Introduction
ALD810024/ALD910024 are members of the ALD8100xx (four-channel) and ALD9100xx (dual-channel) Supercapacitor Auto-Balancing MOSFET (SAB™ MOSFET) series. SAB MOSFETs are manufactured using production-proven EPAD® technology and are designed to address voltage and leakage current balancing in series-connected supercapacitors. Supercapacitors, also known as ultracapacitors or supercaps, when connected in series, can be balanced against leakage current by connecting one or more devices across each supercapacitor bank to prevent overvoltage.
The ALD810024 provides each of its four SAB MOSFET devices with a unique, precisely defined set of operating voltage and current characteristics. It can be used to balance up to four series-connected supercapacitors. The ALD910024 provides each of its two SAB MOSFET devices with its own unique, precisely defined electrical operating characteristics, suitable for up to two series-connected supercapacitors.
Each SAB MOSFET features a precise gate threshold voltage in Vt mode, defined as 2.40V when the gate-drain-source terminals are connected together (VGS = VDS) at a drain-source current of IDS(ON) = 1μA. In this mode, the input voltage VIN = VGS = VDS. Different VIN levels produce output current IOUT = IDS(ON) characteristics, forming an effective variable resistance whose value varies exponentially with VIN. When this VIN is applied across each series-connected supercapacitor, it balances the voltage and current of each supercapacitor within its limits.
When VIN = 2.40V is applied to the ALD810024/ALD910024, IOUT is 1μA. When VIN increases by 100mV to 2.50V, IOUT increases approximately 10-fold. For the ALD910024, when VIN further increases to 2.62V (2.64V for the ALD810024), IOUT increases 100-fold to 100μA. Conversely, when VIN decreases by 100mV to 2.30V, IOUT drops to one-tenth of its previous value, i.e., 0.1μA. A further 100mV reduction in input voltage brings IOUT down to 0.01μA. Therefore, when the ALD810024/ALD910024 SAB MOSFET is connected across a supercapacitor charged to below 2.20V, it consumes essentially no power.
The voltage-dependent on-resistance characteristics of the ALD810024/ALD910024, when connected across a supercapacitor, effectively control the rise of overvoltage across it. In a series-connected supercapacitor bank, when the voltage of one supercapacitor rises, the voltages of the others fall, and the supercapacitor with the highest leakage current exhibits the lowest voltage. The SAB MOSFETs connected across these supercapacitors will exhibit complementary, opposing current levels, generating virtually no additional leakage current beyond that produced by the supercapacitors themselves.
