LT1800
13
1800fa
APPLICATIONS INFORMATION
output voltage is at half of either supply voltage (or the
maximum swing is less than 1/2 supply voltage). P
DMAX
is given by:
P
DMAX
= (V
S
• I
SMAX
) + (V
S
/2)
2
/R
L
Example: An LT1800 in a SOT-23 package operating on ±5V
supplies and driving a 50Ω load, the worst-case power
dissipation is given by:
P
DMAX
= (10 • 4mA) + (2.5)
2
/50 = 0.04 + 0.125 = 0.165W
The maximum ambient temperature that the part is al-
lowed to operate is:
T
A
= T
J
– (P
DMAX
• 250°C/W)
= 150°C – (0.165W • 250°C/W) = 108°C
Input Offset Voltage
The offset voltage will change depending upon which
input stage is active. The PNP input stage is active from
the negative supply rail to 1.2V of the positive supply rail,
then the NPN input stage is activated for the remaining
input range up to the positive supply rail during which
the PNP stage remains inactive. The offset voltage is
typically less than 75µV in the range that the PNP input
stage is active.
Input Bias Current
The LT1800 employs a patent-pending technique to trim
the input bias current to less than 250nA for the input
common mode voltage of 0.2V above negative supply
rail to 1.2V of the positive rail. The low input offset volt-
age and low input bias current of the LT1800 provide the
precision performance especially for high source imped-
ance applications.
Output
The LT1800 can deliver a large output current, so the short-
circuit current limit is set around 50mA to prevent damage
to the device. Attention must be paid to keep the junction
temperature of the IC below the absolute maximum rating
of 150°C (refer to the Power Dissipation section) when the
output is continuously short-circuited. The output of the
amplifi er has reverse-biased diodes connected to each sup-
ply. If the output is forced beyond either supply, unlimited
current will fl ow through these diodes. If the current is
transient and limited to several hundred mA, and the total
supply voltage is less than 12.6V, the absolute maximum
rating, no damage will occur to the device.
Overdrive Protection
When the input voltage exceeds the power supplies, two
pairs of crossing diodes D1 to D4 will prevent the output
from reversing polarity. If the input voltage exceeds either
power supply by 700mV, diode D1/D2 or D3/D4 will turn
on to keep the output at the proper polarity. For the phase
reversal protection to perform properly, the input current
must be limited to less than 10mA. If the amplifi er is
severely overdriven, an external resistor should be used
to limit the overdrive current.
The LT1800’s input stages are also protected against a
large differential input voltage of 1.4V or higher by a pair
of back-back diodes D5/D8 to prevent the emitter-base
breakdown of the input transistors. The current in these
diodes should be limited to less than 10mA when they are
active. The worst-case differential input voltage usually
occurs when the input is driven while the output is shorted
to ground in a unity gain confi guration. In addition, the
amplifi er is protected against ESD strikes up to 3kV on
all pins by a pair of protection diodes on each pin that are
connected to the power supplies as shown in Figure 1.
Capacitive Load
The LT1800 is optimized for high bandwidth, low power
and precision applications. It can drive a capacitive load
of about 75pF in a unity gain confi guration, and more for
higher gain. When driving a larger capacitive load, a resistor
of 10Ω to 50Ω should be connected between the output
and the capacitive load to avoid ringing or oscillation. The
feedback should still be taken from the output so that the
resistor will isolate the capacitive load to ensure stability.
Graphs on capacitive loads indicate the transient response
of the amplifi er when driving capacitive load with a speci-
fi ed series resistor.
