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LT3485 Datasheet(PDF) 11 Page - Linear Technology |
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LT3485 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC3226 11 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: V CPO = VCPO _ FB •1+ R CP1 R 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: R PROG = 10,500 • 1V I VIN(ILIM) , I VIN(ILIM) = 10,500 • 1V R 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: V IN(PFO _ HI_ LO) = VPFI •1+ R PF1 R 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: V IN(PFO _ LO _ HI) = VPFI + VPFI(HYS) ()•1+RPF1 R 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 R H |
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