11
LT1492/LT1493
APPLICATIONS INFORMATION
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Supply Voltage
The LT1492/LT1493 op amps are fully functional and all
internal bias circuits are in regulation with 2.1V of supply.
The amplifiers will continue to function with as little as
1.6V, although the input common mode range and the
phase margin are about gone. The minimum operating
supply voltage is guaranteed by the PSRR tests that are
done with the input common mode equal to 500mV and a
minimum supply voltage of 2.5V.
The positive supply pin of the LT1492/LT1493 should be
bypassed with a small capacitor (about 0.01µF) within an
inch of the pin. When driving heavy loads and for good
settling time, an additional 4.7µF capacitor should be
used. When using split supplies, the same is true for the
negative supply pin.
Power Dissipation
The LT1492/LT1493 combine high speed and large output
drive in small packages. Because of the wide supply
voltage range, it is possible to exceed the maximum
junction temperature under certain conditions. Maximum
junction temperature (T
J
) is calculated from the ambient
temperature (T
A
) and power dissipation (P
D
) as follows:
LT1492CN8: T
J
= T
A
+ (P
D
• 130°C/W)
LT1492CS8: T
J
= T
A
+ (P
D
• 190°C/W)
LT1493CS: T
J
= T
A
+ (P
D
• 150°C/W)
Worst-case power dissipation occurs at the maximum
supply current and when the output voltage is at 1/2 of
either supply voltage (or the maximum swing if less than
1/2 supply voltage). For each amplifier P
DMAX
is:
P
DMAX
= (V
+
– V
–
)(I
SMAX
) + (V
+
/2)
2
/R
L
Example: LT1493 at 70°C, V
S
= ±15V, R
L
= 1k
P
DMAX
= (30V)(930µA) + (7.5V)
2
/1kΩ = 84.2mW
T
JMAX
= 70°C + (4 • 84.2mW)(150°C/W) = 121°C
Inputs
Typically, at room temperature, the inputs of the LT1492/
LT1493 can common mode 400mV below ground (V
–
)
and to within 1.2V of the positive supply with the amplifier
still functional. However, the input bias current will shift as
shown in the characteristic curves. For full precision
performance, the common mode range should be limited
between ground (V
–
) and 1.5V below the positive supply.
When either of the inputs is taken below ground (V
–
) by
more than about 700mV, that input current will increase
dramatically. The current is limited by internal 100Ω
resistors between the input pins and diodes to each
supply. The output will remain low (no phase reversal) for
inputs 1.3V below ground (V
–
). If the output does not have
to sink current, such as in a single supply system with a 1k
load to ground, there is no phase reversal for inputs up to
8V below ground.
There are no clamps across the inputs of the LT1492/
LT1493 and therefore each input can be forced to any
voltage between the supplies. The input current will re-
main constant at about 50nA over most of this range.
When an input gets closer than 1.5V to the positive supply,
that input current will gradually decrease to zero until the
input goes above the supply, then will increase due to the
previously mentioned diodes. If the inverting input is held
more positive than the noninverting input by 200mV or
more, while at the same time the noninverting input is
within 300mV to ground (V
–
), then the supply current will
increase by 1mA and the noninverting input current will
increase to about 10µA. This should be kept in mind in
comparator applications where the inverting input stays
above ground (V
–
) and the noninverting input is at or near
ground (V
–
).
Output
The output of the LT1492/LT1493 will swing to within
0.65V of the positive supply and to within 3mV of the
negative supply with no load. Under loaded conditions see
the Typical Performance curves for the output saturation
voltage characteristics.
The output of the LT1492/LT1493 has reverse-biased
diodes to each supply. If the output is forced beyond either
supply, unlimited currents will flow. If the current is
transient and limited to several hundred milliamps, no
damage will occur.
