Introduction
ALD810021/ALD910021 are members of the ALD8100xx (quad-channel) and ALD9100xx (dual-channel) Supercapacitor Auto-Balancing MOSFET (SAB MOSFET) family. SAB MOSFETs are fabricated 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, can achieve leakage current balancing in series connections by connecting one or more devices across each supercapacitor bank to prevent overvoltage.
The ALD810021 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 ALD910021 provides each of its two SAB MOSFET devices with its own unique, precisely defined operating electrical characteristics, suitable for up to two series-connected supercapacitors.
Each SAB MOSFET features a precise gate threshold voltage in Vt mode, defined as 2.10V when the gate-drain-source terminals (VGS = VDS) are connected together 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 effectively variable resistance whose value varies exponentially with VIN. When this VIN is connected across each series supercapacitor, it balances the voltage and current of each supercapacitor within its rated limits.
When VIN = 2.10V is applied to the ALD810021/ALD910021, IOUT is 1μA. When VIN increases by 100mV to 2.20V, IOUT increases approximately 10-fold. For the ALD910021, when VIN further increases to 2.32V (2.34V for the ALD810021), IOUT increases 100-fold to 100μA. Conversely, when VIN decreases by 100mV to 2.00V, IOUT drops to one-tenth of its previous value, i.e., 0.1μA. A further 100mV decrease in input voltage reduces IOUT to 0.01μA. Therefore, when the ALD810021/ALD910021 SAB MOSFET is connected across a supercapacitor charged to below 1.90V, it consumes essentially no power.
The voltage-dependent on-resistance characteristics of the ALD810021/ALD910021 are highly effective in controlling overvoltage rise across connected supercapacitors. In a series-connected supercapacitor bank, when the voltage across one supercapacitor rises, the voltages across the others decrease, with the supercapacitor exhibiting the highest leakage current having 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.
