Understanding linear regulators and their key performance parameters: Page 3 of 6

February 01, 2016 //By Sitthipong Angkititrakul and Dhananjay Singh
Understanding linear regulators and their key performance parameters
Sitthipong Angkititrakul and Dhananjay Singh, Intersil Corporationtakes a look at  key performance parameters of LDOs and their impact on delivering clean output voltage to the various devices inside an electronic system.
elements are typically found in LDO regulator designs: NPN transistor-based regulators, PNP transistor-based regulators, N-channel MOSFET-based regulators and P-channel MOSFET-based regulators.
   
        

(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

Design category: 

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