The first step in a driver design is to compute the peak-to-peak output voltage from the target specifications. This calculation is done using the following equations:

Equation 2.

With P_L power and V_RMS voltage at the load, and R_L load impedance, this calculation gives:

Equation 3.

Equation 4.

With V_P peak voltage at the load and the crest factor, CF:

Equation 5.

With V_LPP: peak-to-peak voltage at the load.

Consolidating Equation 2 through Equation 5 allows the required peak-to-peak voltage at the load function of the crest factor, the load impedance, and the power in the load to be expressed. Thus:

Equation 6.

This V_LPP is usually computed for a nominal line impedance and can be taken as a fixed design target.

The next step for the driver is to compute the individual amplifier output voltage and currents as a function of V_PP on the line and transformer turns ratio. As the turns ratio changes, the minimum allowed supply voltage also changes. The peak current in the amplifier is given by:

Equation 7.

With V_LPP defined in Equation 6 and R_M defined in Equation 8.

Equation 8.

The peak current is computed in Figure 8-9 by noting that the total load is 4R_M and that the peak current is half of the peak-to-peak calculated using V_LPP.

Figure 8-9 Driver Peak Output Model

With the required output voltage and current versus turns ratio set, an output stage headroom model allows the required supply voltage versus turns ratio to be developed.

The headroom model (see Figure 8-10) can be described with the following set of equations:

First, as available output voltage for each amplifier:

Equation 9.

Or, second, as required single-supply voltage:

Equation 10.

The minimum supply voltage for a set of power and load requirements is given by Equation 10, where V₁, V₂, R₁, and R₂ are internal to the OPA2673.

Table 8-1 gives V₁, V₂, R₁, and R₂ for +12-V operation of the OPA2673.

Figure 8-10 Line Driver Headroom Model