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LM7705
SNVS420D – NOVEMBER 2008 – REVISED MAY 2018
LM7705 Low-Noise Negative Bias Generator
1 Features
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1
3 Description
Regulated Output Voltage −0.232 V
Output Voltage Tolerance 5%
Output Voltage Ripple 4 mVPP
Supply Voltage 3 V to 5.25 V
Conversion Efficiency Up to 98%
Quiescent Current 78 µA
Shutdown Current 20 nA
Turnon Time 500 µs
Operating Temperature Range −40°C to 125°C
8-Pin VSSOP Package
2 Applications
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True Zero Amplifier Outputs
Portable Instrumentation
Low-Voltage Split-Power Supplies
The LM7705 device is a switched capacitor voltage
inverter with a low noise, −0.23 V fixed negative
voltage regulator. This device is designed to be used
with low voltage amplifiers to enable the amplifiers
output to swing to zero volts. The −0.23 V is used to
supply the negative supply pin of an amplifier while
maintaining less then 5.5 V across the amplifier. Railto-Rail output amplifiers cannot output zero volts
when operating from a single-supply voltage and can
result in error accumulation due to amplifier output
saturation voltage being amplified by following gain
stages. A small negative supply voltage will prevent
the amplifiers output from saturating at zero volts and
will help maintain an accurate zero through a signal
processing chain. Additionally, when an amplifier is
used to drive an input of the ADC, the amplifier can
output a zero voltage signal and the full input range
of an ADC can be used. The LM7705 device has a
shutdown pin to minimize standby power
consumption.
Device Information(1)
PART NUMBER
LM7705
PACKAGE
VSSOP (8)
BODY SIZE (NOM)
3.00 mm × 3.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
+V
+V
+ In
+
- In
-V
CFLY
CF+
VDD
shutdown
SD
VOUT
LM7705
VSS
true zero
output voltage
CF-0.23V
COUT
CRES
CRES
low voltage
amplifier
VSS
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.
LM7705
SNVS420D – NOVEMBER 2008 – REVISED MAY 2018
www.ti.com
Table of Contents
1
2
3
4
5
6
7
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
3
3
4
4
4
5
6
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
3.3-V Electrical Characteristics .................................
5-V Electrical Characteristics ....................................
Typical Characteristics ..............................................
Detailed Description ............................................ 10
7.1 Overview ................................................................. 10
7.2 Functional Block Diagram ....................................... 10
7.3 Feature Description................................................. 10
7.4 Device Functional Modes........................................ 10
8
Application and Implementation ........................ 14
8.1 Application Information............................................ 14
8.2 Typical Application .................................................. 16
9 Power Supply Recommendations...................... 18
10 Layout................................................................... 19
10.1 Layout Guidelines ................................................. 19
10.2 Layout Examples................................................... 19
11 Device and Documentation Support ................. 20
11.1
11.2
11.3
11.4
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
20
20
20
20
12 Mechanical, Packaging, and Orderable
Information ........................................................... 20
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision C (September 2015) to Revision D
Page
•
Deleted 'Maximum Output Current 26 mA' from Features list ............................................................................................... 1
•
Deleted IO_MAX spec from 3.3-V Electrical Characteristics and 5-V Electrical Characteristics tables.................................. 4
Changes from Revision B (March 2013) to Revision C
•
Added Pin Configuration and Functions section, ESD Ratings table, Feature Description section, Device Functional
Modes, Application and Implementation section, Power Supply Recommendations section, Layout section, Device
and Documentation Support section, and Mechanical, Packaging, and Orderable Information section .............................. 1
Changes from Revision A (November 2008) to Revision B
•
2
Page
Page
Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 19
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SNVS420D – NOVEMBER 2008 – REVISED MAY 2018
5 Pin Configuration and Functions
DGK Package
8-Pin VSSOP
Top View
8
1
LM7705
4
5
Pin Functions
PIN
NAME
NO.
TYPE
DESCRIPTION
CF+
1
Analog
CFLY Positive Capacitor Connection
VSS
2
Ground
Power Ground
SD
3
Input
VDD
4
Power
Positive Supply Voltage
VSS
5
Ground
Power Ground
VOUT
6
Output
Output Voltage
CRES
7
Analog
Reserve Capacitor Connection
CF-
8
Analog
CFLY Negative Capacitor Connection
Shutdown Pin
If SD pin is LOW, device is ON
If SD pin is HIGH, device is OFF
6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1)
MIN
MAX
UNIT
5.75
V
VDD + 0.3
VSS – 0.3
V
150
°C
260
°C
150
°C
Supply voltage VDD - VSS
SD
Junction temperature (2)
Mounting temperature
Infrared or Convection (20 sec)
−65
Storage temperature, Tstg
(1)
(2)
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.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
6.2 ESD Ratings
VALUE
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001
V(ESD)
(1)
(2)
(3)
Electrostatic discharge
(1)
UNIT
±2000
Charged-device model (CDM), per JEDEC specification JESD22C101 (2)
±750
Machine model (3)
±200
V
Human Body Model, applicable std. MIL-STD-883, Method 3015.7.
Field induced Charge-Device Model, applicable std. JESD22–C101–C. (ESD FICDM std of JEDEC).
Machine model, applicable std JESD22–A115–A (ESSD MM srd of JEDEC).
