TLC27M4_15 Datasheet, PDF (32/52 Pages) Texas Instruments – PRECISION QUAD OPERATIONAL AMPLIFIERS

TLC27M4, TLC27M4A, TLC27M4B, TLC27M4Y, TLC27M9
SLOS093D – OCTOBER 1987 – REVISED OCTOBER 2012
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Output Characteristics
The output stage of the TLC27M4 and TLC27M9 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.
All operating characteristics of the TLC27M4 and TLC27M9 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 41). In many cases,
adding a small amount of resistance in series with the load capacitance alleviates the problem.
(a) C_L = 20 pF, R_L = NO LOAD
(b) C_L = 170 pF, R_L = NO LOAD
2.5 V
(c) C_L = 190 pF, R_L = NO LOAD
V_O
V_I
C_L
T_A = 25°C
f = 1 kHz
V_IPP = 1 V
– 2.5 V
(d) TEST CIRCUIT
Figure 41. Effect of Capacitive Loads and Test Circuit
Although the TLC27M4 and TLC27M9 possess excellent high-level output voltage and current capability,
methods for boosting this capability are available, if needed. The simplest method involves the use of a pullup
resistor (RP) connected from the output to the positive supply rail (see Figure 42). 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 RP, a voltage offset from 0 V at the output occurs. Second, pullup resistor RP 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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