TLC27L1_08 Datasheet, PDF (26/32 Pages) Texas Instruments – LinCMOSE LOW-POWER OPERATIONAL AMPLIFIERS

TLC27L1, TLC27L1A, TLC27L1B
LinCMOS LOWĆPOWER
OPERATIONAL AMPLIFIERS
SLOS154B− DECEMBER 1995 − REVISED JUNE 2005
APPLICATION INFORMATION
output characteristics
The output stage of the TLC27L1 is designed to
sink and source relatively high amounts of current
(see Typical Characteristics). If the output is
subjected to a short-circuit condition, this high
current capability can cause device damage
under certain conditions. Output current capability
increases with supply voltage (see Figure 43).
All operating characteristics of the TLC27L1 were
measured using a 20-pF load. The devices drive
higher capacitive loads; however, as output load
capacitance increases, the resulting response
pole occurs at lower frequencies, thereby causing
ringing, peaking, or even oscillation (see Figure
44). In many cases, adding some compensation
in the form of a series resistor in the feedback loop
alleviates the problem.
VI
ÎÎÎÎÎÎÎÎÎÎ^2.5ÎÎÎÎÎ^V ÎÎÎÎÎ
VO
CL
TA = 25°C
f = 1 kHz
VI(PP) = 1 V
− 2.5 V
Figure 43. Test Circuit for Output
Characteristics
(a) CL = 20 pF, RL = NO LOAD
(b) CL = 260 pF, RL = NO LOAD
(c) CL = 310 pF, RL = NO LOAD
Figure 44. Effect of Capacitive Loads in Low-Bias Mode
Although the TLC27L1 possesses excellent high-level output voltage and current capability, methods are
available for boosting this capability, if needed. The simplest method involves the use of a pullup resistor (R_P)
connected from the output to the positive supply rail (see Figure 45). There are two disadvantages to the use
of this circuit. First, the NMOS pulldown transistor, N4 (see equivalent schematic) must sink a comparatively
large amount of current. In this circuit, N4 behaves like a linear resistor with an on-resistance between
approximately 60 Ω and 180 Ω, depending on how hard the operational amplifier input is driven. With very low
values of R_P, a voltage offset from 0 V at the output occurs. Secondly, pullup resistor R_P acts as a drain load
to N4 and the gain of the operational amplifier is reduced at output voltage levels where N5 is not supplying the
output current.
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