Adjustable voltage regulator LM317T structure diagram analysis

The LM317T from National Semiconductor is a commonly used adjustable voltage regulator that can provide an output voltage of 1.25V to 37V at a maximum current of 1.5A. The output voltage can be adjusted with a potentiometer. The circuit in Figure 1 replaces the potentiometer with an analog voltage, which can be controlled by a PWM (pulse width modulation) signal. This signal can be controlled by a microcontroller or any other digital circuit. The same microcontroller can also dynamically monitor and adjust the output of the LM317T.

Through an RC low-pass filter and an op amp, the PWM signal can be converted to a DC level for adjusting the output voltage of the LM317T. By changing the pulse width of the input signal, an analog voltage of 0V to 5V can be generated at the output of the low-pass filter. The op amp multiplies the voltage to obtain the required voltage range.

If the input voltage is multiplied by 2, the voltage at the regulating end of the LM317T is 0V~10V. Its output range is 1.25V~11.25V. The output voltage formula of the LM317T is VOUT=VADJ+1.25V. By selecting the appropriate values ​​of R4 and R2, the gain of the op amp can be changed. The offset voltage of the op amp must be eliminated. The selection of the capacitor and resistor values ​​in the RC low-pass filter depends on the frequency of the PWM signal.