LT1638 LINER [Linear Technology], LT1638 Datasheet - Page 8

• Manufacturer Part Number: LT1638
• Description: 1.2MHz, 0.4V/us Over-The-TopTM Micropower Rail-to-Rail Input and Output Op Amps
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LT1638/LT1639
Supply Voltage
The positive supply pin of the LT1638/LT1639 should be
bypassed with a small capacitor (typically 0.1 F) within an
inch of the pin. When driving heavy loads an additional
4.7 F electrolytic capacitor should be used. When using
split supplies, the same is true for the negative supply pin.
The LT1638/LT1639 are protected against reverse battery
voltages up to 18V. In the event a reverse battery condition
occurs, the supply current is less than 1nA.
The LT1638/LT1639 can be shut down by removing V
this condition the input bias current is less than 0.1nA,
even if the inputs are 44V above the negative supply.
At temperatures greater than 70 C, when operating the
LT1638/LT1639 on total supplies of 10V or more, the
supply must not be brought up faster than 1V/ s. Increas-
ing the bypass capacitor and/or adding a small resistor in
series with the supply will limit the rise time.
Inputs
The LT1638/LT1639 have two input stages, NPN and PNP
(see the Simplified Schematic), resulting in three distinct
operating regions as shown in the Input Bias Current vs
Common Mode typical performance curve.
For input voltages about 0.8V or more below V
input stage is active and the input bias current is typically
– 20nA. When the input common mode voltage is within
0.5V of the positive rail, the NPN stage is operating and the
input bias current is typically 40nA. Increases in tempera-
ture will cause the voltage at which operation switches
from the PNP input stage to the NPN input stage to move
towards V
untrimmed and is typically 600 V.
A Schottky diode in the collector of each NPN transistor
allow the LT1638/LT1639 to operate over the top, with
either or both of its inputs above V
V
bias current is typically 8 A at room temperature. The
input offset voltage is typically 2mV when operating above
V
above V
8
+
+
. The LT1638/LT1639 will operate with its inputs 44V
the NPN input transistor is fully saturated and the input
–
regardless of V
+
. The input offset voltage of the NPN stage is
U
INFORMATION
+
U
.
+
W
. At about 0.3V above
+
U
, the PNP
+
. In
The inputs are protected against excursions as much as
22V below V
input and a diode from the input to the negative supply.
The input stage of the LT1638/LT1639 incorporates phase
reversal protection to prevent the output from phase
reversing for inputs up to 22V below V
clamping diodes between the inputs and the maximum
differential input voltage is 44V.
Output
The output of the LT1638/LT1639 can swing within 20mV
of the positive rail with no load, and within 3mV of the
negative rail with no load. When monitoring voltages
within 20mV of the positive rail or within 3mV of the
negative rail, gain should be taken to keep the output from
clipping. The LT1638/LT1639 are capable of sinking and
sourcing over 40mA on 15V supplies; sourcing current
capability is reduced to 20mA at 5V total supplies as noted
in the electrical characteristics.
The LT1638/LT1639 are internally compensated to drive
at least 200pF of capacitance under any output loading
conditions. A 0.22 F capacitor in series with a 150
resistor between the output and ground will compensate
these amplifiers for larger capacitive loads, up to 1000pF,
at all output currents.
Distortion
There are two main contributors of distortion in op amps:
output crossover distortion as the output transitions from
sourcing to sinking current and distortion caused by non-
linear common mode rejection. If the op amp is operating
inverting there is no common mode induced distortion. If
the op amp is operating in the PNP input stage (input is not
within 0.8V of V
When the LT1638 switches between input stages there is
significant nonlinearity in the CMRR. Lower load resis-
tance increases the output crossover distortion, but has
no effect on the input stage transition distortion. For low-
est distortion the LT1638/LT1639 should be operated single
supply, with the output always sourcing current and with
the input voltage swing between ground and (V
See the Typical Performance Characteristics curves.
–
by an internal 1k resistor in series with each
+
), the CMRR is very good, typically 98dB.
–
. There are no
+
– 0.8V).