12
FN7111.4
May 4, 2007
FIGURE 22. OPERATION WITH RAIL-TO-RAIL INPUT AND
OUTPUT
Short Circuit Current Limit
The EL5127, EL5227, EL5327, and EL5427 will limit the
short circuit current to ±120mA if the output is directly
shorted to the positive or the negative supply. If an output is
shorted indefinitely, the power dissipation could easily
increase such that the device may be damaged. Maximum
reliability is maintained if the output continuous current never
exceeds ±30mA. This limit is set by the design of the internal
metal interconnects.
Output Phase Reversal
The EL5127, EL5227, EL5327, and EL5427 are immune to
phase reversal as long as the input voltage is limited from
V
S
- -0.5V to V
S
+ +0.5V. Figure 23 shows a photo of the
output of the device with the input voltage driven beyond the
supply rails. Although the device's 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 23. OPERATION WITH BEYOND-THE-RAILS INPUT
Power Dissipation
With the high-output drive capability of the EL5127, EL5227,
EL5327, and EL5427 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
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 Total number of buffers
V
S
= Total supply voltage
I
SMAX
= Maximum quiescent 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. 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 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.
OUTPUT INPUT
5V
V
S
=±5V
T
A
=25°C
V
IN
=10V
P-P
5V
10µs
1V
V
S
=±2.5V
T
A
=25°C
V
IN
=6V
P-P
1V
10µs
P
DMAX
T
JMAX
- T
AMAX
JA
---------------------------------------------
=
P
DMAX
iV
S
I
SMAX
V
S
+ - V
OUT
i I
LOAD
i +=
P
DMAX
iV
S
I
SMAX
V
OUT
i - V
S
- I
LOAD
i+=
EL5127, EL5227, EL5327, EL5427
