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TL431A-Q1, TL431B-Q1
SGLS302E – MARCH 2005 – REVISED NOVEMBER 2016
TL431-Q1 Adjustable Precision Shunt Regulator
1 Features
3 Description
•
•
The TL431-Q1 is a three-pin adjustable shunt
regulator with specified thermal stability over
applicable automotive temperature ranges. The
output voltage can be set to any value from VREF
(approximately 2.5 V) to 36 V, with two external
resistors (see Figure 28). This device has a typical
output impedance of 0.2 Ω. Active output circuitry
provides a sharp turnon characteristic, making this
device an excellent replacement for Zener diodes in
many applications, such as onboard regulation,
adjustable power supplies, and switching power
supplies.
1
•
•
•
•
•
•
Qualified for Automotive Applications
AEC-Q100 Test Guidance With the Following:
– Device Temperature Grade 1: –40°C to 125°C
Ambient Operating Temperature Range
Reference Voltage Tolerance at 25°C:
– 1% (A Grade)
– 0.5% (B Grade)
Typical Temperature Drift:
– 14 mV (Q Temp)
Low Output Noise
0.2-Ω Typical Output Impedance
Sink-Current Capability: 1 mA to 100 mA
Adjustable Output Voltage: VREF to 36 V
2 Applications
•
•
•
•
•
Device Information(1)
PART NUMBER
PACKAGE
BODY SIZE (NOM)
TL431A-Q1
SOT-23 (5)
2.90 mm × 1.60 mm
TL431A-Q1,
TL431B-Q1
SOT-23 (3)
2.92 mm × 1.30 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Adjustable Voltage and Current Referencing
Secondary Side Regulation in Flyback SMPSs
Zener Replacement
Voltage Monitoring
Comparator With Integrated Reference
Simplified Schematic
VKA
Input
IKA
Vref
Copyright © 2016, Texas Instruments Incorporated
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
TL431A-Q1, TL431B-Q1
SGLS302E – MARCH 2005 – REVISED NOVEMBER 2016
www.ti.com
Table of Contents
1
2
3
4
5
6
7
8
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
3
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
3
3
3
4
4
4
5
6
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics: TL431-Q1 ........................
Electrical Characteristics: TL431A-Q1 ......................
Electrical Characteristics: TL431B-Q1 ......................
Typical Characteristics ..............................................
Parameter Measurement Information .................. 9
Detailed Description ............................................ 11
8.1 Overview ................................................................. 11
8.2 Functional Block Diagram ....................................... 11
8.3 Feature Description................................................. 12
8.4 Device Functional Modes........................................ 12
9
Application and Implementation ........................ 13
9.1 Application Information............................................ 13
9.2 Typical Applications ................................................ 13
10 Power Supply Recommendations ..................... 18
11 Layout................................................................... 18
11.1 Layout Guidelines ................................................. 18
11.2 Layout Example .................................................... 18
12 Device and Documentation Support ................. 19
12.1
12.2
12.3
12.4
12.5
12.6
12.7
Documentation Support ........................................
Related Links ........................................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
19
19
19
19
19
19
19
13 Mechanical, Packaging, and Orderable
Information ........................................................... 19
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision D (March 2013) to Revision E
Page
•
Added Applications section, Device Information table, Pin Configuration and Functions section, Specifications
section, ESD Ratings table, Detailed Description section, Application and Implementation section, Power Supply
Recommendations section, Layout section, Device and Documentation Support section, and Mechanical,
Packaging, and Orderable Information section ...................................................................................................................... 1
•
Deleted Ordering Information table; see Package Option Addendum at the end of the data sheet ...................................... 1
•
Added Thermal Information table ........................................................................................................................................... 4
•
Changed RθJA values for 5-pin DBV (SOT-23) From: 206 To: 215 and for 3-pin DBZ (SOT-23) From: 206 To: 334.7......... 4
2
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SGLS302E – MARCH 2005 – REVISED NOVEMBER 2016
5 Pin Configuration and Functions
DBV Package
5-Pin SOT-23
Top View
NC
1
1
2
CATHODE
3
NC
(1)
DBZ Package
3-Pin SOT-23
Top View
5
ANODE
4
REF
CATHODE
1
REF
2
3
ANODE
Pin 2 is connected internally to ANODE
(die substrate) and must be left floating or
connected to ANODE.
