9
FN7426.1
May 4, 2007
output will not change phase, the input's overvoltage should
be avoided. If an input voltage exceeds supply voltage by
more than 0.6V, electrostatic protection diodes placed in the
input stage of the device begin to conduct and overvoltage
damage could occur.
FIGURE 19. OPERATION WITH BEYOND-THE-RAILS INPUT
Power Dissipation
With the high-output drive capability of the EL5027 buffer, it
is possible to exceed the +125°C 'absolute-maximum
junction temperature' under certain load current conditions.
Therefore, it is important to calculate the maximum junction
temperature for the application to determine if load
conditions need to be modified for the buffer to remain in the
safe operating area.
The maximum power dissipation allowed in a package is
determined according to:
where:
T
JMAX
= Maximum junction temperature
T
AMAX
= Maximum ambient temperature
Θ
JA
= Thermal resistance of the package
P
DMAX
= Maximum power dissipation in the package
The maximum power dissipation actually produced by an IC
is the total quiescent supply current times the total power
supply voltage, plus the power in the IC due to the loads, or:
when sourcing, and:
when sinking.
where:
i = 1 to 2 for dual buffer
V
S
= Total supply voltage
I
SMAX
= Maximum supply current per channel
V
OUT
i = Maximum output voltage of the application
I
LOAD
i = Load current
If we set the two P
DMAX
equations equal to each other, we
can solve for R
LOAD
i to avoid device overheat. Figure 20
and Figure 21 provide a convenient way to see if the device
will overheat. The maximum safe power dissipation can be
found graphically, based on the package type and the
ambient temperature. By using the previous equation, it is a
simple matter to see if P
DMAX
exceeds the device's power
derating curves.
Unused Buffers
It is recommended that any unused buffer have the input tied
to the ground plane.
Driving Capacitive Loads
The EL5027 can drive a wide range of capacitive loads. As
load capacitance increases, however, the -3dB bandwidth of
the device will decrease and the peaking increase. The
buffers drive 10pF loads in parallel with 10kΩ with just 1.5dB
of peaking, and 100pF with 6.4dB of peaking. If less peaking
is desired in these applications, a small series resistor
(usually between 5Ω and 50Ω) can be placed in series with
the output. However, this will obviously reduce the gain
slightly. Another method of reducing peaking is to add a
"snubber" circuit at the output. A snubber is a shunt load
consisting of a resistor in series with a capacitor. Values of
150Ω and 10nF are typical. The advantage of a snubber is
that it does not draw any DC load current or reduce the gain.
Power Supply Bypassing and Printed Circuit
Board Layout
The EL5027 can provide gain at high frequency. As with any
high frequency device, good printed circuit board layout is
necessary for optimum performance. Ground plane
construction is highly recommended, lead lengths should be
as short as possible, and the power supply pins must be well
bypassed to reduce the risk of oscillation. For normal single
supply operation, where the V
S
- pin is connected to ground,
a 0.1µF ceramic capacitor should be placed from V
S
+ to pin
to V
S
- pin. A 4.7µF tantalum capacitor should then be
connected in parallel, placed in the region of the buffer. One
4.7µF capacitor may be used for multiple devices. This same
capacitor combination should be placed at each supply pin
to ground if split supplies are to be used.
1V
1V
10µs
V
S
=±2.5V
T
A
=25°C
V
IN
=6V
P-P
P
DMAX
T
JMAX
- T
AMAX
Θ
JA
---------------------------------------------
=
P
DMAX
ΣiV[
S
I
SMAX
V
S
+( - V
OUT
i) I
LOAD
i]×+×=
P
DMAX
ΣiV[
S
I
SMAX
V(
OUT
i - V
S
-) I
LOAD
i×+×]=
EL5027
