OPA2674_17 Datasheet, PDF (18/36 Pages) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

OPA2674
SBOS270C − AUGUST 2003 − REVISED AUGUST 2008
OPA2674 HDSL2 UPSTREAM DRIVER
Figure 6 shows an HDSL2 implementation of a single-
supply driver.
20Ω
+12V
1/2
O PA 26 74
0.1µF
324Ω
I_P = 185mA
11.5Ω _1:2.4
AFE _+6V
2kΩ
2V_PP
Max
Assumed 0.1µF 2kΩ
82.5Ω
1µF
324Ω
17.7V_PP
11.5Ω
Z_Line
135Ω
20Ω
1/2
O PA 26 74
I_P = 185mA
Figure 6. HDSL2 Upstream Driver
The two designs differ by the values of the matching im-
pedance, the load impedance, and the ratio turns of the
transformers. All of these differences are reflected in the
higher peak current and thus, the higher maximum power
dissipation in the output of the driver.
LINE DRIVER HEADROOM MODEL
The first step in a driver design is to compute the peak-to-
peak output voltage from the target specifications. This is
done using the following equations:
P_L + 10
log
V _RMS ²
(1mW) R_L
(3)
With P_L power and V_RMS voltage at the load, and R_L load
impedance, this gives:
Ǹ _{VRMS +}
(1mW)
R_L
PL
10 10
(4)
V_P + CrestFactor V_RMS + CF V_{RMS (5)}
with V_P peak voltage at the load and CF Crest Factor;
V_LPP + 2 CF V_RMS
(6)
with V_LPP: peak-to-peak voltage at the load.
www.ti.com
Consolidating Equations 3 through 6 allows the required
peak-to-peak voltage at the load function of the crest fac-
tor, the load impedance, and the power in the load to be ex-
pressed. Thus:
V_LPP + 2
CF
Ǹ_(1mW)
R_L
10
PL
10
(7)
This V_LPP is usually computed for a nominal line imped-
ance and may be taken as a fixed design target.
The next step for the driver is to compute the individual am-
plifier 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:
"
I_P
+
1
2
2 V_LPP
n
1
4R_M
(8)
With V_LPP defined in Equation 7 and R_M defined in Equa-
tion 2. The peak current is computed in Figure 7 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.
±I_P R_M
1:n
2V_LPP
n
V_LPP
n
R_M
±I_P
R_L
V_LPP
Figure 7. Driver Peak Output Model
With the required output voltage and current versus turns
ratio set, an output stage headroom model will allow the re-
quired supply voltage versus turns ratio to be developed.
The headroom model (see Figure 8) can be described with
the following set of equations:
First, as available output voltage for each amplifier:
V_OPP + V_CC * (V₁ ) V₂) * I_P (R₁ ) R₂) ₍₉₎
Or, second, as required single-supply voltage:
V_CC + V_OPP ) (V₁ ) V₂) ) I_P (R₁ ) R₂) ₍₁₀₎
The minimum supply voltage for a power and load require-
ment is given by Equation 10.
18