LM7321MA NSC [National Semiconductor], LM7321MA Datasheet - Page 19

• Manufacturer Part Number: LM7321MA
• Manufacturer: NSC [National Semiconductor]

SETTLING TIME WITH LARGE CAPACITIVE LOADS
Figure 9 below, shows a typical application where the
LM7321/LM7322 is used as a buffer amplifier for the V
signal employed in a TFT LCD flat panel:
Figure 10 shows the time domain response of the amplifier
when used as a V
application, the op amp loop will try and maintain its output
voltage based on the voltage on its non-inverting input
(V
load. As long as this load current is within the range tolerable
by the LM7321/LM7322 (45 mA sourcing and 65 mA sinking
for ±5V supplies), the output will settle to its final value within
less than 2 μs.
FIGURE 8. Output Sinking Characteristics with Load
REF
) despite the current injected into the TFT simulated
FIGURE 9. V
COM
COM
buffer/driver with V
Driver Application Schematic
Lines
REF
at ground. In this
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OUTPUT SHORT CIRCUIT CURRENT AND DISSIPATION
ISSUES
The LM7321/LM7322 output stage is designed for maximum
output current capability. Even though momentary output
shorts to ground and either supply can be tolerated at all op-
erating voltages, longer lasting short conditions can cause the
junction temperature to rise beyond the absolute maximum
rating of the device, especially at higher supply voltage con-
ditions. Below supply voltage of 6V, the output short circuit
condition can be tolerated indefinitely.
With the op amp tied to a load, the device power dissipation
consists of the quiescent power due to the supply current flow
into the device, in addition to power dissipation due to the load
current. The load portion of the power itself could include an
average value (due to a DC load current) and an AC compo-
nent. DC load current would flow if there is an output voltage
offset, or the output AC average current is non-zero, or if the
op amp operates in a single supply application where the out-
put is maintained somewhere in the range of linear operation.
Therefore:
where:
I
V
V
V
P
P
P
P
S
S
O
r
: Supply Current
TOTAL
Q
DC
AC
: V
: Total Supply Voltage (V
: Average Output Voltage
FIGURE 10. V
= I
+
= See Table 1 below
= I
S
for sourcing and V
O
= P
· V
· (V
S
Q
r
+ P
- V
DC
o
COM
)
+ P
Driver Performance Scope Photo
AC
−
for sinking current
+
− V
−
Op Amp Quiescent Power
)
DC Load Power
AC Load Power
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Dissipation