AN-1048 application note ?2011 cymbet corporation ? tel: +1-763-633-1780 ? www.cymbet.com doc an-72-1048 rev a page 1 of 3 enerchip? cc as backup power for a ds1340 real-time clock introduction cymbet enerchips are solid state rechargeable batteries having distinguishing characteristics compared to conventional rechargeable batteries. enerchips have a high charge/discharge cycle life; low self-discharge; simple voltage controlled charging; fat discharge voltage profle; have no fammable solvents to leak or catch fre; are solder refow tolerant; and are offered in low profle surface mount packages. the enerchip cc is a surface mount package containing a rechargeable solid state battery - the enerchip - with integrated battery management that includes charge control, discharge cutoff circuitry, and a battery switchover (i.e., supply supervisory) function. enerchip storage (discharge) capacity varies by product. enerchips are used in applications requiring backup, bridging, or transition power to maintain real-time clock (rtc) operation or sram data retention in the event of main power interruption; wireless sensing as the main power source when energy can be harvested from the ambient power and used to constantly trickle charge the enerchip; and as a power source to perform housekeeping for microcontrollers (mcus) and peripherals when main power is interrupted, to ensure an orderly shutdown or transition to low power modes. many applications demand ultra low power consumption from the various system components, particularly when system power is derived from primary or rechargeable batteries. moreover, in systems utilizing rtcs it is essential that when main power is interrupted, the standby power of the rtc be exceedingly low in order to achieve maximum run time from the backup power source. a commonly used rtc is the dallas/ maxim ds1340. the device has a vbackup pin to which a backup power source such as an enerchip can be connected. because the device has low operating current in backup mode - typically 800na to maintain the rtc and 25na data retention current - a miniature enerchip device with integrated power management can provide up to several days of backup power when main power is interrupted. this application note describes circuits utilizing the enerchip cc as a backup power source for the ds1340 rtc. a typical application circuit is shown in figure 1. main power is fed to both the cbc31xx and ds1340. when the supply voltage drops below a specifc threshold as detected by the cbc31xx, the enerchip solid state battery embedded in the enerchip cc delivers power to the rtc through the cbc31xx vout pin. figure 1. typical application circuit using the enerchip cc and ds1340 real-time clock.
AN-1048: enerchip? cc as backup power for a ds1340 rtc ?2011 cymbet corporation ? tel: +1-763-633-1780 ? www.cymbet.com doc an-72-1048 rev a page 2 of 3 circuit description and operation under normal operation, the rtc derives its power from the main system supply, which might be a large coin cell, prismatic cell, cylindrical batteries, or indirectly from wall power through a voltage regulator. because the enerchip has very low self-discharge, it needs to be charged only after it has been used in the performance of its duty as a backup power source and periodically to compensate for the enerchip cc discharge cutoff current drain of 30na (typical). such a low charging duty cycle presents a negligible parasitic load to the main power source, as the enerchip charges quickly and its charging current decays to just a few tens of nanoamperes when fully charged. it is important that the ds1340 registers assigned to the trickle charge function for the backup power source not be enabled. the enerchip cc controls the enerchip charging function internally. when main power is interrupted - as during line power outages or when the main battery is removed for recharging or replacement, the backup power source provides enough energy to maintain operation of the real- time clock. in applications where the cbc31xx vout line is also connected to vdd of a cpu or mcu, it might be necessary to connect the cbc31xx reset/ line to the gate of an n-channel fet placed between the cpu/ mcu ground pin and system ground. this confguration ensures that no esd i/o structures in the cpu/mcu are biased through the interface bus lines when in backup mode, resulting in a parasitic drain on the enerchip. the enerchip cc is a surface mount device that contains a rechargeable battery with integrated battery management that performs the charge control, discharge control, threshold voltage detection, and supply supervisory functions all in one low profle package. it operates over the range of 2.5v to 5.5v. a block diagram of the enerchip cc is shown in figure 2. enerchip cc includes the following functional elements: internal charge control circuit that converts a wide range of input voltages to a tightly ? regulated battery charging voltage; battery cutoff circuit that disconnects the load from the embedded enerchip when the ? discharge voltage reaches a preset limit; output pin (reset/) that can be used to drive an interrupt line on an mcu to indicate that the ? system is operating in battery-backed mode; a voltage detection circuit allowing a user-selectable threshold trip voltage; ? a control line (enable) that enables and disables the internal charge pump. enable can be ? tied directly to vdd or controlled by the system mcu to reduce power consumption when the internal enerchip does not need to be charged.
AN-1048: enerchip? cc as backup power for a ds1340 rtc ?2011 cymbet corporation ? tel: +1-763-633-1780 ? www.cymbet.com doc an-72-1048 rev a page 3 of 3 all of these features have been combined to create a backup power solution that eliminates the need for a dedicated supply supervisory circuit, battery holder or socket, separate battery management circuitry, and a conventional power source such as a coin cell or supercapacitor. the result is a compact, single-chip solution to backup and bridging power. the power source - the enerchip - is a component class, rechargeable solid state battery that has no organic solvents or harmful chemicals that accompany coin cells and supercapacitors. given the low operating current of many rtcs, the enerchip cc can provide several hours to several days of backup time. backup time for the ds1340 is calculated based on the typical operating current of 800na in rtc backup mode (25na data retention current), plus the ds1340 battery leakage current of 100na and the enerchip cc battery cutoff circuit current of 30na (typical). the following table illustrates the amount of backup time afforded by the enerchip cc devices - cbc3105, cbc3112, and cbc3150 - when connected to the ds1340 rtc. cbc3105 5 x 5 x 0.9 6 32 cbc3112 7 x 7 x 0.9 14 76 cbc3150 9 x 9 x 0.9 60 320 given that the vast majority power interruptions - whether due to grid power outages in wall-powered systems or in mobile devices during main battery recharging and replacement - are less than one day, either enerchip cc device can be used in most applications. once the minimum backup run time specifcation is defned, device selection can be made accordingly and in conjunction with other considerations such as package footprint. the enerchip cc is also confgured to allow additional surface mount enerchips to be connected to increase backup run time. in cases where an mcu must also be maintained in a low power state (as opposed to being completely disconnected from the circuit) during battery-backed operation, the mcu sleep current must be factored into the backup run time equation. to maintain favorable backup run time, the designer can choose from a variety of mcus, some having sleep currents as low as 20na. when using mcus with such exceptionally low sleep currents, the backup run time is not reduced signifcantly. during operation, the mcu can also be programmed to drive the enerchip cc enable line low when enerchip battery charging is not required. doing so will reduce the overhead current associated with operating the charge pump internal to the enerchip cc. conclusion many systems utilizing rtcs require power supply backup in the event of main power interruption. conventional backup power sources are primary coin cells, rechargeable coin cells, and supercapacitors. traditional backup power sources have long charge times, high self-discharge, require relatively high steady state parasitic charging current, and often require external components for charge control and discharge cutoff. the enerchip - with its low self-discharge, low parasitic current draw when charged, and fast recharge time - is a superior solution to supercapacitors and coin cell batteries in backup power applications. when implemented in systems using real-time clocks, the enerchip cc delivers a robust backup power source with high cycle life and integrated battery management in a low profle, compact surface mount package.
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