ALD810023, ALD810024, ALD810025,

Advanced Linear Devices, Inc.

8 of 17

ALD810026, ALD810027, ALD810028

GENERAL DESCRIPTION (cont.)

when it is at 2.7V. The primary benefit here is that this process of

leakage balancing is fully automatic and works for a variety of

supercaps, each with a different leakage characteristic profile of its

own.

A second benefit to note is that with ~2.4V and ~2.6V across the

two supercaps, in this example, the actual current level difference

between the top and the bottom SAB MOSFETs is at about a 100:1

ratio (~2 orders of magnitude). The net additional leakage current

contributed by the ALD8110026 in the design example above would,

therefore, be approximately 0.01µA. In this case, the difference in

leakage currents between the two supercaps can have a ratio of

100:1 and could still have charge balancing and voltage regulation.

The dynamic response of a SAB MOSFET circuit is very fast, and

the typical response time is determined by the R C time constant of

the equivalent ON resistance value of the SAB MOSFET and the

capacitance value of the supercap. In many cases the R value is

small initially, responding rapidly to a large voltage transient by

having a smaller R C time constant. As the voltages settle down,

the equivalent R increases. As these R and C values can become

very large, it can take a long time for the voltages across the

supercaps to settle down to steady state leakage current levels.

The direction of the voltage movements across the supercap,

however, would indicate the trend that the supercap voltages are

moving away from the voltage limits.

PARALLEL-CONNECTED AND SERIES-CONNECTED SAB

MOSFETS

In the previous design example, note that the ALD810026 is a quad

pack, with four SAB MOSFETs in a single SOIC package. For a

standard configuration of two supercaps connected in series, the

ALD9100xx dual SAB MOSFET is recommended for charge

balancing. If a two-stack supercap requires charge balancing, then

there is also an option to parallel-connect two SAB MOSFETs of a

quad ALD8100xx for each of the two supercaps. Parallel-connec-

tion generally means that the drain, gate and source terminals of

each of two SAB MOSFETs are connected together to form a

MOSFET with a single drain, a single gate and a single source

terminal with twice the output currents. In this case, at a nominal

operating voltage of 2.50V, the additional leakage current contribu-

tion by the SAB MOSFET is equal to 2 x 0.1µA = 0.2µA. The total

current for the supercaps and the SB MOSFET is = 2.5µA + 0.2µA

~= 2.7µA @ 2.50V operating voltage. At max. voltage of 2.70V

current of 2 x 10µA = 20µA. So this configuration would be chosen

to increase max. charge balancing leakage current at 2.70V to 20µA,

at the expense of an additional 0.1µA leakage at 2.50V.

This method also extends to four supercaps in series, although this

may require two separate ALD810026 packages, if the maximum

voltage ratings of the SAB MOSFET are exceeded.

For stacks of series-connected supercaps consisting of more than

three or four supercaps, it is possible to use a single SAB MOSFET

array for every three or four supercap stacks connected in series.

Multiple SAB MOSFET arrays can be arrayed across multiple

supercap stacks to operate at higher operating voltages. It is

important to limit the voltage across any two pins within a single

SAB MOSFET array package to be less than its absolute

maximum voltage and current ratings.

ENERGY HARVESTING APPLICATIONS

Supercaps offer an important benefit for energy harvesting appli-

cations from a low energy source, buffering and storing such

energy to drive a higher power load.

For energy harvesting applications, supercap leakage currents are

a critical factor, as the average energy harvesting input charge must

exceed the average supercap internal leakage currents in order for

any net energy to be harvested and saved. Often times the input

energy is variable, meaning that its input voltage and current

magnitude is not constant and may be dependent upon a whole set

of other parameters such as the source energy availability, energy

sensor conversion efficiency, etc.

For these types of applications, it is essential to pick supercaps

with low leakage specifications and to use SAB MOSFETs that

minimize the amount of energy loss due to leakage currents.

For up to 90% of the initial voltages of a supercap used in energy

harvesting applications, supercap charge loss is lower than its

maximum leakage rating, at less than its max. rated voltage. SAB

MOSFETs used for charge balancing, due to their high input thresh-

old voltages, would be completely turned off, consuming zero drain

current while the supercap is being charged, maximizing any

energy harvesting gathering efforts. The SAB MOSFET would not

become active until the supercap is already charged to over 90%

of its max. rated voltage. The trickle charging of supercaps with

energy harvesting techniques tends to work well with SAB MOSFETs

as charge balancing devices, as it is less likely to have high

transient energy spurts resulting in excessive voltage or current

excursions.

If an energy harvesting source only provides a few µA of current,

the power budget does not allow wasting any of this current on

capacitor leakage currents and power dissipation of resistor or

operational amplifier based charge-balancing circuits. It may also

be important to reduce long term leakage currents, as energy

harvesting charging at low levels may take up to many days.

In summary, in order for an energy harvesting application to be

successful, the input energy harvested must exceed all the energy

required due to the leakages of the supercaps and the charge-

balancing circuits, plus any load requirements. With their unique

balancing characteristics and near-zero charge loss, SAB MOSFETs

are ideal devices for use in supercap charge-balancing in energy

harvesting applications.