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A low-cost, constant, on-time, synchronous buck regulator in fly-buck configuration with an external transformer
(LM5017) generates the isolated power supply. The LM5017 has a wide input supply range, making this device
ideal for accepting a 24-V industrial supply. This transformer can accept up to 100 V, thereby making reliable
transient protection of the input supply more easily achievable. The fly-buck power supply isolates and steps the
input voltage down to 6 V. The supply then provides that voltage to the
generate 5 V to power the ADS8688A and other circuitry. The LM5017 also features a number of other safety
and reliability functions, such as undervoltage lockout (UVLO), thermal shutdown, and peak current limit
protection.
Input analog signals are protected against high-voltage, fast-transient events often expected in an industrial
environment. The protection circuitry makes use of the transient voltage suppressor (TVS) and ESD diodes. The
RC low-pass mode filters are used on each analog input before the input reaches the ADS8688A, thus
eliminating any high-frequency noise pickups and minimizing aliasing.
For a step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, see
(TIDU365).
10 Power-Supply Recommendations
The device uses two separate power supplies: AVDD and DVDD. The internal circuits of the device operate on
AVDD; DVDD is used for the digital interface. AVDD and DVDD can be independently set to any value within the
permissible range.
The AVDD supply pins must be decoupled with AGND by using a minimum 10-µF and 1-µF capacitor on each
supply. Place the 1-µF capacitor as close to the supply pins as possible. Place a minimum 10-µF decoupling
capacitor very close to the DVDD supply to provide the high-frequency digital switching current. The effect of
using the decoupling capacitor is illustrated in the difference between the power-supply rejection ratio (PSRR)
performance of the device.
PSRR improves when the decoupling capacitors are used, as shown in
150
---- ± 2.5*V
---- ] 1.25*V
REF,
------]0.3125*V
130
REF,
---- + 1.25*V
REF,
110
90
70
50
30
0.001
0.01
Input Frequency (MHz)
Code output near 32,769
Figure 126. PSRR Without a Decoupling Capacitor
Copyright © 2015, Texas Instruments Incorporated
16-Bit, 8-Channel, Integrated Analog Input Module for Programmable Logic Controllers (PLCs)
Figure 126
shows the PSRR of the device without using a decoupling capacitor. The
---- ] 0.625*V
REF,
REF,
-------]0.156 V
---- + 2.5*V
REF,
REF
---- + 0.625*V
---- + 0.3125*V
REF,
REF
0.1
1
Product Folder Links:
TPS70950
Figure
140
---- ± 2.5*V
------]0.3125*V
120
---- + 1.25*V
100
80
60
40
10
0.001
C063
Code output near 32,768
Figure 127. PSRR With a Decoupling Capacitor
ADS8684A ADS8688A
ADS8684A, ADS8688A
SBAS680 – JULY 2015
(the low dropout regulator) to
127.
---- ] 1.25*V
---- ] 0.625*V
REF,
REF,
REF,
-------]0.156 V
---- + 2.5*V
REF,
REF,
---- + 0.625*V
---- + 0.3125*V
REF,
REF,
0.01
0.1
1
Input Frequency (MHz)
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REF
REF
10
C062
69
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