UA78L: Quiescent Current = Bias current?

Hello Kei,

The Bias and quiescent may be referred to the same current  in a bipolar LDO, but in modern LDOs they are different. 

In modern LDO's, the bias current is inherent of NMOS LDOs as they do require a higher voltage bias rail to power their internal circuitry. Think of it in terms of the pass device, a MOSFET, where the LDO's input is the drain, the output is the source, and the gate is driven internally by some circuitry. Since VGS > 0, we need a higher voltage at the gate than the output. This is where an additional supply pin, the bias, comes into the picture. TPS748 (https://www.ti.com/lit/ds/symlink/tps748a-q1.pdf) is a good example of an LDO with a bias pin.

Not all of the NMOS LDOs have an external bias pin. Some of them have an internal charge pump to boost the input voltage and drive the pass element. The disadvantage is that noise is higher due to the switching frequency of the charge pump. TPS7A57 is a good example, where a charge pump is integrated but also a bias pin is present. https://www.ti.com/lit/ds/symlink/tps7a57.pdf 

The quiescent current is a small amount of current that flows through the regulator itself to power its internal circuitry. It is the minimum current consumed by the LDO to remain operational and maintain regulation, even when the output is not supplying current to a load.

In the specific case of UA78L, since this is a bipolar design, the quiescent current is referred as the bias current.

Best regards,

Daniel Esparza