8
FN7506.0
February 14, 2005
Choice of Feedback Resistor and Gain Bandwidth
Product for VCOM Amplifier
For applications that require a gain of +1, no feedback
resistor is required. Just short the output pin to the inverting
input pin. For gains greater than +1, the feedback resistor
forms a pole with the parasitic capacitance at the inverting
input. As this pole becomes smaller, the amplifier's phase
margin is reduced. This causes ringing in the time domain
and peaking in the frequency domain. Therefore, RF has
some maximum value that should not be exceeded for
optimum performance. If a large value of RF must be used, a
small capacitor in the few Pico farad range in parallel with RF
can help to reduce the ringing and peaking at the expense of
reducing the bandwidth.
As far as the output stage of the amplifier is concerned, the
output stage is also a gain stage with the load. RF and RG
appear in parallel with RL for gains other than +1. As this
combination gets smaller, the bandwidth falls off.
Consequently, RF also has a minimum value that should not
be exceeded for optimum performance. For gain of +1, RF =
0 is optimum. For the gains other than +1, optimum
response is obtained with RF between 1k to 5k.
The VCOM amplifier has a gain bandwidth product of
20MHz. For gains
≥5, its bandwidth can be predicted by the
following equation:
Output Drive Capability
The EL5624A does not have internal short-circuit protection
circuitry. The buffer will limit the short circuit current to over
250mA and the VCOM amplifier will limit the short circuit
current to ±200mA if the outputs are directly shorted to the
positive or the negative supply. If the output is shorted
indefinitely, the power dissipation could easily increase such
that the part will be destroyed. Maximum reliability is
maintained if the output continuous current never exceeds
±30mA for the buffers and ±60mA for the VCOM amplifier.
These limits are set by the design of the internal metal
interconnections.
The Unused Buffers
It is recommended that any unused buffers should have their
inputs tied to ground plane.
Power Dissipation
With the high-output drive capability of the EL5624A, 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:
TJMAX = Maximum junction temperature
TAMAX = Maximum ambient temperature
θJA = Thermal resistance of the package
PDMAX = 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 total number of buffers
VS = Total supply voltage of buffer and VCOM
ISMAX = Total quiescent current
VOUTi = Maximum output voltage of the application
VOUT = Maximum output voltage of VCOM
ILOADi = Load current of buffer
ILA = Load current of VCOM
If we set the two PDMAX equations equal to each other, we
can solve for the RLOAD's to avoid device overheat. The
package power dissipation curves 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 PDMAX exceeds the
device's power derating curves.
Power Supply Bypassing and Printed Circuit
Board Layout
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 VS- pin is
connected to ground, one 0.1F ceramic capacitor should be
Gain BW
20MHz
=
×
PDMAX
TJMAX - TAMAX
Θ
JA
---------------------------------------------
=
PDMAX
VS IS ΣiVS+
(
[
VOUTi) ILOADi]
VS+
(
VOUT) ILA
×
–
+
×
–
×
+
×
=
PDMAX
VS IS ΣiV
(
OUTi
[
VS-) ILOADi]
VOUT
(
VS-) ILA
×
–
+
×
–
×
+
×
=
EL5624A