OPA2673 Datasheet, PDF (30/40 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier with Active Off-Line Control

OPA2673
SBOS382A – JUNE 2008 – REVISED OCTOBER 2008..................................................................................................................................................... www.ti.com
Power Control Operation
The OPA2673 provides a power control feature that
may be used to reduce system power. The four
modes of operation for this power control feature are
100% bias, 75% bias, 50% bias, and power
shutdown. These four operating modes are set
through two logic lines A0 and A1. Table 3 shows the
different modes of operation.
Table 3. Operating Modes
MODE OF
OPERATION
A1
A0
100% bias
0
0
75% bias
0
1
50% bias
1
0
Shutdown
1
1
The 100% bias mode is used for normal operating
conditions. The 75% bias mode brings the quiescent
power to 24mA. The 50% bias mode brings the
quiescent power to 16mA. The shutdown mode has a
high output impedance as well as the lowest
quiescent power (5.5mA).
If the A0 and A1 pins are left unconnected, the
OPA2673 operates normally (100% bias).
To change the power mode, the control pins (either
A0 or A1) must be asserted low. This logic control is
referenced to the ground supply, as shown in the
simplified circuit of Figure 86.
+V_S
Control
A0 or A1
1.4V
GND
Figure 86. Supply Power Control Circuit
space
space
The shutdown feature for the OPA2673 is a
ground-supply referenced, current-controlled
interface. For voltage output logic interfaces, the
on/off voltage levels described in the Electrical
Characteristics apply only for either the ground pin
RGV package or the –V_S pin used for the
single-supply specifications.
THERMAL ANALYSIS
As a result of the high output power capability of the
OPA2673, heat-sinking or forced airflow may be
required under extreme operating conditions. The
maximum desired junction temperature sets the
maximum allowed internal power dissipation,
described below. In no case should the maximum
junction temperature be allowed to exceed +150°C.
Operating junction temperature (T_J) is given by T_A +
P_D × θ_JA. The total internal power dissipation (P_D) is
the sum of quiescent power (P_DQ) and additional
power dissipation in the output stage (P_DL) to deliver
load power. Quiescent power is the specified no-load
supply current times the total supply voltage across
the part. P_DL depends on the required output signal
and load; for a grounded resistive load, however, P_DL
is at a maximum when the output is fixed at a voltage
equal to 1/2 of either supply voltage (for equal bipolar
supplies). Under this condition, P_DL = V_S2/(4 × R_L),
where R_L includes feedback network loading.
Note that it is the power in the output stage and not
into the load that determines internal power
dissipation.
As a worst-case example, compute the maximum T_J
using an OPA2673 QFN-16 in the circuit of Figure 76
operating at the maximum specified ambient
temperature of +85°C with both outputs driving a
grounded 20Ω load to +2.5V.
P_D = 12V × 32mA + 2 × [5²/(4 × [20Ω 535Ω])] =
1.03W
Maximum T_J = +85°C + (1.03 × 45°C/W) = 131°C
Although this value is still well below the specified
maximum junction temperature, system, reliability
considerations may require lower tested junction
temperatures.The highest possible internal dissipation
occurs if the load requires current to be forced into
the output for positive output voltages, or sourced
from the output for negative output voltages. This
condition puts a high current through a large internal
drop in the output transistors. The output V-I plot in
the Typical Characteristics (Figure 6) includes a
boundary for 2W maximum internal power dissipation
under these conditions.
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