Datasheet
THERMAL ANALYSIS
25kW
110kW
15kW
I
S
Control
-V
S
+V
S
V
DIS
Q1
P =10V 42.6mA+3 5 /(4 (100 ||1.2k ))=629mW´ W
D
´ ´ W
2
MaximumT =+85 C+(0.629W 80 C/W)=135 C°
J
´ ° °
OPA3695
SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008 ...............................................................................................................................................
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Turn-on time is very quick from the shutdown
condition (typically < 25ns). Turn-off time strongly
The OPA3695 does not require heatsinking or airflow
depends on the selected gain configuration and load,
in most applications. Maximum desired junction
but is typically 1 µ s for the circuit of Figure 35 . To shut
temperature sets the maximum allowed internal
down, the control pin must be asserted low. This logic
power dissipation as described here. In no case
control is referenced to the positive supply, as the
should the maximum junction temperature be allowed
simplified circuit of Figure 44 shows.
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 dissipated in the output stage (P
DL
) to deliver
load power. Quiescent power is simply the specified
no-load supply current times the total supply voltage
across the part. P
DL
depends on the required output
signal and load but would, for a grounded resistive
load, be at a maximum when the output is fixed at a
voltage equal to 1/2 either supply voltage (for equal
bipolar supplies). Under this worst-case condition,
P
DL
= V
S
2
/(4 × R
L
) where R
L
includes feedback
network loading. This value is the absolute highest
power that can be dissipated for a given R
L
. All actual
applications dissipate less power in the output stage.
Note that it is the power in the output stage and not
into the load that determines internal power
Figure 44. Simplified Disable Control Circuit
dissipation.
In normal operation, base current to Q1 is provided As a worst-case example, compute the maximum T
J
through the 110k Ω resistor while the emitter current using an OPA3695IDBQ (SSOP-16 package) in the
through the 15k Ω resistor sets up a voltage drop that circuit of Figure 35 operating at the maximum
is inadequate to turn on the two diodes in the Q1 specified ambient temperature of +85 ° C and driving a
emitter. As V
DIS
is pulled low, additional current is grounded 100 Ω load at V
S
/2. Maximum internal
pulled through the 15k Ω resistor, eventually turning power is:
on these two diodes ( ≈ 80 µ A). At this point, any
further current pulled out of V
DIS
goes through those
diodes, holding the emitter-base voltage of Q1 at
approximately 0V. This sequence shuts off the
collector current out of Q1, turning the amplifier off.
Actual applications operate at a lower junction
The supply current in the shutdown mode is only that
temperature than the +135 ° C computed above. This
required to operate the circuit of Figure 44 .
condition is because the RMS voltage of the output
The shutdown feature for the OPA3695 is a
signals vary, along with the fact that part of the
positive-supply-referenced, current-controlled
quiescent current is steered to the output, thus
interface. Open-collector (or drain) interfaces are
reducing the 10V × 42.6mA dominant term. Compute
most effective, as long as the controlling logic can
the actual output stage power to get an accurate
sustain the resulting voltage (in the open mode) that
estimate of maximum junction temperature, or use
appears at the V
DIS
pin. That voltage is one diode
the results shown here as an absolute worst case
below the positive supply voltage applied to the
maximum scenario.
OPA3695. For voltage output logic interfaces, the
on/off voltage levels described in the Electrical
Characteristics apply only for a +5V positive supply
on the OPA3695. An open-drain interface is
recommended for shutdown operation using a higher
positive supply for the OPA3695 and/or logic families
with inadequate high-level voltage swings.
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