(A) NPN transistor-based regulator (B) PNP transistor-based regulator
(C) N-Channel MOSFET-based regulator (D) P-Channel MOSFET-based regulator
Figure 2: Four different types of transistors used in LDO regulators
In general, transistor-based regulators have higher dropout voltage compared to MOSFET-based regulators. Additionally, a transistor-based regulator’s base driving current of the transistor pass element is proportional to the output current. This directly impacts the transistor-based regulator’s quiescent current. By comparison, the MOSFET pass element uses voltage driven on the isolated gate to make its quiescent current significantly lower than the transistor-based regulator.
Key LDO Performance Parameters
1) Dropout Voltage
Dropout voltage is defined as the difference between the input and output voltages at the point when a further decrease in input voltage causes output voltage regulation to fail. In the dropout condition, the pass element operates in the linear region and behaves like a resistor. For the modern LDO, the pass element is typically implemented with PMOS or NMOS FETs, which can achieve a dropout voltage as low as 30mV to 500mV.
Figure 3 shows the dropout voltage of the ISL80510 LDO, which uses a PMOS FET as the pass element.
Figure 3: ISL80510 dropout voltage
2) Load Regulation
Load regulation is defined as the output voltage change for a given load change. This is typically from no load to full load, given by Equation 1:
Load regulation indicates the performance of the pass element and the closed-loop DC gain of the regulator. The higher the closed-loop DC gain, the better the load regulation.
3) Line Regulation
Line regulation is the output voltage change for a given input voltage change, as defined in Equation 2:
Since line regulation is also dependent on the performance of the pass