Basic Query on LDO operation

[ssundar.shan]

Hi Friends,
I have some basic doubts on LDO, please help to clarify

In the block diagram of LDO, how the Vref is generated internally? For eg: if i am using a LDO for 5V to 3.3V @ 0.5A, then the LDO IC should have circuits capable of providing 3.3V constant output voltage for reference voltage. How its achieved?

Normally we are considering LDO as a normal resistor, is this resistor is analogy to pass element (BJT / FET) of LDO internal block?

Thanks,
Sundar

 

[AMS012]

You can generate the reference voltage through band gap reference or similar circuits.

 

[d123]

Hi,

Adjustable LDOs have, for example, a 1.2V reference fed to one of the error amplifiers inputs, and the feedback resistors are scaled to create 1.2V at the other input to the error amplifier. As AMSO12 said, it always seems to be a bandgap reference. I don't know how fixed regulators do this as I was researching adjustable, I think it will be the same method. There are a lot of application notes about LDOs if you have a quick search, well worth reading.

slva072: "Technical Review of Low Dropout Voltage Regulator Operation and Performance"
"LDO operation can be explained using the NMOS series pass element I-V characteristics shown in Figure 2. NMOS devices are not widely used in LDO designs, but they simplify the explanation of LDO performance. Figure 2 (a) shows the two regions of operation—linear and saturation. In the linear region, the series pass element acts like a series resistor. In the saturation region, the device becomes a voltage-controlled current source. Voltage regulators usually operate in the saturation region."

 

[RetroTechie]

As d123 hinted at: a linear voltage regulator is better seen as a combo of a voltage reference, and an opamp that amplifies the difference between an output voltage and that reference. Result is used to steer open or close a pass transistor.

LDO regulators are no different there. Actual implementation may be different for each type, but that reference-opamp-pass transistor setup is a good way to visualize what it does.