LDK120M-R

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LDK120M-R is a positive adjustable linear voltage regulator and can output a stable voltage 5V to 1.2V adjustable. The LDK120M-R is a three terminal adjustable output regulator requires two external resistor to set the output voltage. It can replace LM117 in many applications.

LDK120M-R Typical Applications

• Post regulator for switched mode power supplies
• On-card regulation for elimination of noise and distribution problems
• Local on card regulation
• Microcomputer and logic power supplies
• Test and control equipment

LDK120M-R Working Principle

The LDK120M-R operates as a series regulator by providing a constant voltage output, the input voltage setting depending upon the external resistor. The integrated pass transistor switch and a highly stabilized reference provide a series controlled voltage output. The current into the ADJ pin is proportional to the voltage set point and by adjusting the bias current at this point the voltage on the output can be adjusted. In this way, up to a certain maximum output current which depends on temperature, the exact output voltage is almost independent of load variations and line ripples. Ripple rejection is first order, about 50dB. By increasing the value of the external resistors, the output voltage can be reduced up to 1.2V.

LDK120M-R is initially set to 5V output through two external resistors R1 and R2. The output voltage VO is maintained constant regardless of the input voltage variations and load variations as given by the following equation:

VO = 5V × ( 1 + R2/R1 )

When the load increases, the output voltage will decrease, and the voltage drop across the Adjustment (ADJ) pin will increase, causing the pass transistor to increase the current flow, thus providing more current and the output voltage will become the same.

When the output voltage rises above the reference voltage, the output transistor turns off and the current flow is reduced, and the output again will drop to its controlling value.

Current limiting is provided by the internal current limiting transistor, which begins to conduct when the voltage on the adjust pin, trying to increase the output voltage. The transistor limits the current flow to a safe level. The current limiting transistor is limited to a maximum current of 1.6A.

One of the most important features of the LDK120M-R is its current limiting circuit, which helps protect the device from over-current conditions. With the current limiting feature, the output voltage is maintained at a predetermined voltage, even if the current draw exceeds the maximum current rating.

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