How to understand the adjustable current of Lingte?

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How to understand the adjustable current of Lingte? [Copy link]

How to understand the adjustable current of Lingte?

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What's there to watch?

The LT1965 is a low noise, low dropout linear regulator that provides 1.1A of output current and a typical dropout voltage of 310mV.

This is an adjustable constant current source circuit.

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A capacitor C3 is added to the feedback end to prevent instantaneous short-circuit current protection, right?

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This LT1965 is an LDO chip. The current is regulated through the ADJ pin and the external op amp LT1366 is used for coordination. This is all due to the internal characteristics of the LDO.

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The diagram above shows an adjustable current source circuit, and the word "source" cannot be omitted.

Its working principle is: R5 is a current sampling resistor, the op amp is connected in the form of an open-loop comparator, and the threshold is set by R1. When the current increases, the voltage drop of R5 increases, the voltage at the inverting input of the comparator decreases, and the comparator output is the power rail after it is lower than the threshold. The voltage at the ADJ terminal of LT1965 increases, and then its output voltage decreases, and the output current also decreases until it stabilizes at the set value. Conversely, when the current decreases, the voltage drop of R5 decreases, the voltage at the inverting input of the comparator increases, and the comparator output is ground level after it is higher than the threshold. The voltage at the ADJ terminal of LT1965 decreases, and then its output voltage increases, and the output current also increases. It can be seen that this is a negative feedback process, and the presence of C3 can make the comparator output smooth (AC negative feedback), and finally the output current can be stabilized.

The most difficult part of this circuit to understand is the state of the op amp near the current stabilization point. The AC negative feedback channel formed by C3 is crucial here.