Do You Have a Plan B?

Tony Armstrong, Linear Technology


Systems that must always be available should have a backup power source

Being a Sci-Fi geek, I am a fan of the late 1950’s Hollywood “cheesy” low budget Ed Wood movies typical of this genre. One of my favorite films is “Plan 9 from Outer Space,” in which aliens come to Earth to stop humanity from building a doomsday weapon that could destroy the entire universe. However, things did not go quite as planned and the aliens were annihilated. Clearly, the aliens did not a have a plan B, just in case plan 9 did not work!

A corollary can readily be drawn with this plot line to that of an electronic system which must continue to remain operational, regardless of its external operating conditions. Said another way, any glitch in its power supply, whether momentary, or seconds, or even minutes, must be taken into account during its design process.

The most common way of dealing with such circumstances is to use uninterruptible power supplies (UPS) to cover these brief downtimes, thus ensuring high reliability continuous operation of the system. Similarly, many of today’s emergency and standby systems are used to provide backup power for building systems to provide assurance that safety systems and critical equipment can maintain their operation during a power outage – whatever its cause.

Beyond the macro

Another obvious example can be readily found in the ubiquitous handheld electronic devices which are used in our everyday lives. Because dependability is paramount, handhelds are carefully engineered with lightweight power sources for reliable use under normal conditions. The problem is that no amount of careful engineering can prevent the mistreatment they will undergo at the hands of humans (or even aliens).

For example, what happens when a factory worker drops a bar code scanner, causing its battery to drop out? Such events are electronically unpredictable, and important data stored in volatile memory would be lost without some form of safety net – namely a short-term power holdup system that stores sufficient energy to supply standby power until the battery can be replaced or the data can be stored in permanent memory.

These examples clearly demonstrate the need for an alternative form of power source to be available, just in case there is an interruption of the primary power source. In other words, a backup plan is necessitated in the event that the main power is not present – whatever the reason. I call this plan B.

Forms of storage

Having acknowledged the need for backup power for any given system, the question then arises: what can be used as a storage medium for this power? Traditionally, the choices have been capacitors and batteries.

I think that is it fair to say that capacitor technology has played a major role in power transmission and delivery applications for multiple decades. For example, traditional thin film and oil based capacitor designs performed a variety of functions, such as power factor correction and voltage balancing.

However, in the past decade there has been substantial research and development which has led to significant advances in capacitor design and capabilities. These have been called Supercapacitors (also known as Ultracapacitors) and they are ideal for use in battery energy storage and backup power systems. Supercapacitors maybe limited in terms of their total energy storage; nevertheless, they are “energy dense.” Furthermore, they possess the ability to discharge high levels of energy quickly and recharge rapidly.

Supercapacitors are also compact, robust, and reliable and can support the requirements of a backup system for short-term power-loss events such as the ones already described above. They can easily be paralleled, or stacked in series or even a combination of both to deliver the necessary voltage and current demand by the end application.

More than a big capacitor

Nevertheless, a supercapacitor is more than just a capacitor with a very high level of capacitance. Compared to standard ceramic, tantalum or electrolytic capacitors, supercapacitors offer higher energy density and higher capacitance in a similar form factor and weight. And, although supercapacitors require some “care and feeding,” they are augmenting or even replacing batteries in data storage applications requiring high current/short duration backup power.

Furthermore, 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 longer charge cycle life over a wider operating temperature range. Supercapacitor lifetime can be maximized 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. Table 1 shows a summary of the pros and cons between supercapacitors, capacitors and batteries.

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Table 1. Supercapacitor Comparison vs. Capacitors & Batteries

Now that we have established that either supercapacitors, batteries and/or a combination of both are candidates for use as a backup power supply in almost any electronic system, what are the IC solutions available? It turns out that Linear Technology has a broad range of ICs which were specifically designed to address this application need. Three examples to highlight here are the LTC4040, LTC3643 and the LTC3110.

The LTC4040

The LTC4040 is a complete lithium battery backup power management system for 3.5V to 5V supply rails that must be kept active during a main power failure.  Batteries provide considerably more energy than supercapacitors, making them superior for applications which require backup for extended periods of time. The LTC4040 uses an on-chip bidirectional synchronous converter to provide high efficiency battery charging as well as high current, high efficiency backup power. When external power is available, the device operates as a step-down battery charger for single-cell Li-Ion or LiFePO4 batteries while giving preference to the system load. 

When the input supply drops below the adjustable Power-Fail Input (PFI) threshold the LTC4040 operates as a step-up regulator capable of delivering up to 2.5A to the system output from the backup battery. During a power fail event, the device’s PowerPath control provides reverse blocking and a seamless switchover between input power and backup power. Typical applications for the LTC4040 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.

