Hello, I'm Alan Martin, and welcome to our topic of linear regulator fundamentals, this module covering NMOS transistors used as the pass element inside the linear regulator. All linear regulators look like the following diagram. There's a pass element that is controlled by an error amplifier. The error amplifier is fed by an internal voltage reference and a voltage divider that is sampling the output voltage. Together, these control the control terminal on the pass transistor-- in this case, an n-channel FET. So let's look at the schematic and show the losses using the standard NMOS topology. The losses are actually quite few. With an NMOS transistor, there is essentially no gate current requirement. So it's superior to the NPN bipolar equivalent, in that regard. As a result, you can have very little wasted power in the control section of the linear regulator. As shown here, there's a current source feeding the internal reference, and there is a control amplifier driving the gate of the pass transistor, its other input terminal tied to the output voltage sampling resisted divider. This shows the particular characteristics of driving the n-channel pass element. Notice that one of the limitations in this topology is that the VGS voltage of the pass transistor is higher, a higher burden on the ultimate dropout voltage of the regulator, when compared to the NPN equivalent. But still it has advantages, and it should be considered. Let's look at the drive current requirement in the case of the 50 milliamp application on the left and increasing the load current to three amps on the right. If you look at the numbers closely, you'll find that there's no difference in the amount of current that it takes to drive between these two modes of operation. As a result, they're very efficient in the use of the internal quiescent supply current for controlling the linear regulator operation. One disadvantage of the n-channel FET is, because of the VGS requirement being higher, it in fact would be easier if you provide an external bias rail to run the control amplifier. This may appear like a disadvantage, but the result is quite impressive. In this case, the VIN rail could be, say, 1.8 volts. The output rail could be 1.5 or even higher, even closer to the 1.8 input rail. But you would need a bias supply, say 5 volts, and that's typically available in most systems. As a result, you can have a very good combination of performance characteristics. You can have extremely low dropout voltage, very good bandwidth, very low quiescent current into the bias pin running the control element, very high power-supply rejection ratio, because the VIN terminal is looking into the drain of the MOSFET, and that looks like a current source, a very high impedance. And the list goes on. So the advantages of using a n-channel FET is that, if you look at it on the substrate level, an N-FET has a lower ON resistance than the P-FET equivalent, when implemented in silicon. Therefore you can implement a higher load current LDO using an n-channel FET than you would with the equivalent p-channel-type pass element. This allows for very low VIN and VOUT values, the lower output impedance reduces the effect of the load pull, and, as a result, the external capacitor is not nearly an issue as you find with p-type pass elements. As we mentioned, the low ground-pin current is not a function of low current, as it is in bipolar-type devices. And the resulting topology gives you very high DC gain and good bandwidth that results in good load regulation and very good transient response. It has the following characteristics, as well. The input voltage could be higher than the output voltage by the VGS requirement of the pass transistor. As a result, that's not a low-dropout type of regulator. But, if you do apply the trick with the external bias input, then it does become a low-dropout configuration. Ground-pin current does not vary with output load current, as we mentioned. And, while there are particular data sheets that say they don't require an output capacitor for operation, we recommend you use one anyway. Thank you for joining us on this topic.

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