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ALD110808PCL Datasheet(PDF) 4 Page - Advanced Linear Devices |
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ALD110808PCL Datasheet(HTML) 4 Page - Advanced Linear Devices |
4 / 11 page ALD110808/ALD110808A/ALD110908/ALD110908A Advanced Linear Devices 4 of 11 SUB-THRESHOLD REGION OF OPERATION Low voltage systems, namely those operating at 5V, 3.3V or less, typically require MOSFETs that have threshold voltage of 1V or less. The threshold, or turn-on, voltage of the MOSFET is a voltage below which the MOSFET conduction channel rapidly turns off. For analog designs, this threshold voltage directly affects the operating signal voltage range and the operating bias current levels. At or below threshold voltage, an EPAD MOSFET exhibits a turn- off characteristic in an operating region called the subthreshold re- gion. This is when the EPAD MOSFET conduction channel rapidly turns off as a function of decreasing applied gate voltage. The con- duction channel induced by the gate voltage on the gate electrode decreases exponentially and causes the drain current to decrease exponentially. However, the conduction channel does not shut off abruptly with decreasing gate voltage, but decreases at a fixed rate of approximately 116mV per decade of drain current decrease. Thus if the threshold voltage is +0.20V, for example, the drain current is 1uA at VGS = +0.20V. At VGS = +0.09V, the drain current would decrease to 0.1uA. Extrapolating from this, the drain current is 0.01uA (10nA) at VGS = -0.03V, 1nA at VGS = -0.14V, and so forth. This subthreshold characteristic extends all the way down to cur- rent levels below 1nA and is limited by other currents such as junc- tion leakage currents. At a drain current to be declared “zero current” by the user, the Vgs voltage at that zero current can now be estimated. Note that using the above example, with VGS(th) = +0.20V, the drain current still hovers around 20nA when the gate is at zero volt, or ground. LOW POWER AND NANOPOWER When supply voltages decrease, the power consumption of a given load resistor decreases as the square of the supply voltage. So one of the benefits in reducing supply voltage is to reduce power consumption. While decreasing power supply voltages and power consumption go hand-in-hand with decreasing useful AC bandwidth and at the same time increases noise effects in the circuit, a circuit designer can make the necessary tradeoffs and adjustments in any given circuit design and bias the circuit accordingly. With EPAD MOSFETs, a circuit that performs a specific function can be designed so that power consumption can be minimized. In some cases, these circuits operate in low power mode where the power consumed is measure in micro-watts. In other cases, power dissipation can be reduced to nano-watt region and still provide a useful and controlled circuit function operation. ZERO TEMPERATURE COEFFICIENT (ZTC) OPERATION For an EPAD MOSFET in this product family, there exist operating points where the various factors that cause the current to increase as a function of temperature balance out those that cause the cur- rent to decrease, thereby canceling each other, and resulting in net temperature coefficient of near zero. One of this temperature stable operating point is obtained by a ZTC voltage bias condition, which is 0.55V above a threshold voltage when VGS = VDS, resulting in a temperature stable current level of about 68uA. For other ZTC op- erating points, see ZTC characteristics. PERFORMANCE CHARACTERISTICS Performance characteristics of the EPAD MOSFET product family are shown in the following graphs. In general, the threshold voltage shift for each member of the product family causes other affected electrical characteristics to shift with an equivalent linear shift in VGS(th) bias voltage. This linear shift in VGS causes the subthresh- old I-V curves to shift linearly as well. Accordingly, the subthreshold operating current can be determined by calculating the gate volt- age drop relative from its threshold voltage, VGS(th). RDS(ON) AT VGS=GROUND Several of the EPAD MOSFETs produce a fixed resistance when their gate is grounded. For ALD110800, the drain current at VDS = 0.1V is at 1uA at VGS = 0.0V. Thus just by grounding the gate of the ALD110800, a resistor with RDS(ON) = ~100KOhm is produced. When an ALD114804 gate is grounded, the drain current IDS = 18.5 uA@ VDS = 0.1V, producing RDS(ON) = 5.4KOhm. Similarly, ALD114813 and ALD114835 produces 77uA and 185uA, respec- tively, at VGS = 0.0V, producing RDS(ON) values of 1.3KOhm and 540Ohm, respectively. MATCHING CHARACTERISTICS A key benefit of using matched-pair EPAD MOSFET is to maintain temperature tracking. In general, for EPAD MOSFET matched pair devices, one device of the matched pair has gate leakage currents, junction temperature effects, and drain current temperature coeffi- cient as a function of bias voltage that cancel out similar effects of the other device, resulting in a temperature stable circuit. As men- tioned earlier, this temperature stability can be further enhanced by biasing the matched-pairs at Zero Tempco (ZTC) point, even though that could require special circuit configuration and power consump- tion design consideration. PERFORMANCE CHARACTERISTICS OF EPAD® PRECISION MATCHED PAIR MOSFET FAMILY (cont.) |
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