Datasheet

R
F
750W
±150mVOutputAdjustment
= = 2- -
Power-supplydecoupling
notshown.
5kW
5kW
226W
0.1 Fm
R
G
324W
V
I
20kW
10kW
0.1 Fm
-5V
+5V
OPA890
+5V
-5V
V
O
V
O
V
I
R
F
R
G
200kW
2MW
80kW
I
S
Control
-
S
V
+V
S
V
DIS
Q1
OPA890
SBOS369B –MAY 2007–REVISED DECEMBER 2009 www.ti.com
current out of Q1, turning the amplifier off. The supply
current in the disable mode is only that required to
operate the circuit of Figure 54. Additional circuitry
ensures that turn-on time occurs faster than turn-off
time (make-before-break).
When disabled, the output and input nodes go to a
high-impedance state. If the OPA890 is operating at a
gain of +1V/V, it shows a very high impedance at the
output and exceptional signal isolation. If operating at
a gain greater than +1V/V, the total feedback network
resistance (R
F
+ R
G
) appears as the impedance
looking back into the output, but the circuit still shows
very-high forward and reverse isolation. If configured
as an inverting amplifier, the input and output are
connected through the feedback network resistance
(R
F
+ R
G
) and the isolation is very poor, as a result.
Figure 53. DC-Coupled, Inverting Gain of -2V/V,
with Offset Adjustment
THERMAL ANALYSIS
Maximum desired junction temperature sets the
DISABLE OPERATION
maximum allowed internal power dissipation, as
described below. In no case should the maximum
The OPA890 provides an optional disable feature that
junction temperature be allowed to exceed +150°C.
may be used either to reduce system power or to
implement a simple channel multiplexing operation. If
Operating junction temperature (T
J
) is given by T
A
+
the DIS control pin is left unconnected, the OPA890
P
D
× θ
JA
. The total internal power dissipation (P
D
) is
operates normally. To disable the OPA890, the
the sum of quiescent power (P
DQ
) and additional
control pin must be asserted low. Figure 54 shows a
power dissipated in the output stage (P
DL
) to deliver
simplified internal circuit for the disable control
load power. Quiescent power is simply the specified
feature.
no-load supply current times the total supply voltage
across the part. P
DL
depends on the required output
signal and load, but for a grounded resistive load 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
S
2
/(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 OPA890IDBV (SOT23-6 package) in the
circuit of Figure 46 operating at the maximum
specified ambient temperature of +85°C and driving a
grounded 100Ω load.
P
D
= 10V × 1.25mA + 5
2
/(4 × (100Ω || 1.5kΩ)) =
Figure 54. Simplified Disable Control Circuit
79mW
Maximum T
J
= +85°C + (79W × 150°C/W) = +97°C.
In normal operation, base current to Q1 is provided
through the 2MΩ resistor, while the emitter current
Although this result is still well below the specified
through the 80kΩ resistor sets up a voltage drop that
maximum junction temperature, system reliability
is inadequate to turn on the two diodes in the Q1
considerations may require lower operating junction
emitter. As V
DIS
is pulled low, additional current is
temperatures. The highest possible internal
pulled through the 80kΩ resistor, eventually turning
dissipation occurs if the load requires current to be
on those two diodes ( 15μA). At this point, any
forced into the output for positive output voltages, or
further current pulled out of V
DIS
goes through those
sourced from the output for negative output voltages.
diodes, holding the emitter-base voltage of Q1 at
This configuration puts a high current through a large
approximately 0V. This process shuts off the collector
internal voltage drop in the output transistors.
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