ALD8100XX/ALD9100XX SUPERCAPACITOR

Advanced Linear Devices, Inc.

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AUTO BALANCING (SAB) MOSFET ARRAY FAMILY

CHOOSING A SPECIFIC SAB MOSFET

In choosing SAB MOSFETs for a specific application, go to the

SAB MOSFET selection table, (Table 1 for ALD8100xx devices,

Table 2 for ALD9100xx devices) where each SAB MOSFET Part

Number and its respective parameters are listed. First, select an

Table(s). Next, look down that column to the row that contains the

is in the first column of that row. The part number of an SAB

MOSFET references its rated threshold voltage, but that is not nec-

essarily the desired operating voltage where the auto-balancing

supercapacitor operates. Generally, the recommended maximum

family is about 1mA. When supercapacitor leakage current exceeds

1mA, the effectiveness of the SAB MOSFET auto-balancing gradu-

ally diminishes and there is additional leakage current contribution

techsupport@aldinc.com for more information or technical assis-

tance.

A DESIGN EXAMPLE

A single 5V power supply using two 2.7V rated supercapacitors

connected in series and a single ALD810026 SAB MOSFET array

package (using two of the four devices in the package).

For a supercapacitor with:

1) max. operating voltage = 2.70V and

3) At 2.50V, the supercapacitor max. leakage current = 2.5µA at

tion by the ALD810026 is therefore ~= 0.1µA. The total leakage

current for the supercapacitor and the SAB MOSFET = 2.5µA +

0.1µA ~= 2.6µA @ 2.50V operating voltage. When operating volt-

age becomes 2.40V, additional ALD810026 leakage current con-

tribution decreases to about 0.01µA.

10µA is also the max. leakage current design margin, the differ-

ence between top and bottom supercapacitor leakage currents that

can be compensated.

If a higher max. leakage current margin is desired, then SAB

MOSFET selection may need to go to the next SAB MOSFET part

down in Table 1, which is ALD810025. For ALD810025 operating

at a max. rated voltage of 2.70V, the max. leakage current margin

average current consumption for the series-connected stack. The

total current for the supercapacitor and the SAB MOSFET is = 2.5µA

+ 1µA ~= 3.5µA @ 2.50V operating voltage.

Because an SAB MOSFET is always active and always in “on”

mode, there is no circuit switching or sleep mode involved. This

may become an important factor when the time interval between

the supercap discharging or recharging, and other events happen-

ing in the application, is long, unknown or variable. The circuit op-

eration is also greatly simplified.

In real life situations, the actual circuit behavior is a little different,

further reducing overall leakage currents from both supercapacitors

and SAB MOSFETs, due to the automatic compensation for differ-

ent leakage currents from the supercapacitors by themselves and

in combination with the SAB MOSFETs. Take an example of two

supercapacitors in series, assuming that the top supercapacitor is

leaking 10µA and the bottom one is leaking 4µA (both at the rated

2.7V max.) while the power supply remains at 5V DC. The actual

voltage across the top supercapacitor tends to be less than 2.5V

(50% of 5.0V), due to its higher internal leakage current, and re-

sults in a lowered current level than 10µA because the current tends

to be lower at less than 2.7V. As the total voltage across both

supercapacitors is still 5.0V, each supercapacitor would experience

a lowered voltage than its maximum rated voltage of 2.7V, thereby

resulting in reduced overall leakage currents in each of the two

supercapacitors.

These leakage currents are then further regulated by the SAB

MOSFETs connected across each of the supercapacitors. The end

result is a compensated condition where, for example, the top

the top supercapacitor is bypassed across the bottom SAB

MOSFET. Meanwhile the top SAB MOSFET, with ~2.4V across it,

supercapacitor is now biased at ~2.4V and, therefore, would expe-

rience less current leakage than when it is at 2.7V. The key point

here is that this process of leakage current balancing is fully auto-

matic and works for a variety of supercapacitors, each with its own

different leakage current characteristic profile.

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

supercapacitors, as in this example, the actual current level differ-

ence 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 ALD810026 in the design example above

would, therefore, be approximately 0.01µA. In this case, leakage

currents between the two supercapacitors can be at a ratio of 100:1

and still experience charge balancing and voltage regulation. If a

range of supercapacitor leakge currents can be determined or se-

lected for a particular model of supercapacitor across different pro-

duction batches, then a SAB MOSFET part can be specified that

further minimizes any SAB leakage currents and still maintains bal-

anced supercapacitor voltages within a narrow range.

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

the typical response time is determined by the RC time constant of

the capacitance value C of the supercapacitor. In many cases the

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 lev-

els. The direction of the voltage movements across the

supercapacitor, however, can indicate that the supercapacitor volt-

ages are moving away from the voltage limits.

A HIGH LEAKAGE CURRENT DESIGN EXAMPLE

A nominal 12V DC power supply connects across a supercapacitor

series stack consisting of six 2.0V supercapacitor cells. Each cell

has a nominal operating voltage of 2.0V and is rated at 2.5V max.

Maximum voltage across the stack is 13.92V, which results in a

per-cell voltage of 2.32V. The max. leakage current for the

supercapacitor is rated at 1mA at 2.5V.

Next, we choose a maximum acceptable supercapacitor in-balance

stack voltage of 2.42V, which allows for temperature and aging ef-

fects, among other factors. When we look down the column of

the corresponding ALD part number to be ALD810019.

ALD8100XX/ALD9100XX FAMILY GENERAL DESCRIPTION (cont.)