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LTC3226

3226fa

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:

CPO = VCPO _ FB •1+

CP1

CP2

⎛

⎝⎜

⎞

⎠⎟

where VCPO_FB is 1.2V. See the block diagram in Figure 1.

Typical values for RCP1 and RCP2 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:

PROG = 10,500 •

VIN(ILIM)

, I

VIN(ILIM) = 10,500 •

PROG

where IVIN(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 VIN pin to GND via the

PFI pin such that:

IN(PFO _ HI_ LO) = VPFI •1+

PF1

PF2

⎛

⎝⎜

⎞

⎠⎟

where VPFI is 1.2V. See Figure 1. Typical values for RPF1

and RPF2 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, VOUT.

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:

IN(PFO _ LO _ HI) = VPFI + VPFI(HYS)

()•1+RPF1

PF2

⎛

⎝⎜

⎞

⎠⎟

where VPFI(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, RH, 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 VIN 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) = VPFI + VPFI(HYS) –VF

()•RPF1

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