COVER STORY FEATURE
a similar form factor and weight; and, although supercapacitors require some special care, they are augmenting or even replacing batteries in data storage applications requiring high current/short duration backup power. Moreover, they are also finding use in a
variety of high peak power and portable applications in need of high current bursts or momentary battery backup, such as UPS systems. Compared to batteries, supercapacitors provide higher peak power bursts in smaller form factors and feature a longer charge cycle life over a wider operating temperature range. Supercapacitor lifetime can be maximised by reducing the capacitor’s top-off voltage and avoiding high temperatures (>50°C). Batteries, on the other hand, can store
a lot of energy, but are limited in terms of power density and delivery. Due to the chemical reactions that occur within a battery, they have limited life with regard to cycling. As a result, they are most effective when delivering modest amounts of power over a long period of time, since pulling many amps out of them very quickly severely limits their useful operating life.
NEW BACKUP MANAGER POWER SOLUTIONS Now that we have established that either supercapacitors, batteries, and/or a combination of both are candidates for use as a backup power sources in almost any electronic system, what are some of the solutions available? First of all, any IC solution would need to be a complete lithium ion battery backup power management system with the capability to keep 3.5V to 5V supply rails active during a main power failure event. Since batteries provide considerably more energy than supercapacitors, they are superior for applications that require backup for extended periods of time. Accordingly, any IC solution would need
to have an on-chip bidirectional synchronous converter to provide high efficiency charging of the backup battery, as well as be able to deliver high current backup power to the downstream load should an interruption on the main power rail occur. Thus, when external power is available, the device would operate as a step-down battery charger for single-cell Li-ion or LiFePO4
batteries while giving preference
to the system load. However, if the input supply was to suddenly drop below the adjustable power-fail input (PFI) threshold the IC would need to operate as a step-up regulator, capable of delivering multiple amps to the system
/ ELECTRONICS
output from the backup battery. Accordingly, if a power failure were to occur, then the IC would need power path control to provide reverse blocking and a seamless switchover between input power and the backup power source. Typical applications for such an IC
would include fleet and asset tracking, automotive GPS data loggers, automotive telematics systems, toll collection systems, security systems, communications systems, industrial backup, and USB-powered devices. Figure 1 shows a typical application schematic for this purpose using Analog Devices’ Power by Linear LTC4040 lithium ion battery backup manager. This device also includes optional overvoltage protection (OVP) that protects the IC from input voltages greater than 60V with an external FET. Its adjustable input current limit function enables operation from a current limited source while prioritising system load current over battery charge current. An external disconnect switch isolates the primary input supply from the system during backup. The LTC4040’s 2.5A battery charger provides eight selectable charge voltages optimised for Li-ion and LiFePO4
batteries. The device also
includes input current monitoring, an input power loss indicator, and a system power loss indicator. Analogous to batteries are supercapacitors. However, instead of supporting long time intervals of power loss, they are an excellent choice for systems that need high power, short duration backup power. Accordingly, any IC that supports this type of application would typically require the capability to
Figure 1:
A backup supply using the LTC4040 with a user set PFI threshold
support 2.9V to 5.5V supply rails during a main power interruption. It is well known that supercapacitors have higher power density than batteries, making them an ideal choice for systems whose applications require high peak power backup for brief time intervals. By way of example, the LTC4041 from Analog Devices’ Power by Linear product line uses an on-chip bidirectional synchronous converter to provide high efficiency step-down supercapacitor charging, as well as high current, high efficiency boost backup power. When external power is available, the device operates as a step-down battery charger for one or two supercapacitor cells while giving preference to the system load. When the input supply drops below the adjustable PFI threshold, the LTC4041 switches to step-up mode operation and can deliver up to 2.5A to the system load from the supercapacitor(s). During a power fail event, the device’s PowerPath control provides reverse blocking and a seamless switchover from input power to backup power. Typical applications include ride-through “dying gasp” supplies, high current ride-through 3V to 5V UPSs, power meters, industrial alarms, servers, and solid-state drives. Figure 2 shows a typical LTC4041 application schematic.
Figure 2: The LTC4041
supercapacitor backup application schematic
The LTC4041 includes an optional OVP function using an external FET that can protect the IC from input voltages greater than 60V. An internal supercapacitor balancing circuit maintains equal voltages across each supercapacitor and limits the maximum voltage of each supercapacitor to a predetermined value. Its adjustable input current limit function enables operation from a current limited source while prioritising system load current over battery charge current. An external disconnect switch isolates the primary input supply from the system during backup. The device also includes input current monitoring, an input power fail indicator, and a system power fail indicator. Whenever a system must have constant
availability, even if the primary power source should fail or is briefly interrupted, it is always a wise choice to have a backup power source available. These types of ICs allow an easy method to have backup power available if the main power is interrupted or lost, regardless of whether their storage medium is a supercapacitor, an electrolytic capacitor, or even a battery.
Analog Devices Ltd.
www.analog.com 01628 477 066
e:
uksales@linear.com ELECTRONICS | OCTOBER 2018 13
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