Output Loading
Applications Information Ð Contd.
and outputs will have to have their levels shifted
above ground to accommodate the lack of nega-
tive supply.
The output stage of the EL4452 is very powerful.
It can typically source 80 mA and sink 120 mA.
Of course, this is too much current to sustain and
the part will eventually be destroyed by excessive
dissipation or by metal traces on the die opening.
The metal traces are completely reliable while de-
livering the 30 mA continuous output given in
the Absolute Maximum Ratings table in this
data sheet, or higher purely transient currents.
The power dissipation of the EL4452 increases
with power supply voltage, and this must be
compatible with the package chosen. This is a
close estimate for the dissipation of a circuit:
e
c
c
a
I , max (V
b
V ) V /R
PAR
c
P
D
2
V
S
S
S
O O
Gain changes only 0.2% from no load to a 100X
load. Heavy resistive loading will degrade fre-
where I , max is the maximum supply current
S
k
quency response and distortion for loads 100X.
g
supply voltage (assumed
V
is the
S
equal)
Capacitive loads will cause peaking in the fre-
quency response. If capacitive loads must be driv-
en, a small-valued series resistor can be used to
isolate it. 12X to 51X should suffice. A 22X series
resistor will limit peaking to 1 dB with even a
220 pF load.
V
is the output voltage
O
R
is the parallel of all resistors loading
PAR
the output
For instance, the EL4452 draws a maximum of
%
18mA. With light loading, RPAR x and the
g
dissipation with 5V supplies is 180 mW. The
maximum supply voltage that the device can run
AGC Circuits
The basic AGC (automatic gain control) loop is
this:
on for a given P and other parameters is
D
2
e
V , max (P
a
a
)/(2I V /R
S O
V
O
/R
)
PAR
S
D
PAR
The maximum dissipation a package can offer is
e
b
A
P , max
D
(T , max T , max) / i
J JA
Where T , max is the maximum die tempera-
J
ture, 150 C for reliability, less to re-
tain optimum electrical performance
§
4452–24
T , max is the ambient temperature,
A
Basic AGC Loop
70 C for commercial and 85 C for in-
dustrial range
§
§
A multiplier scales the input signal and provides
necessary gain and buffers the signal presented
to the output load, a level detector (shown sche-
matically here as a diode) converts some measure
of the output signal amplitude to a DC level, a
low-pass filter attenuates any signal ripple pres-
ent on that DC level, and an amplifier compares
that level to a reference and amplifies the error to
create a gain-control voltage for the multiplier.
The circuitry is a servo that attempts to keep the
output amplitude constant by continuously ad-
justing the multiplier’s gain control input.
i
is the thermal resistance of the
mounted package, obtained from
data sheet dissipation curves
JA
The more difficult case is the SO-14 package.
With a maximum die temperature of 150 C and a
§
maximum ambient temperature of 85 C, the 65 C
§
§
temperature rise and package thermal resistance
of 120 C/W gives a dissipation of 542 mW at
§
85 C. This allows the full maximum operating
§
supply voltage unloaded, but reduced if loaded.
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
