LT1800 Linear Technology, LT1800 Datasheet - Page 12

• Manufacturer Part Number: LT1800
• Description: 80MHz/ 25V/ms Low Power Rail-to-Rail Input and Output Precision Op Amp
• Manufacturer: Linear Technology

LT1800
APPLICATIO S I FOR ATIO
of Q16 is nominally equal to the base current of the input
devices. The base current of Q16 is then mirrored by
devices Q17-Q19 to cancel the base current of the input
devices Q1-Q2.
A pair of complementary common emitter stages Q14/Q15
that enable the output to swing from rail to rail constructs
the output stage. The capacitors C2 and C3 form the local
feedback loops that lower the output impedance at high
frequency. These devices are fabricated on Linear
Technology’s proprietary high-speed complementary bi-
polar process.
Power Dissipation
The LT1800 amplifier is offered in a small package, SOT-23,
which has a thermal resistance of 250 C/W,
is a need to ensure that the die’s junction temperature
should not exceed 150 C. Junction temperature T
calculated from the ambient temperature T
pation P
The power dissipation in the IC is the function of the supply
voltage, output voltage and the load resistance. For a given
supply voltage, the worst-case power dissipation P
occurs at the maximum supply current and the output
voltage is at half of either supply voltage (or the maximum
swing is less than 1/2 supply voltage). P
Example: An LT1800 in a SOT-23 package operating on
power dissipation is given by:
The maximum ambient temperature that the part is al-
lowed to operate is:
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
12
5V supplies and driving a 50
T
P
P
T
J
A
DMAX
DMAX
= T
= T
= 150 C – (0.165W • 250 C/W) = 108 C
D
A
J
= (10 • 4mA) + (2.5)
= (V
and thermal resistance
+ (P
– (P
S
D
DMAX
• I
•
SMAX
JA
U
• 250 C/W)
)
) + (V
U
2
S
/50 = 0.04 + 0.125 = 0.165W
/2)
2
/R
W
JA
load, the worst-case
L
:
DMAX
A
, power dissi-
JA
is given by:
U
. So there
DMAX
J
is
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 75 V 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 voltage
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 maxi-
mum rating of 150 C (refer to the Power Dissipation
section) when the output is continuously short circuited.
The output of the amplifier has reverse-biased diodes
connected to each supply. If the output is forced beyond
either supply, unlimited current will flow 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 amplifier 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
1800f