Pin Functions
PIN
NAME
I/O
DESCRIPTION
DBV
DBZ
ANODE
5
3
O
Common pin, normally connected to ground.
CATHODE
3
1
I/O
Shunt current or voltage input
NC
1, 2
—
—
No connection (1)
REF
4
2
I
(1)
Threshold relative to common anode
Pin 2 of the 5-pin DBV (SOT-23) package is connected internally to ANODE (die substrate) and must be left floating or connected to
ANODE.
6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1)
MIN
MAX
Cathode voltage (2)
Continuous cathode current
–100
Low
Reference input current
mA
µA
High
Storage temperature, Tstg
(2)
V
150
–50
Operating junction temperature, TJ
(1)
UNIT
37
–65
10
mA
150
°C
150
°C
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
Voltage values are with respect to the ANODE pin, unless otherwise noted.
6.2 ESD Ratings
VALUE
Human-body model (HBM), per AEC Q100-002
V(ESD)
(1)
Electrostatic discharge
(1)
UNIT
±2500
Charged-device model (CDM), per AEC Q100-011
±1000
Machine model (MM)
±200
V
AEC Q100-002 indicates that HBM stressing shall be in accordance with the ANSI/ESDA/JEDEC JS-001 specification.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
VKA
Cathode voltage
IKA
Cathode current
TA
Operating free-air temperature
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MIN
MAX
VREF
36
V
1
100
mA
–40
125
°C
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UNIT
3
TL431A-Q1, TL431B-Q1
SGLS302E – MARCH 2005 – REVISED NOVEMBER 2016
www.ti.com
6.4 Thermal Information
TL431-Q1
THERMAL METRIC (1)
DBV (SOT-23)
DBZ (SOT-23)
5 PINS
3 PINS
UNIT
215
334.7
°C/W
135.2
113.5
°C/W
43
67.6
°C/W
RθJA
Junction-to-ambient thermal resistance
RθJC(top)
Junction-to-case (top) thermal resistance
RθJB
Junction-to-board thermal resistance
ψJT
Junction-to-top characterization parameter
19.6
6.7
°C/W
ψJB
Junction-to-board characterization parameter
42.1
65.9
°C/W
(1)
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
6.5 Electrical Characteristics: TL431-Q1
over recommended operating conditions, TA = 25°C (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
2440
2495
2550
mV
14
34
mV
ΔVKA = 10 V – VREF
–1.4
–2.7
ΔVKA = 36 V – 10 V
–1
–2
VREF
Reference voltage
VKA = VREF, IKA = 10 mA, see Figure 20
VI(DEV)
Deviation of reference voltage over full
temperature (1)
VKA = VREF, IKA = 10 mA, TA = –40°C to 125°C,
see Figure 20
ΔVREF/ΔVKA
Ratio of change in reference voltage to the
change in cathode voltage
IKA = 10 mA,
see Figure 21
IREF
Reference current
IKA = 10 mA, R1 = 10 kΩ, R2 = ∞, see Figure 21
2
4
µA
II(DEV)
Deviation of reference current over full
temperature (1)
IKA = 10 mA, R1 = 10 kΩ, R2 = ∞,
TA = –40°C to 125°C, see Figure 21
0.8
2.5
µA
IMIN
Minimum cathode current for regulation
VKA = VREF, see Figure 20
0.4
1
mA
IOFF
OFF-state cathode current
VKA = 36 V, VREF = 0, see Figure 22
0.1
1
µA
IKA = 1 mA to 100 mA, VKA = VREF, f ≤ 1 kHz,
see Figure 20
0.2
0.5
Ω
|ZKA|
(1)
Dynamic impedance
(1)
mV/V
The deviation parameters (VI(DEV) and II(DEV)) are defined as the differences between the maximum and minimum values obtained over
the recommended temperature range.
