Power backup solution for power failure protection of handheld devices

　　introduction

　　Handheld electronic devices play an important role in our daily lives. However, no amount of careful engineering can protect them from being subjected to "rough treatment" in the hands of their users. For example, what happens when a factory worker accidentally drops a barcode scanner and the battery pops out? Such events are unpredictable with electronic methods, and without some form of "safety net" (a short-term power retention system with sufficient energy stored to provide backup power until the battery can be replaced or the data can be stored in permanent storage), important data stored in volatile memory will be lost.

　　Supercapacitors are compact, rugged and reliable, and can support the power requirements of backup systems for short-term power outages . Similar to batteries, they also require careful charging and power regulation at the output. The LTC3226 is a two-series supercapacitor charger with a PowerPath controller that simplifies the design of backup systems. Specifically, the device includes a charge pump supercapacitor charger with programmable output voltage and automatic cell voltage balancing, a low dropout regulator and a power fault comparator for switching between normal mode and backup mode. Low input noise, low quiescent current and compact footprint make the LTC3226 ideal for compact, handheld and battery-powered applications. The device is available in a 3mm x 3mm 16-pin QFN package.

　　Backup power application

　　Figure 1 shows a power-maintaining system using a supercapacitor bank that is capable of providing 165mW of standby power for approximately 45s without battery power. An LDO is responsible for converting the output of the supercapacitor bank into a constant voltage supply during backup mode.

　　Figure 1: Typical power backup system using supercapacitors

　　The LTC3226 makes it easy to design a power backup system. For example, take a device with 150mA operating current and 50mA standby current (ISB) when powered by a single lithium-ion battery. To ensure that a charged battery is connected, the high level trigger point of the power failure comparator (PFI) is set to 3.6V. When the battery voltage reaches 3.15V, the device enters standby mode; when the battery voltage is 3.10V (VBAT (MIN)), the device enters backup mode and the time period (tHU) to maintain power is initially set to about 45s.

　　The standby mode trigger level is controlled by an external comparator circuit, while the backup mode trigger level is controlled by the PFI comparator. When in backup mode, the device must be prohibited from entering full operation mode to avoid discharging the supercapacitor too quickly.

　　The design starts with setting the PFI trigger level. R2 is set to 121k, and the resistance value of R1 should set the PFI trigger level (VPFI) on the PFI pin to 1.2V, which is calculated as follows:

　　Based on this, R1 is set to 191k.

　　The hysteresis on the VIN pin must be extended to meet the 3.6V trigger level. This can be achieved by adding a series combination of a resistor and a diode between the PFI pin and the PFO pin. VIN(HYS) is

　　0.5V, VPFI(HYS) is 20mV, and Vf is 0.4V.

　　R8 is set to 348k.

　　By setting R7 to 80.6k and calculating the R6 resistance, the LDO backup mode output voltage is set to 3.3V. VLDO (FB) is 0.8V.

　　Set R6 to 255k.

　　The fully charged voltage on the series connected supercapacitors is set to 5V. This is done using a voltage divider network between the CPO pin and the CPO_FB pin. R5 is set to 1.21M and the R4 resistance is calculated. VCPO (FB) is 1.21V.

　　Set R4 = 3.83M.

　　In backup mode, when the voltage across the supercapacitor stack begins to approach VOUT, the minimum voltage across the supercapacitor at the end of tHU calculation must take into account the ESR of the two supercapacitors and the output resistance of the LDO. Assuming the ESR of each supercapacitor is 100mΩ and the LDO output resistance is 200mΩ, this will increase VOUT (MIN) by 20mV due to the 50mA standby current. VOUT (MIN) is set to 3.1V, resulting in a 1.88V discharge voltage (ΔVSCAP) across the supercapacitor stack. Now, the size of each supercapacitor can be determined.

　　Each supercapacitor was selected as a 3F/2.7V capacitor (ESHSR-0003C0-002R7) provided by Nesscap.

　　Figure 2 shows the actual backup time of the system with a 50mA load. Since a larger 3F capacitor is used in the actual circuit, the backup time is 55.4s.

　　Figure 2: Backup time to support 50mA load

　　in conclusion

　　The LTC3226 makes it easy to build a complete power backup solution by integrating a two-cell supercapacitor charger, PowerPath controller, an LDO regulator and a power-fail comparator all in a single 3mm x 3mm 16-pin QFN package.