ALD8100XX/ALD9100XX SUPERCAPACITOR

Advanced Linear Devices, Inc.

3 of 17

AUTO BALANCING (SAB) MOSFET ARRAY FAMILY

temperature as well. This current increase compensates for the

leakage current increase within the supercapacitor, reducing the

overall supercapacitor temperature leakage effect and preserving

charge balancing effectiveness. This temperature compensation

assumes that all supercapacitors and SAB MOSFETs operate in

the same temperature environments.

SAB MOSFET LIMITATIONS

During supercapacitor charging, consideration must be paid to limit

the rate of charging so that excessive voltage and current does not

build up across any two pins of the SAB MOSFETs, even momen-

tarily, to exceed their absolute maximum ratings in voltage, operat-

ing current, and power dissipation. In most cases though, this is

not an issue, as other design constraints elsewhere limit the rate of

charging or discharging of the supercapacitors. For many applica-

tions, no further action, other than checking the voltage and cur-

rent excursions, or including a simple current-limiting charging re-

sistor, is necessary.

For each SAB MOSFET, its V+ pin must be connected to the most

positive voltage and its V- and IC pins to the most negative voltage

within the package. SAB MOSFETs have numerous pins required

for its manufacturing process, which must be connected to the

supercapacitors when in use, for proper circuit operation. Multiple

packages can be cascaded for higher system voltages as long as

absolute maximum ratings are observed for each individual pack-

age.

Note that each Drain pin of a SAB MOSFET has an internal re-

verse biased diode to its Source pin, which can become forward-

biased if the Drain voltage should become negative relative to its

Source voltage. This forward-biased diode clamps the Drain volt-

age to limit the negative voltage relative to its Source voltage, and

is limited to a 80mA max. rated current between any two pins.

Each Gate pin also has a reverse biased diode to V-. When for-

ward biased, the maximum diode current must be within the abso-

lute maximum ratings. All other pins must have voltages within V+

and V- voltage limits. Standard ESD protection facilities and han-

dling procedures for static sensitive devices must also be followed

before the SAB MOSFETs are installed. Once the SAB MOSFET

is permanently connected to the supercapacitors, ESD concerns

are relieved because any extraneous electrostatic charge would

be absorbed by the supercapacitor and would not cause exessive

voltage increase.

EXTENDED TEMPERATURE RANGE OPERATION

SAB MOSFETs are built with solid state integrated circuit tehcnology.

They are available for operation over a wide temperature range,

with appropriate derating, screening and packaging. Standard com-

MATCHING SAB™ MOSFETS TO SUPERCAPACITORS

Figure 1 shows a basic connection diagram of two SAB MOSFETs

connected across two supercapacitors, powered by a V+ power

supply with an external (or internal) resistor, with basic equations

of SAB MOSFET and supercapacitor voltages and currents.

The proess of selecting SAB MOSFETs to match specific models

of supercapacitors begins by analyzing the parameters and the re-

quirements of a given set of supercapacitors:

1) For better leakage current matching results, pick the same make

and model of supercapacitors to be connected in a series. If pos-

sible, select supercapacitors from the same production batch. (Note:

SAB MOSFETs are precisely set at the factory and specified such

that their unit-to-unit variation is not a concern.)

2) Determine the max. leakage current of each supercapacitor.

3) Determine the desired nominal operating voltage of the

supercapacitor.

4) Determine the maximum operating voltage rating of the

supercapacitor.

5) Calculate or measure the maximum leakage current of the

supercapacitor at its maximum rated operating voltage.

6) Determine the operating temperature range of the supercapacitor.

7) Determine any additional level of operating leakage current in

the system.

For example, the ALD810025 has a rated Drain Current of 1µA at

2.3V at that current, which produces an equivalent ON resistance

of 2.3V/0.01µA ~= 230MΩ (using the rule of thumb: one decade of

FIGURE 1

V+

VSUPPLY

RSRC

VIN1

VIN2

IOUT1

IOUT2

Basic Equations:

V+ = VC1 + VC2 + RSRC (IC1 + IOUT1)

V+ = VC1 + VC2 + RSRC (IC2 + IOUT2)

VC1 = VIN1

VC2 = VIN2

IC1 + IOUT1 = IC2 + IOUT2

VSUPPLY = VC1 + VC2

+

-

+

-

VC1

VC2

IC1

IC2

+

-

+

-

ALD8100XX/ALD9100XX FAMILY GENERAL DESCRIPTION (cont.)