6.6 Electrical Characteristics: TL431A-Q1
over recommended operating conditions, TA = 25°C (unless otherwise noted)
TEST CONDITIONS
MIN
TYP
MAX
UNIT
VREF
Reference voltage
PARAMETER
VKA = VREF, IKA = 10 mA, see Figure 20
2470
2495
2520
mV
VI(DEV)
Deviation of reference voltage over full
temperature (1)
VKA = VREF, IKA = 10 mA, TA = –40°C to 125°C,
see Figure 20
14
34
mV
ΔVREF/ΔVKA
Ratio of change in reference voltage to the
change in cathode voltage
IKA = 10 mA,
see Figure 21
ΔVKA = 10 V – VREF
–1.4
–2.7
ΔVKA = 36 V – 10 V
–1
–2
IREF
Reference current
IKA = 10 mA, R1 = 10 kΩ, R2 = ∞, see Figure 21
2
4
µA
II(DEV)
Deviation of reference current over full
temperature (1)
IKA = 10 mA, R1 = 10 kΩ, R2 = ∞,
TA = –40°C to 125°C, see Figure 21
0.8
2.5
µA
IMIN
Minimum cathode current for regulation
VKA = VREF, see Figure 20
0.4
0.7
mA
IOFF
OFF-state cathode current
VKA = 36 V, VREF = 0, see Figure 22
0.1
0.5
µA
Dynamic impedance (1)
IKA = 1 mA to 100 mA, VKA = VREF, f ≤ 1 kHz,
see Figure 20
0.2
0.5
Ω
|ZKA|
(1)
4
mV/V
The deviation parameters (VI(DEV) and II(DEV)) are defined as the differences between the maximum and minimum values obtained over
the recommended temperature range.
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6.7 Electrical Characteristics: TL431B-Q1
over recommended operating conditions, TA = 25°C (unless otherwise noted)
TEST CONDITIONS
MIN
TYP
MAX
UNIT
VREF
Reference voltage
PARAMETER
VKA = VREF, IKA = 10 mA, see Figure 20
2483
2495
2507
mV
VI(DEV)
Deviation of reference voltage over full
temperature (1)
VKA = VREF, IKA = 10 mA, TA = –40°C to 125°C,
see Figure 20
14
34
mV
ΔVREF/ΔVKA
Ratio of change in reference voltage to the
change in cathode voltage
IKA = 10 mA,
see Figure 21
ΔVKA = 10 V – VREF
–1.4
–2.7
ΔVKA = 36 V – 10 V
–1
–2
IREF
Reference current
IKA = 10 mA, R1 = 10 kΩ, R2 = ∞, see Figure 21
2
4
µA
II(DEV)
Deviation of reference current over full
temperature (1)
IKA = 10 mA, R1 = 10 kΩ, R2 = ∞,
TA = –40°C to 125°C, see Figure 21
0.8
2.5
µA
IMIN
Minimum cathode current for regulation
VKA = VREF, see Figure 20
0.4
0.7
mA
IOFF
OFF-state cathode current
VKA = 36 V, VREF = 0, see Figure 22
0.1
0.5
µA
Dynamic impedance (1)
IKA = 1 mA to 100 mA, VKA = VREF, f ≤ 1 kHz,
see Figure 20
0.2
0.5
Ω
|ZKA|
(1)
mV/V
The deviation parameters (VI(DEV) and II(DEV)) are defined as the differences between the maximum and minimum values obtained over
the recommended temperature range.
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6.8 Typical Characteristics
2600
2560
5
VREF = 2440 mV
VREF = 2495 mV
VREF = 2550 mV
4
Refrence Current (PA)
Reference Voltage (mV)
2580
2540
2520
2500
2480
2460
2440
3
2
1
2420
2400
-75
-50
-25
0
25
50
75
Free-Air Temperature (qC)
100
0
-75
125
-50
-25
D001
Data is for devices having the indicated value of VREF at IKA = 10 mA,
TA = 25°C.
