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ALD810023 参数 Datasheet PDF下载

ALD810023图片预览
型号: ALD810023
PDF下载: 下载PDF文件 查看货源
内容描述: QUAD超级电容器自动平衡( SABA ?? ¢ ) MOSFET阵列 [QUAD SUPERCAPACITOR AUTO BALANCING (SAB™) MOSFET ARRAY]
分类和应用: 电容器局域网
文件页数/大小: 17 页 / 523 K
品牌: ALD [ ADVANCED LINEAR DEVICES ]
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GENERAL DESCRIPTION (cont.)  
SUPERCAPS  
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.  
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.  
Hence, at V  
= V  
= 2.396V, the ALD810025 has drain current  
= 2.292V, the ALD810025 drain current  
GS  
of 0.1µA. At V  
DS  
= V  
GS  
DS  
GS  
0.001µA. It is apparent that at V  
becomes 0.01µA. At V  
= V  
= 2.188V, the drain current is  
DS  
= VDS 2.10V, the drain leak-  
GS  
age current 0.00014µA, which is essentially zero when compared  
to 1µA initial threshold current. When individual V  
= V volt-  
DS  
GS  
ages fall below 1.9V, the SAB MOSFET leakage current essentially  
goes to zero (~70pA).  
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.  
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.  
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.  
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.  
CHARACTERISTICS OF SUPERCAP AUTO BALANCING  
(SAB™) MOSFETS  
At V  
= V  
voltages of the ALD810025 above its V threshold  
t
GS  
voltage, its drain current behavior has the opposite near-exponen-  
tial effect. At V = V = 2.60V, for example, the ALD810025  
DS  
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.  
GS  
DS  
I
increases tenfold to 10µA. Similarly, I  
becomes  
DS(ON)  
100µA for a V  
DS(ON)  
voltage increase to 2.74V, and 300µA at  
= V  
GS  
2.84V. (See Table 1)  
DS  
As I  
changes rapidly with applied voltage on the Drain-Gate  
DS(ON)  
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.  
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.  
Consider the case when two supercap cells are connected in  
series, each with a SAB MOSFET connected across it in the  
V mode (V  
= V ), charged by a power supply to a voltage  
t
DS  
GS  
equal to 2 x V .  
S
If the top supercap has a higher internal leakage current than the  
bottom supercap, the voltage V across it tends to drop lower  
S(top)  
than that of the bottom supercap. The SAB MOSFET I  
across  
DS(ON)  
the top supercap, sensing this voltage drop, drops off rapidly.  
Meanwhile, the bottom supercap V voltage tends to rise,  
For the ALD8100xx/ALD9100xx family of SAB MOSFETs, the  
S(bottom)  
threshold voltage V of a SAB MOSFET is defined as its drain-gate  
as V  
= (2 x V ) - V  
. This tendency for the voltage  
t
S(bottom)  
rise also increases V  
S
S(top)  
source voltage at a drain-source ON current, I  
its gate and drain terminals are connected together (V  
GS  
= 1µA when  
= V ).  
= V voltage of the SAB MOSFET across  
DS(ON)  
GS  
DS  
the bottom supercap. This increased V  
cause the I  
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-  
= V voltage would  
DS  
DS  
GS  
current of the bottom SAB MOSFET to increase  
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,  
DS(ON)  
V = 2.50V, and I  
t
= 1µA. The SAB MOSFET has a precision  
DS(ON)  
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/  
ing mechanism, the top supercap, V  
while the bottom supercap, V  
creating simultaneously opposing actions of the supercap leakage  
currents.  
, voltage tends to rise  
, voltage tends to drop,  
S(top)  
S(bottom)  
ALD910025 SAB MOSFET, it conducts an I  
= 1µA.  
DS(ON)  
ALD810023, ALD810024, ALD810025,  
ALD810026, ALD810027, ALD810028  
Advanced Linear Devices, Inc.  
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