GENERAL DESCRIPTION (cont.)
SUPERCAPS
Supercaps are typically rated with a nominal recommended
working voltage established for long life at their maximum rated
operating temperature. Excessive supercap voltages that exceed
its rated voltage for a prolonged time period will result in reduced
lifetime and eventual rupture and catastrophic failure. To prevent
such an occurrence, a means of automatically adjusting (charge-
balancing) and monitoring the maximum voltage is required in most
applications having two or more supercaps connected in series,
due to their different internal leakage currents that vary from one
supercap to another.
The supercap leakage current itself is a variable function of its many
parameters such as aging, initial leakage current at zero input
voltage, the material and construction of the supercap. Its leakage
is also a function of the charging voltage, the charging current,
operating temperature range and the rate of change of many of
these parameters. Supercap balancing must accommodate these
changing conditions.
SUPERCAP CHARGING AND DISCHARGING
During supercap charging, consideration must be paid to limit the
rate of supercap charging so that excessive voltage and current do
not build up across any two pins of the SAB MOSFETs, even
momentarily, to exceed their absolute maximum rating. In most
cases though, this is not an issue, as there may be other design
constraints elsewhere in the circuit to limit the rate of charging or
discharging the supercaps. For many types of applications, no
further action, other than checking the voltage and current excur-
sions, or including a simple current-limiting charging resistor, is nec-
essary.
CHARACTERISTICS OF SUPERCAP AUTO BALANCING
(SAB™) MOSFETS
The principle behind the Supercap Auto Balancing MOSFET in
balancing supercaps is basically simple. It is based on the natural
threshold characteristics of a MOSFET device. The threshold volt-
age of a MOSFET is the voltage at which a MOSFET turns on and
starts to conduct a current. The drain current of the MOSFET, at or
below its threshold voltage, is an exponentially non-linear function
of its gate voltage. Hence, for small changes in the MOSFET’s
gate voltage, its on-current can vary greatly, by orders of magni-
tude. ALD’s SAB MOSFETs are designed to take advantage of
this fundamental device characteristic.
SAB MOSFETs can be connected in parallel or in a series, to suit
the desired leakage current characteristics, in order to charge-
balance an array of supercaps. The combined SAB MOSFET and
supercap array is designed to be self-regulating with various
supercap array leakage mismatches and environmental
temperature changes. The SAB MOSFETs can also be used only
in the subthreshold mode, meaning the SAB MOSFET is used
entirely at min., nominal and max. operating voltages in voltage
ranges below its specified threshold voltage.
For the ALD8100xx/ALD9100xx family of SAB MOSFETs, the
threshold voltage V
t
of a SAB MOSFET is defined as its drain-gate
source voltage at a drain-source ON current, I
DS(ON)
= 1µA when
its gate and drain terminals are connected together (V
GS
= V
DS
).
This voltage is specified as xx, where the threshold voltage is in
0.10V increments. For example, the ALD810025 features a 2.50V
threshold voltage MOSFET with drain-gate source voltage,
V
t
= 2.50V, and I
DS(ON)
= 1µA. The SAB MOSFET has a precision
trimmed threshold voltage where the tolerance of the threshold
voltage is very tight, typically 2.50V +/-0.005V. When a 2.50V drain-
gate source voltage bias is applied across an ALD810025/
ALD910025 SAB MOSFET, it conducts an I
DS(ON)
= 1µA.
ALD810023, ALD810024, ALD810025,
ALD810026, ALD810027, ALD810028
As all ALD8100xx and ALD9100xx devices operate the same way,
an ALD810025 is used in the following illustration. At voltages be-
low its threshold voltage, the ALD810025 rapidly turns off at a rate
of approximately one decade of current per 104mV of voltage drop.
Hence, at V
GS
= V
DS
= 2.396V, the ALD810025 has drain current
of 0.1µA. At V
GS
= V
DS
= 2.292V, the ALD810025 drain current
becomes 0.01µA. At V
GS
= V
DS
= 2.188V, the drain current is
0.001µA. It is apparent that at V
GS
= V
DS
≤
2.10V, the drain leak-
age current
≤
0.00014µA, which is essentially zero when compared
to 1µA initial threshold current. When individual V
GS
= V
DS
volt-
ages fall below 1.9V, the SAB MOSFET leakage current essentially
goes to zero (~70pA).
This exponential relationship between the Drain-Gate Source
Voltage and the Drain-Source ON Current is an important
consideration for replacing certain supercap charge balancing
applications currently using fixed resistor or operational amplifier
charge balancing. These other conventional charge-balancing cir-
cuits would continue to dissipate a significant amount of current,
even after the voltage across the supercaps had dropped, because
the current dissipated is a linear function, rather than an exponen-
tial function, of the supercap voltage (I = V/R). For supercap stacks
consisting of more than two supercaps, the challenge of supercap
balancing becomes more onerous.
For other IC circuits that offer charge balancing, active power is still
being consumed even if the supercap voltage falls below 2.0V. For
a four-cell supercap stack, this translates into a 2.0V x 4 ~= 8.0V
power supply for an IC charge-balancing circuit. Even a two-cell
supercap stack would be operating such an IC circuit with
2.0V x 2 = 4V. A supercap stack with SAB MOSFET charge-
balancing, on the other hand, would be the only way to lose
exponentially decreasing amount of charge with time and preserve
by far the greatest amount of charge on each of the supercaps, by
not adding charge loss to the leakages contributed by the supercaps
themselves.
At V
GS
= V
DS
voltages of the ALD810025 above its V
t
threshold
voltage, its drain current behavior has the opposite near-exponen-
tial effect. At V
GS
= V
DS
= 2.60V, for example, the ALD810025
I
DS(ON)
increases tenfold to 10µA. Similarly, I
DS(ON)
becomes
100µA for a V
GS
= V
DS
voltage increase to 2.74V, and 300µA at
2.84V. (See Table 1)
As I
DS(ON)
changes rapidly with applied voltage on the Drain-Gate
to Source pins, the SAB MOSFET device acts like a voltage
limiting regulator with self-adjusting current levels. When this SAB
MOSFET is connected across a supercap cell, the total leakage
current across the supercap is compensated and corrected by the
SAB MOSFET.
Consider the case when two supercap cells are connected in
series, each with a SAB MOSFET connected across it in the
V
t
mode (V
DS
= V
GS
), charged by a power supply to a voltage
equal to 2 x V
S
.
If the top supercap has a higher internal leakage current than the
bottom supercap, the voltage V
S(top)
across it tends to drop lower
than that of the bottom supercap. The SAB MOSFET I
DS(ON)
across
the top supercap, sensing this voltage drop, drops off rapidly.
Meanwhile, the bottom supercap V
S(bottom)
voltage tends to rise,
as V
S(bottom)
= (2 x V
S
) - V
S(top)
. This tendency for the voltage
rise also increases V
GS
= V
DS
voltage of the SAB MOSFET across
the bottom supercap. This increased V
GS
= V
DS
voltage would
cause the I
DS(ON)
current of the bottom SAB MOSFET to increase
rapidly as well. The excess leakage current of the top supercap
would now leak across the bottom SAB MOSFET, reducing the
voltage rise tendency of the lower supercap. With this self-regulat-
ing mechanism, the top supercap, V
S(top)
, voltage tends to rise
while the bottom supercap, V
S(bottom)
, voltage tends to drop,
creating simultaneously opposing actions of the supercap leakage
currents.
6 of 17
Advanced Linear Devices, Inc.
ALD [ ADVANCED LINEAR DEVICES ]
