Adjustable outputs can meet the requirement from different customers and make it easy to power the transmitter since the design may need several voltage rails. In this design, a signal (0 V to 2.5 V) from FPGA is used to control the output voltage by comparing the feedback voltage captured from the positive output. The output of the op amp is connected to the FB pin of the LM51551 device to regulate the V_OUT.

Figure 2-5 shows the diagram of the voltage control circuit. To select the proper value of R18, R19, R20, R21, R22, and R26, see the following equations.

where

• V_ref is the reference voltage of the FB pin in LM51551
• V_ctrl is the desired control voltage range of the customers
• k_FB, k_a, and k_b are the ratio of the resistors which helps to define the value

For example, the following equations show that if k_FB = 1.5 , desired control voltage is 0 V to 2.5 V and the output voltage is 15 V to 75 V: