SBOSAA5B April 2022 – September 2022 OPA2675
PRODUCTION DATA
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:
With P_{L} power and V_{RMS} voltage at the load, and R_{L} load impedance, this calculation gives:
With V_{P} peak voltage at the load and the crest factor, CF:
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:
This V_{LPP} is usually computed for a nominal line impedance and may 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:
With V_{LPP} defined in Equation 6 and R_{M} defined in 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}.
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:
Or, second, as required single-supply voltage:
The minimum supply voltage for a set of power and load requirements is given by Equation 10, where V_{1}, V_{2}, R_{1}, and R_{2} are internal to the OPA2675.
Table 8-1 gives V_{1}, V_{2}, R_{1}, and R_{2} for +12-V operation of the OPA2675.
V_{1} | R_{1} | V_{2} | R_{2} |
---|---|---|---|
0.9 V | 2 Ω | 0.9 V | 2 Ω |