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1.2 Linear regulator fundamentals - NPN
Hello, I'm Alan Martin, and welcome to our topic on Linear Regulator Fundamentals. This module discusses NPN type low dropout regulators where the pass element is an NPN bipolar transistor internal to the IC. Any linear regulator consists of the following diagram. There's a pass transistor that is controlled by internal control circuitry and we're discussing the differences in past transistor characteristics and how that affects the linear regulator performance. All irregulars take on this particular format of having a divider resistor tied to Vout feeding an error amplifier and an internal voltage reference. The difference between those two changes the control signal into the pass transistor, and results in a regulated output voltage. In this particular topology called a Quasi-LDO, the voltage regulator pass element consists of an internal in NPN transistor. That NPN bipolar transistor is in turn driven by a PNP driver transistor that provides base current into the NPN control element. In turn, the PNP is controlled by circuitry pulling base current from the input supply through the base emitter junction of the PNP toward ground. Now this ground current is a loss element, which must be taken into account in certain designs. So there are losses due to this. Fortunately, you've got current gain associated with each of the transistors. There's typically a gain of about 80 in the NPNs and a gain of about 40 in the PNP. And we've drawn the path of the particular drive currents and the betas of the transistors illustrating the gain of those transistors in this diagram here. Now let's look at the actual drive current requirements under two different load current situations. In the first case, we've got a drive current for a 5 milliamp output current, and in the second case we've increased the load current by a factor of 100 to 500 milliamps. And what you see is that the ground current also increases a factor of 100, and that is a loss element for this particular approach. In summary, it has the following characteristics. The transistor configuration allows the input voltage to be as close as 9/10 of a bolt from the required output voltage. So it is, in fact, a low dropout regulator. But at highest currents the burden requirement or the overhead required is that you may need as much as a volt and a half before the transistor configuration drops out of regulation. This approach has higher ground current than NPN-Darlington, but less than the PNP type configuration. It does require an output capacitor. In fact, all the linear regulators require some sort of output capacitance. But it's superior in that ESR requirements are not nearly so restrictive as they are in PNP and P channel type approaches. Thank you for joining us on this topic.
Watch the next video in the series:
Linear regulator fundamentals - PNP
Details
Date:
February 25, 2013
This presentation provides an overview of NPN voltage regulators.
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