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ALD810024 Datasheet(PDF) 4 Page - Advanced Linear Devices |
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ALD810024 Datasheet(HTML) 4 Page - Advanced Linear Devices |
4 / 17 page ALD8100XX/ALD9100XX SUPERCAPACITOR Advanced Linear Devices, Inc. 4 of 17 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 SAB MOSFET IOUT Current horizontally across the top row of the Table(s). Next, look down that column to the row that contains the maximum desired VIN voltage. The appropriate ALD part number 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 supercapacitor IOUT auto-balancing for the ALD8100xx/ALD9100xx 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 from the SAB MOSFET itself as its VIN increases. Please contact 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 2) max. leakage current = 10µA at 70°C. 3) At 2.50V, the supercapacitor max. leakage current = 2.5µA at 25°C. Next, pick ALD810026, a SAB MOSFET with Vt = 2.60V. For this device, at VIN = 2.60V, the nominal IOUT = 1µA. See Table 1, at VIN = 2.50V, IOUT ~= 0.1µA. At a nominal VIN of 2.50V, the additional leakage current contribu- 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. At VIN of 2.70V across the ALD810026 SAB MOSFET, IOUT = 10µA. 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 is ~= 50µA. For this device, IOUT at 2.50V is ~= 1µA, which is the 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 supercapacitor has VIN of ~2.4V across it and the bottom supercapacitor has VIN of ~2.6V. The excess leakage current of the top supercapacitor is bypassed across the bottom SAB MOSFET. Meanwhile the top SAB MOSFET, with ~2.4V across it, is biased to conduct very little IOUT. Note also that the top 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 equivalent ON resistance value RON of the SAB MOSFET and the capacitance value C of the supercapacitor. In many cases the RON value is small initially, responding rapidly to a large voltage transient by having a smaller RONC time constant. As the volt- ages settle down, the equivalent RON increases. As these RON 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 1000µA (1mA) in Table 1 to locate a VIN voltage of 2.42V, we find the corresponding ALD part number to be ALD810019. ALD8100XX/ALD9100XX FAMILY GENERAL DESCRIPTION (cont.) |
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