFI
•
⎛
⎝⎜
1+
R_PF1
R_{PF 2}
⎞
⎠⎟
where V_PFI is 1.2V. See Figure 1. Typical values for R_PF1
and R_PF2 are in the range of 40k to 5M. For a smooth
transition from normal to back-up mode, the PFI threshold
should be set 50mV to 100mV above the programmed
LDO output voltage, V_OUT.
The input voltage above which the power-fail status pin
PFO is high impedance and the supercapacitor charger
and the ideal diode are enabled is:
( ) ^V_{IN(PFO _LO _HI)} ⁼
V_PFI + V_PFI(HYS)
•
⎛
⎝⎜
1+
R_PF1
R_{PF 2}
⎞
⎠⎟
where V_PFI(HYS) is the hysteresis of the PFI comparator. It
is internally set to a typical value of 20mV.
The hysteresis can be increased externally by adding a
resistor, R_H, in series with a diode, D1, between the PFO and
PFI pins as shown in Figure 2. This network will increase
the low-to-high V_IN threshold for PFO while keeping the
high-to-low threshold intact. The increase in hysteresis
at the input can be calculated as shown:
( ) ^ΔV_IN(HYS) ⁼
V_PFI + V_PFI(HYS) – V_F
• ^RPF1
R_H
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