ALD8100XX/ALD9100XX SUPERCAPACITOR

Advanced Linear Devices, Inc.

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AUTO BALANCING (SAB) MOSFET ARRAY FAMILY

In the graph titled “Input Voltage vs. Output Current”, locate the

SAB MOSFET Selection Table, which is 1.90V. Next, subtract 1.90V

voltage variation as a function of temperature. If the temperature

variation allowance is 60mV, then the maximum supercap inbalance

voltage is 2.48V (2.42V + 0.06V) across temperature.

In cases where the supercapacitor leakage current is 1mA max.,

the ALD810019 is suggested. In cases where supercapacitor leak-

age currents are up to 3mA, then a part such as the ALD81016

can be used, although this may cause increased leakage current

through the SAB MOSFET itself. Another way to reduce leakage

currents would be to parallel connect mulitple ALD810019 devices

to auto-balance leakage currents greater than 1mA.

A 4.2V SUPERCAPACITOR STACK DESIGN EXAMPLE

A supply voltage of 4.2V across two supercapacitors gives 2.1V

across each supercapacitor cell. With a maximum leakage current

responding ALD part number is ALD910020SAL, a dual 8L SOIC

package.

leakage current ratio between two supercapacitor cell units of 100µA

to 1µA, a 100 to 1 ratio, would produce one cell voltage of 2.22V

and the other cell voltage of 1.98V, which adds up to 4.20V. Simi-

larly, a lower supply voltage than 4.2V would be divided between

the two supercapacitors corresponding to their respective leakage

currents. Consider the case when the supply voltage is 4.10V, each

with an ALD910020 connected to it. If the leakage current ratio

between the supercapacitors remains the same, then one cell would

be biased at 2.22V (100µA) and the other would be biased at 1.88V

(4.10V - 2.22V). This would cause the ALD910020 to have a max.

leakage current contribution of less than 0.1µA.

PARALLEL-CONNECTED AND SERIES-CONNECTED SAB

MOSFETS

In the first design example on the previous page, note that the

ALD810026 is a quad pack, with four SAB MOSFETs in a single

SOIC package. For applications where two supercapacitors are

connected in series, the ALD9100xx dual SAB MOSFET is recom-

mended for charge balancing. If a two-stack supercapacitor re-

quires charge balancing, then there is also an option to parallel-

connect two additional SAB MOSFETs of the quad ALD8100xx for

each of the two supercapacitors. Parallel-connection means that

the drain, gate and source terminals of each of the two SAB

MOSFETs are connected together to form a single MOSFET with

twice the output current and twice the output current sensitivity to

equal to 2 x 0.1µA = 0.2µA. The total current for the combined

supercapacitor and SAB MOSFET is = 2.5µA + 0.2µA ~= 2.7µA @

2.50V operating voltage. At max. voltage of 2.70V across the SAB

this configuration would be chosen to increase max. supercapacitor

charge balancing leakage current at 2.70V to 20µA, at the expense

For stacks of series-connected supercapacitors consisting of more

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

array for every supercapacitor stack (up to 4 cells) connected in

series. Multiple SAB MOSFET arrays can be arrayed across mul-

tiple supercapacitor stacks to operate at higher operating voltages.

It is only 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.

LOW LEAKAGE ENERGY HARVESTING APPLICATIONS

Supercapacitors offer an important benefit in energy harvesting ap-

plications with a high impedance energy source, in buffering and

storing such energy to drive a higher power load.

For energy harvesting applications, supercapacitor leakage cur-

rents are a critical design parameter, as the average energy har-

vesting input charge must exceed the average supercapacitor in-

ternal leakage currents in order for any net energy to be harvested.

When the input energy is a variable, meaning that its input voltage

and current magnitude is not constant and dependent upon other

parameters such as the source energy availability (energy sensor

conversion efficiency, etc.), the energy harvested and stored must

supply and exceed the necessary leakage currents, which tend to

be steady DC currents.

In these types of applications, in order to minimize the amount of

energy loss due to leakage currents, it is essential to choose

supercapacitors with low leakage specifications and to use SAB

MOSFETs to balance them.

For the first 90% of the initial voltages of a supercapacitor used in

energy harvesting applications, supercapacitor charge loss is lower

than its maximum leakage rating, at less than its max. rated volt-

age. SAB MOSFETs, used for charge balancing, would be com-

pletely turned off, consuming zero leakage current while the

supercapacitor is being charged, maximizing any energy harvest-

ing gathering efforts. The SAB MOSFET would not become active

until the supercapacitor is already charged to over 90% of its max.

rated voltage. The trickle charging of supercapacitors 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 would not allow wasting any of this current on

capacitor leakage currents, and on many other conventional charge

balancing methods. Resistors or operational amplifiers used as

charge-balancing circuits would dissipate far more energy than

desired. It may also be an important consideration to reduce long

term DC 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

spent, due to the leakages of the supercapacitors and the charge-

balancing circuits, plus any load requirements. With their unique

balancing characteristics and near-zero charge loss, SAB MOSFETs

are ideal devices to use for supercapacitor charge-balancing within

energy harvesting applications.

LONG TERM BACKUP BATTERY APPLICATIONS

Similar to energy harvesting applications, any low leakage long-

term application, such as a long-term backup battery requiring

supercapacitors at the output to reduce output impedance and to

boost its output power, would benefit from SAB MOSFET deploy-

ment. Over a long time span, reducing leakge currents is an impor-

tant design parameter. For example, a low DC leakage current of

just 1µA over 5 years translates into 44.8mAhr of energy lost.

ALD8100XX/ALD9100XX FAMILY GENERAL DESCRIPTION (cont.)