GENERAL DESCRIPTION (cont.)
A DESIGN EXAMPLE
With appropriate design and selection of a specific SAB MOSFET
device for a given pair of supercaps, it is now possible to have
regulation and balancing of two series-connected supercaps, at
essentially no extra leakage current, since the SAB MOSFET only
conducts the difference in leakage current between the two
supercaps.
A single 5V power supply using two 2.7V rated supercaps con-
nected in a series and a single SAB MOSFET array package.
For a supercap with:
1) max. operating voltage = 2.70V and
2) max. leakage current = 10µA at 70°C.
Likewise, the case of the bottom supercap having a higher leakage
current than that of the top supercap works in similar fashion, with
3) At 2.50V, the supercap max. leakage current = 2.5µA at 25°C.
the tendency of the bottom supercap, V
, voltage to drop,
S(bottom)
compensated by the tendency of the top supercap, V
, voltage
S(top)
Next, pick ALD810026, a SAB MOSFET with V = 2.60V. For this
t
to drop as well, effected by the top SAB MOSFET. This SAB
MOSFET charge balancing scheme also extends to up to four
supercaps in a series network by using four SAB MOSFETs in a
single ALD8100xx SAB MOSFET package.
device, at V
= V = 2.60V, the nominal I
= 1µA. Per the
DS(ON)
= 2.50V, I ~= 0.1µA.
DS(ON)
GS
leakage current table, at V
DS
= V
GS
DS
At a nominal operating voltage of 2.50V, the additional leakage
current contribution by the ALD810026 is therefore 0.1µA. The
total current for the supercap and the SAB MOSFET = 2.5µA +
0.1µA ~= 2.6µA @ 2.50V operating voltage. At an operating
voltage of 2.40V, the additional ALD810026 leakage current
decreases to about 0.01µA.
As ambient temperature increases, the supercap leakage current,
as a function of temperature, increases. The SAB MOSFET thresh-
old voltage is reduced with temperature increase, which causes the
drain current to increase with temperature as well. This drain
current increase compensates for the leakage current increase within
the supercap, reducing the overall supercap temperature leakage
effect and preserving charge balancing effectiveness. This tem-
perature compensation assumes that all the supercaps and the SAB
MOSFETs are in the same temperature environments.
At a max. voltage of 2.70V across the ALD810026 SAB MOSFET,
V
GS
= V
= 2.70V results in I = 10µA. 10µA is also the
DS
DS(ON)
max. leakage current margin, the difference between top and bot-
tom supercap leakage currents that can be compensated.
Each drain pin of a SAB MOSFET has an internal reverse biased
diode to its source pin, which can become forward biased if the
drain voltage should become negative relative to its source pin. This
forward-biased diode clamps the drain voltage to limit the negative
voltage relative to its source voltage, and is limited to 80mA max.
rated current between any two pins.
If a higher max. leakage current margin is desired for an applica-
tion, then the selection may need to go to the next SAB MOSFET
down in the series, ALD810025. For an ALD810025 operating at a
max. rated voltage of 2.70V, the max. leakage current margin is
~= 50µA. For this device, the nominal operating current at 2.50V is
~= 1µA, which is the average current consumption for the series-
connected stack. The total current for the supercap and the SAB
MOSFET is = 2.5µA + 1µA ~= 3.5µA @ 2.50V operating voltage.
SPECIFYING SAB™ MOSFETS
Because the 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 process of selecting SAB MOSFETs begins by analyzing the
parameters and the requirements of a given selection of supercaps:
1) For better leakage current matching results, pick the same make
and model of supercaps to be connected in a series. If possible,
select supercaps from the same production batch. (Note: SAB
MOSFETs are precisely set at the factory and specified such that
their lot-to-lot and MOSFET-to-MOSFET variation is not a concern.)
In real life situations, the actual circuit behavior is a little different,
further reducing overall leakage currents from both supercaps and
SAB MOSFETs, due to the automatic compensation for different
leakage current levels by both the supercaps themselves and in
combination with the SAB MOSFETs. Take the above example of
two supercaps in series, assuming that the top supercap is leaking
10µA and the bottom one leaking 4µA (both at the rated 2.7V max.)
while the power supply remains at 5V DC. The actual voltage across
the top supercap tends to be less than 50% of 5.0V, due to its
internal leakage current, and results in a lowered current level be-
cause the voltage across it tends to be lower as well. The total
voltage across both supercaps is still 5.0V, so each supercap would
experience a lowered voltage at less than maximum rated voltage
of 2.7V, thereby resulting in reduced overall leakage currents in
each of the two supercaps.
2) Determine the leakage current range of the supercaps.
3) Determine the desired nominal operating voltage of the supercaps.
4) Determine the maximum operating voltage rating of the supercaps.
5) Calculate or measure the maximum leakage current of the
supercap at the maximum rated operating voltage.
6) Determine the operating temperature range of the supercaps.
7) Determine any additional level of operating leakage current in
the system.
These leakage currents are then further regulated by the SAB
MOSFETs connected across each of the supercaps. The end re-
sult is a compensated condition where the top supercap has ~2.4V
and the bottom cap has a voltage of ~2.6V. The excess leakage
current of the top supercap is bypassed across the bottom SAB
MOSFET, so that there is little or no net additional leakage current
introduced by the bottom SAB MOSFET. Meanwhile the top SAB
MOSFET, with ~2.4V across it, is biased to conduct (or leak) very
little drain current. Note also that the top supercap is now biased at
~2.4V and, therefore, would experience less current leakage than
Next, determine the normalized drain current of a SAB MOSFET at
a pre-selected operating voltage. For example, theALD810025 has
a rated leakage, or drain, current of 1µA at applied drain-gate source
voltage of 2.50V. If the desired normalized drain current is 0.01µA,
then the ALD810025 would give a bias drain-gate source voltage of
approximately 2.3V at that current, which produces an equivalent
ON resistance of 2.3V/0.01µA ~= 230MΩ (using the rule of thumb
of one decade of current change per 0.10V of V
= V change).
GS
DS
ALD810023, ALD810024, ALD810025,
ALD810026, ALD810027, ALD810028
Advanced Linear Devices, Inc.
7 of 17
ALD [ ADVANCED LINEAR DEVICES ]