Figure 1. Reference Voltage vs Free-air Temperature
0
25
50
75
Free-Air Temperature (qC)
100
125
D002
Figure 2. Reference Current vs Free-air Temperature
150
800
125
600
Cathode Current (PA)
Cathode Current (mA)
100
75
50
25
0
-25
-50
Imin
400
200
0
-75
-100
-2
-1
0
1
Cathode Voltage (V)
2
-200
-1
3
Figure 3. Cathode Current vs Cathode Voltage
2
3
D004
-0.85
-0.95
2
'VKA - mV/V
Off-State Cathode Current (PA)
1
Cathode Voltage (V)
Figure 4. Cathode Current vs Cathode Voltage
2.5
1.5
1
-1.05
-1.15
-1.25
0.5
0
-75
-1.35
-50
-25
0
25
50
75
Free-Air Temperature (qC)
100
Figure 5. OFF-State Cathode Current vs
Free-air Temperature
6
0
D003
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125
-1.45
-75
-50
-25
D005
0
25
50
75
Free-Air Temperature (qC)
100
125
D006
Figure 6. Ratio of Delta Reference Voltage to Delta Cathode
Voltage vs Free-air Temperature
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Typical Characteristics (continued)
6
Equivalent Input Noise Voltage (PV)
Equivalent Input Noise Voltage (nV/ÖHz)
260
240
220
200
180
160
140
120
100
10
4
3
2
1
0
-1
-2
-3
-4
-5
-6
100
1k
Frequency (Hz)
10k
100k
0
50
50
30
20
Reference Impedance (:)
100
40
3
4
30
20
10
5000 10000
100000
Frequency (Hz)
5
6
Time (s)
7
8
9
10
D008
10
5
3
2
1
0.5
0.3
0.2
0.1
1000
1000000
10000
100000
Frequency (Hz)
D001
Figure 9. Small-Signal Voltage Amplification vs Frequency
1000000
1E+7
D001
Figure 10. Reference Impedance vs Frequency
6
100
90
5
80
Cathode Current (mA)
Input and Output Voltage (V)
2
Figure 8. Equivalent Input Noise Voltage Over a 10-s Period
60
0
1000 2000
1
D007
Figure 7. Equivalent Input Noise Voltage vs Frequency
Small-Signal Voltage Amplification (dB)
5
4
3
2
A VKA = Vref
B VKA = 5 V
C VKA = 10 V
D VKA = 15 Vf
70
Stable
60
Stable
50
40
30
20
1
0
-1
10
0
1
2
3
4
Time (Ps)
5
6
7
D011
Figure 11. Pulse Response
0
0.001
0.01
0.1
Load Capacitance (PF)
1
10
D012
The areas under the curves represent conditions that may cause the
device to oscillate. For curves B, C, and D, R2 and VREF were
adjusted to establish the initial VKA and IKA conditions with CL = 0.
VBATT and CL then were adjusted to determine the ranges of stability
(see Figure 18 and Figure 19 for test circuits).
Figure 12. Stability Boundary Conditions for All TL431 and
TL431A Devices (Except for SOT23-3, SC-70,
and Q-TEMP Devices)
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Typical Characteristics (continued)
100
A VKA = Vref
B VKA = 5 V
C VKA = 10 V
D VKA = 15 Vf
90
Cathode Current (mA)
80
70
60
Stable
Stable
50
40
30
20
10
0
0.001
0.01
0.1
Load Capacitance (PF)
1
10
D013
The areas under the curves represent conditions that may cause the device to oscillate. For curves B, C, and D, R2 and VREF were adjusted
to establish the initial VKA and IKA conditions with CL = 0. VBATT and CL then were adjusted to determine the ranges of stability (see
Figure 18 and Figure 19 for test circuits).