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Figure 1. 4.5V Backup Supply with a 4.22V PFI Threshold

The LTC4040 also includes optional overvoltage protection (OVP) which 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 prioritizing 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 optimized for Li-Ion and LiFePO4 batteries. The device also includes input current monitoring, an input power loss indicator and a system power loss indicator.

The LTC3643

The LTC3643 is a bi-directional, high voltage boost capacitor charger that automatically turns into a buck regulator for system backup. The proprietary single-inductor topology, with integrated PowerPath functionality, does the work of two separate switching regulators - saving size, cost, and complexity.

The LTC3643 operates in 2 modes, boost charge mode and buck backup mode. The charging mode efficiently charges an electrolytic capacitor array up to 40V at up to 2A continuously from an input supply between 3V to 17V. In backup mode - when the input supply falls below the programmable PFI threshold - the step-up charger operates in reverse as a synchronous step-down regulator to power and hold up the system rail from the backup capacitor(s).

During backup, current limit can be programmed from 2A to 4A, making this device ideal for high energy, relatively short duration backup capacitor systems, power failure backup systems, solid-state drives and battery stack charging applications. Figure 2 shows a simplified application schematic.

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Figure 2. Simplified LTC3643 Schematic Using an Electrolytic Capacitor for Backup

When charging the backup capacitor, an external low-value sense resistor can be used by the LTC3643 to maintain an accurate current limit from the input supply while prioritizing power delivery to the system load. Input current limit can be programmed with a 50mV threshold sense resistor, preventing system power source overload while minimizing capacitor recharge time. The converter operates at a 1MHz frequency, minimizing the size of external components.

Low quiescent-current Burst-Mode operation during regulation maximizes the energy usage from the backup capacitor. The LTC3643 provides ideal diode operation at the input supply by providing a gate drive signal to an external PMOS switch, allowing efficient power delivery while providing complete isolation between the input supply and the system load during backup mode.

The LTC3110

The LTC3110 is a bidirectional, programmable input current buck-boost Supercapacitor charger with active charge balancing for 1- or 2-series supercapacitors. Its proprietary low noise buck-boost topology does the work of two separate switching regulators, saving size, cost and complexity. The LTC3110 operates in two modes, backup and charge mode. In backup mode, the device maintains a system voltage, VSYS, of 1.71V to 5.25V, powered from the supercapacitor stored energy.

The Supercapacitor storage input, VCAP, features a wide practical operating range from 5.5V down to 0.1V. This ensures that all practical stored Supercapacitor energy is utilized, thereby extending backup times or shrinking the storage capacitors. Alternatively, in charge mode when the main power system is active, the LTC3110 can autonomously or through a user command, reverse the direction of power flow using the regulated system voltage to charge and balance the supercapacitors.  VCAP is efficiently charged to above or below VSYS by the buck-boost PWM.

The device also features a charge-mode average input current limit that can be programmed up to 2A with +/-2% accuracy, preventing system power source overload while minimizing capacitor recharge time. Figure 3 illustrates the functionality of the LTC3110 in a backup power source application.

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Figure 3. Simplified LTC3310 Schematic Showing Its Bi-directional Functionality

The LTC3110’s active charge balancing eliminates the constant drain of dissipative external ballast resistors, ensuring charging even with mismatched capacitors and less frequent recharge cycles.  Programmable maximum capacitor voltage regulation actively balances and limits the voltage across each capacitor in the series stack to one-half of the programmed value, ensuring reliable operation as capacitors age and develop mismatched capacities.  The low RDS(ON), low gate-charge synchronous switches provide high-efficiency conversion to minimize the charging time of storage elements. 

The LTC3110’s input current limit and maximum capacitor voltage is resistor programmable. Average input current is accurately controlled over a 0.125A to 2A programming range.  Pin-selectable Burst Mode operation improves light-load efficiency and reduces standby current to only 40µA, and shutdown current to less than 1µA.

Other features of the LTC3110 include high 1.2MHz switching frequency to minimize external component size, thermal overload protection, and two voltage supervisors for direction control and end of charge and one general purpose comparator with an open-collector output for interfacing with a microcontroller or microprocessor.

Always have a backup

Whenever your design calls for a system to be always available even if the primary power source should fail, it is always a good idea to have a backup power source. Fortunately, there are many IC options available that can allow an easy backup power supply, whether that storage medium is a Supercapacitor, an electrolytic capacitor or even a battery. So, don’t be like an alien – make sure you have a plan B.

Linear Technology