REF5020-EP Datasheet, PDF (15/29 Pages) Texas Instruments – LOW-NOISE, VERY LOW DRIFT, PRECISION VOLTAGE REFERENCE

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Typical Applications (continued)
REF5020-EP, REF5025-EP, REF5040-EP, REF5050-EP
SBOS471B – APRIL 2010 – REVISED JUNE 2015
Figure 29. REF50xx FFT Plot of the Noise
With 10-µF Load Capacitor and 10-µΩ ESR
Every capacitor can be represented with a complicated equivalent model, which is voltage and frequency
dependent with a large number of passive components. For the purposes of this design, this model is limited to
the few components. The biggest impact on the creation of the low-pass filter and stability analysis is the
simplified model of equivalent series inductance and resistance. Considering good layout practice and inherently
low equivalent series inductance of today’s components, this model in the future analysis will be presented only
by equivalent capacitance and series resistance.
V_CC
ƒ_P
=
2p
•
(R
+
1
ESR) •
C_L
R_O
ESR
C_L
ƒ_Z
=
1
2p • ESR •
C_L
Figure 30. Equivalent SCH Of REF50xx With Load Capacitor for Stability Analysis
When evaluating the impact of ESR and C_L on the performance the reference voltage, it is important to include
the effect of the open-loop output resistance (R_O) of the output amplifier. The combination of R_O, ESR, and C_L
modifies the open-loop response curve by introducing one pole (f_P) and one zero (f_Z). The values R_O, ESR, and
C_L determine the corner frequency of the added pole f_P; and the values of ESR and C_L determine the corner
frequency of the added zero.
The introduction of the external ESR-CL on the output of the reference voltage modifies the output amplifier
open-loop gain curve. The added pole modifies the open-loop gain curve of the reference voltage output amplifier
by introducing a –20 dB/decade change at the frequency f_P to the already –20 dB/decade slope of the open-loop
gain curve, making the slope equal to –40 dB/decade. The added zero at frequency f_Z changes the open-loop
gain curve back to –20 dB/decade.
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