Figure 13. Stability Boundary Conditions for All TL431B, TL432, SOT-23, SC-70, and Q-TEMP Devices
8
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7 Parameter Measurement Information
19.1 V
1 kΩ
500 µF
910 Ω
2000 µF
VCC
TL431
(DUT)
VCC
1 µF
TLE2027
AV = 10 V/mV
+
−
16 kΩ
16 kΩ
1 µF
To
Oscilloscope
−
16 Ω
160 kΩ
22 µF
TLE2027
+
820 Ω
33 kΩ
AV = 2 V/V
0.1 µF
33 kΩ
VEE
VEE
Copyright © 2016, Texas Instruments Incorporated
Figure 14. Test Circuit for Equivalent Input Noise Voltage
Output
15 k Ω
220 Ω
IKA
Output
232 Ω
9 µF
Pulse
Generator
f = 100 kHz
+
−
50 Ω
8.25 k Ω
GND
GND
Figure 15. Test Circuit for Voltage Amplification
Figure 17. Test Circuit for Pulse Response
150 Ω
1 kΩ
Output
IKA
+
IKA
CL
50 Ω
VBATT
−
−
+
GND
Figure 16. Test Circuit for Reference Impedance
Figure 18. Test Circuit for Curve A
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150 Ω
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VKA
Input
IKA
IKA
R1
+
CL
Iref
VBATT
−
Vref
R2
Figure 19. Test Circuit for Curves B, C, and D
VKA
Input
R1 ö
æ
VKA = Vref ç 1 +
+ Iref ´ R1
R2 ÷ø
è
Figure 21. Test Circuit for VKA > VREF
VKA
Input
Ioff
IKA
Vref
Figure 22. Test Circuit for IOFF
Figure 20. Test Circuit for VKA = VREF
10
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8 Detailed Description
8.1 Overview
This device has proven ubiquity and versatility across a wide range of applications, ranging from power to signal
path. This is due to its key components containing an accurate voltage reference and op amp, which are
fundamental analog building blocks. The TL431-Q1 can be used as a single voltage reference, error amplifier,
voltage clamp, or comparator with integrated reference.
The TL431-Q1 can be operated and adjusted to cathode voltages from 2.5 V to 36 V, making this part optimum
for a wide range of end equipments in industrial, auto, telecommunications, and computing. For this device to
behave as a shunt regulator or error amplifier, at least 1 mA (IMIN(MAX)) must be supplied to the cathode pin.
Under this condition, feedback can be applied from the CATHODE and REF pins to create a replica of the
internal reference voltage.
Various reference voltage options can be purchased with initial tolerances (at 25°C) of 0.5% and 1%. These
reference options are denoted by B (0.5%) or A (1%) in the part number (TL431x-Q1).
8.2 Functional Block Diagram
CATHODE
+
REF
_
V ref
ANODE
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Figure 23. Equivalent Schematic
CATHODE
800 Ω
800 Ω
20 pF
REF
150 Ω
3.28 kΩ
2.4 kΩ
7.2 kΩ
10 kΩ
4 kΩ
20 pF
1 kΩ
800 Ω
ANODE
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All component values are nominal.
Figure 24. Detailed Schematic
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8.3 Feature Description
The TL431-Q1 consists of an internal reference and amplifier that outputs a sink current based on the difference
between the reference pin and the virtual internal pin. The sink current is produced by the internal Darlington
pair, shown in Figure 24. A Darlington pair is used to allow this device to sink a maximum current of 100 mA.
When operated with enough voltage headroom (at least 2.5 V) and cathode current (IKA), the TL431-Q1 forces
the reference pin to 2.5 V. However, the reference pin can not be left floating, as IREF must be at least 4 µA (see
Specifications). This is because the reference pin is driven into an NPN, which requires base current to operate
properly.
When feedback is applied from the CATHODE and REF pins, the TL431-Q1 behaves as a Zener diode,
regulating to a constant voltage dependent on current being supplied into the cathode. This is due to the internal
amplifier and reference entering the proper operating regions. The same amount of current required in the above
feedback situation must be applied to this device in open loop, servo, or error amplifying implementations for it to
be in the proper linear region giving the device enough gain.
