SLVAFE7 Application note

SLVAFE7 September 2022 LM51551

2.3 Voltage Control by External Signal

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 2-4 Output Voltage Control by External Signal

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.

Figure 2-5 Voltage Control Circuit Diagram

Equation 1.

V_{o u t} = V_{r e f} (k_{F B} + 1)

Equation 2.

\frac{R_{1}}{R_{1} + R_{2}} = k_{a} \dots, \frac{R_{f}}{R_{3}} = k_{b}

Equation 3.

k_{F B} + 1 = {k_{a} \times V}_{o p} (k_{b} + 1) - k_{b} V_{c t r l}

Equation 4.

k_{F B} + 1 = {k_{a} \times V}_{o p} (k_{b} + 1) - k_{b} V_{c t r l}

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:

Equation 5.

1.5 + 1 = k_{a} \times 15 \times (k_{b} + 1)

Equation 6.

1.5 + 1 = k_{a} \times 75 \times (k_{b} + 1) - 2.5 k_{b}

Equation 7.

k_{a} = 0.033, k_{b} = 4