TLV521 Datasheet, PDF (12/27 Pages) Texas Instruments – TLV521 NanoPower, 350nA, RRIO, CMOS Input, Operational Amplifier

TLV521
SNOSD26 – MAY 2016
8 Applications and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The TLV521 is specified for operation from 1.7 V to 5.5 V (±0.85 V to ±2.275 V). The TLV521 features rail to rail
input and rail-to-rail output swings while consuming only nanowatts of power. Parameters that can exhibit
significant variance with regard to operating voltage or temperature are presented in the Typical Characteristics
section.
8.1.1 Driving Capacitive Load
The TLV521 is internally compensated for stable unity gain operation, with a 6-kHz, typical gain bandwidth.
However, the unity gain follower is the most sensitive configuration to capacitive load. The combination of a
capacitive load placed at the output of an amplifier along with the amplifier’s output impedance creates a phase
lag, which reduces the phase margin of the amplifier. If the phase margin is significantly reduced, the response
will be under damped which causes peaking in the transfer and, when there is too much peaking, the op amp
might start oscillating.
-
V_IN
+
R_ISO
C_L
V_OUT
Figure 30. Resistive Isolation of Capacitive Load
In order to drive heavy capacitive loads, an isolation resistor, R_ISO, should be used, as shown in Figure 30. By
using this isolation resistor, the capacitive load is isolated from the amplifier’s output. The larger the value of
R_ISO, the more stable the amplifier will be. If the value of R_ISO is sufficiently large, the feedback loop will be
stable, independent of the value of C_L. However, larger values of R_ISO result in reduced output swing and
reduced output current drive.
Recommended minimum values for R_ISO are given in the following table, for 5-V supply. Figure 31 shows the
typical response obtained with the C_L = 50 pF and R_ISO = 154 kΩ. The other values of R_ISO in the table were
chosen to achieve similar dampening at their respective capacitive loads. Notice that for the TLV521 with larger
C_L a smaller R_ISO can be used for stability. However, for a given C_L a larger R_ISO will provide a more damped
response. For capacitive loads of 20 pF and below no isolation resistor is needed.
Table 1. Capacitive Loads vs. Needed Isolation Resistors
C_L
0 – 20 pF
50 pF
100 pF
500 pF
1 nF
5 nF
10 nF
R_ISO
not needed
154 kΩ
118 kΩ
52.3 kΩ
33.2 kΩ
17.4 kΩ
13.3 kΩ
12
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