Unlike many linear regulators, the TL431-Q1 is internally compensated to be stable without an output capacitor
between the cathode and anode. However, if it is desired to use an output capacitor Figure 24 can be used as a
guide to assist in choosing the correct capacitor to maintain stability.
8.4 Device Functional Modes
8.4.1 Open Loop (Comparator)
When the cathode or output voltage or current of the TL431-Q1 is not being fed back to the reference or input pin
in any form, the device operates in open loop. With proper cathode current (IKA) applied to this device, the
TL431-Q1 has the characteristics shown in Figure 24. With such high gain in this configuration, the device is
typically used as a comparator. The integrated reference makes TL431 the prefered choice when trying to
monitor a certain level of a single signal.
8.4.2 Closed Loop
When the cathode or output voltage or current of the TL431-Q1 is being fed back to the reference or input pin in
any form, the device operates in closed loop. The majority of applications involving the TL431-Q1 use it in this
manner to regulate a fixed voltage or current. The feedback enables this device to behave as an error amplifier,
computing a portion of the output voltage and adjusting it to maintain the desired regulation. This is done by
relating the output voltage back to the reference pin in a manner to make it equal to the internal reference
voltage, which can be accomplished through resistive or direct feedback.
REF
ANODE
CATHODE
Figure 25. Logic Symbol
12
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9 Application and Implementation
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.
9.1 Application Information
As this device has many applications and setups, there are many situations that this data sheet can not
characterize in detail. The linked application notes help the make the best choices when using this part.
Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet provides a deeper
understanding of this devices stability characteristics and aid the user in making the right choices when choosing
a load capacitor. Setting the Shunt Voltage on an Adjustable Shunt Regulator assists designers in setting the
shunt voltage to achieve optimum accuracy for this device.
9.2 Typical Applications
9.2.1 Comparator Application
Copyright © 2016, Texas Instruments Incorporated
Figure 26. Comparator Application Schematic
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Product Folder Links: TL431A-Q1 TL431B-Q1
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TL431A-Q1, TL431B-Q1
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Typical Applications (continued)
9.2.1.1 Design Requirements
For this design example, use the parameters listed in Table 1 as the input parameters.
Table 1. Design Parameters
PARAMETER
VALUE
Input voltage
0 V to 5 V
Input resistance
10 kΩ
Supply voltage
24 V
Cathode current, IK
5 mA
Output voltage level
Approximately 2 V to VSUP
Logic input thresholds, VIH / VIL
VL
9.2.1.2 Detailed Design Procedure
When using the TL431-Q1 as a comparator with reference, determine the following:
• Input voltage range
• Reference voltage accuracy
• Output logic input high and low level thresholds
• Current source resistance
9.2.1.2.1 Basic Operation
In the configuration shown in Figure 26 the TL431-Q1 behaves as a comparator, comparing the REF pin voltage
to the internal virtual reference voltage. When provided a proper cathode current (IKA), the TL431-Q1 has enough
open loop gain to provide a quick response. This is shown in Figure 27, where the RSUP = 10 kΩ (IKA = 500 µA)
situation responds much slower than RSUP = 1 kΩ (IKA = 5 mA). With the TL431-Q1's maximum operating current
(IMIN) being 1 mA, operation below that could result in low gain, leading to a slow response.
9.2.1.2.2 Overdrive
Slow or inaccurate responses can also occur when the reference pin is not provided enough overdrive voltage.
This is the amount of voltage that is higher than the internal virtual reference. The internal virtual reference
voltage is within the range of 2.5 V ± (0.5%, 1%, or 1.5%) depending on which version is being used. The more
overdrive voltage provided, the faster the TL431-Q1 responds.
For applications where the TL431-Q1 is being used as a comparator, it is best to set the trip point to greater than
the positive expected error (for example: +1% for the A version). For fast response, setting the trip point to at
least 10% of the internal VREF should suffice.
For minimal drop or difference from VINREF to the REF pin, TI recommends using an input resistor
