10
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 EL5524, EL5624,
EL5724, and EL5824, 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
placed from the VS+ pin to ground. A 4.7F tantalum
capacitor should then be connected from the VS+ pin to
ground. One 4.7F 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.
Important Note: The metal plane used for heat sinking of
the device is electrically connected to the negative
supply potential (VS-). If VS- is tied to ground, the
thermal pad can be connected to ground. Otherwise, the
thermal pad must be isolated from any other power
planes.
P
DMAX
T
JMAX - TAMAX
Θ
JA
---------------------------------------------
=
P
DMAX
V
S
I
S
ΣiV
S+
(
[
V
OUTi )
I
LOADi ]
V
S+
(
V
OUT )
I
LA
×
–
+
×
–
×
+
×
=
P
DMAX
V
S
I
S
ΣiV
(
OUTi
[
V
S- )
I
LOAD i ]
V
OUT
(
V
S- )
I
LA
×
–
+
×
–
×
+
×
=
EL5524, EL5624, EL5724, EL5824