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6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
Supply voltage (VDD to GND)
MIN
MAX
UNIT
V
3
5.25
Supply voltage (VDD wrt VOUT)
3.23
5.48
V
Temperature range
−40
125
°C
6.4 Thermal Information
LM77005
THERMAL METRIC (1)
DGK (VSSOP)
UNIT
8 PINS
RθJA
(1)
Junction-to-ambient thermal resistance
253
°C/W
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
6.5 3.3-V Electrical Characteristics
Unless otherwise specified, all limits are ensured for TA = 25°C, VDD = 3.3 V, VSS = 0 V, SD = 0 V, CFLY= 5 µF, CRES = 22 µF,
COUT = 22 µF.
PARAMETER
TEST CONDITIONS
TA = 25°C
−0.24
2
−40°C to 125°C
−0.25
1
TA = 25°C
−0.24
2
−40°C to 125°C
−0.25
1
IOUT = 0 mA
VOUT
MIN (1)
Output Voltage
IOUT = −20 mA
TYP (2)
MAX (1)
−0.232
UNIT
−0.219
−0.209
V
−0.226
−0.219
−0.209
VR
Output Voltage Ripple
IOUT = −20 mA
IS
Supply Current
No Load
ISD
Shutdown Supply Current
SD = VDD
ηPOWER
Current Conversion Efficiency
−5 mA ≤ IOUT ≤
−20 mA
98%
ηPOWER
Current Conversion Efficiency
IOUT = −5 mA
98%
tON
Turnon Time
IOUT = −5 mA
500
μs
t OFF
Turnoff Time
IOUT = −5 mA
700
μs
tOFF CP
Turnoff Time Charge Pump
IOUT = −5 mA
11
ZOUT
Output Impedance
fOSC
Oscillator Frequency
−1 mA ≤ IOUT ≤
−20 mA
50
78
−40°C to 125°C
mVPP
100
150
20
TA = 25°C
0.23
−40°C to 125°C
μs
0.8
1.3
VIH
Shutdown Input High
IC
Shutdown Pin Input Current
SD = VDD
Load Regulation
0 mA ≤ IOUT ≤
−20 mA
1.25
TA = 25°C
1.85
−40°C to 125°C
2.15
−40°C to 125°C
V
V
50
TA = 25°C
Ω
kHz
1.6
−40°C to 125°C
μA
nA
92
Shutdown Input Low
4
TA = 25°C
TA = 25°C
VIL
(1)
(2)
4
0.12
pA
0.6
0.85
%/mA
All limits are specified by testing or statistical analysis.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not specified on shipped
production material.
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3.3-V Electrical Characteristics (continued)
Unless otherwise specified, all limits are ensured for TA = 25°C, VDD = 3.3 V, VSS = 0 V, SD = 0 V, CFLY= 5 µF, CRES = 22 µF,
COUT = 22 µF.
PARAMETER
Line Regulation
TEST CONDITIONS
3 V ≤ VDD ≤ 5.25 V TA = 25°C
(No Load)
−40°C to 125°C
MIN (1)
–0.2
TYP (2)
MAX (1)
0.29
0.7
1.1
UNIT
%/V
6.6 5-V Electrical Characteristics
Unless otherwise specified, all limits are ensured for TA = 25°C, VDD = 5.0V, VSS = 0V, SD = 0V, CFLY = 5 µF, CRES = 22 µF,
COUT = 22 µF.
PARAMETER
MIN (1)
TYP (2)
MAX (1)
TA = 25°C
−0.24
2
−0.233
−0.219
−40°C to 125°C
−0.25
1
TA = 25°C
−0.24
2
−40°C to 125°C
−0.25
1
TEST CONDITIONS
IOUT = 0 mA
VOUT
Output Voltage
IOUT = −20 mA
UNIT
−0.209
V
−0.226
−0.219
−0.209
VR
Output Voltage Ripple
IOUT = −20 mA
IS
Supply Current
No Load
ISD
Shutdown Supply Current
SD = VDD
ηPOWER
Current Conversion Efficiency
−5 mA ≤ IOUT ≤ −20 mA
98%
ηPOWER
Current Conversion Efficiency
IOUT = −5 mA
98%
tON
Turnon Time
IOUT = −5 mA
200
μs
t OFF
Turnoff Time
IOUT = −5 mA
700
μs
tOFF CP
Turnoff Time Charge Pump
IOUT = −5 mA
11
ZOUT
Output Impedance
−1 mA ≤ IOUT
≤
−20 mA
fOSC
Oscillator Frequency
4
TA = 25°C
103
135
240
20
TA = 25°C
0.26
−40°C to 125°C
μs
0.8
1.3
91
−40°C to 125°C
1.95
VIH
Shutdown Input High
IC
Shutdown Pin Input Current
SD = VDD
Load Regulation
0 mA ≤ IOUT ≤
−20 mA
TA = 25°C
3 V ≤ VDD ≤
5.25 V (No
Load)
TA = 25°C
Line Regulation
TA = 25°C
2.8
−40°C to 125°C
50
0.14
−40°C to 125°C
−40°C to 125°C
V
V
3.25
pA
0.6
0.85
−0.2
Ω
kHz
2.55
Shutdown Input Low
μA
nA
TA = 25°C
VIL
(1)
(2)
60
−40°C to 125°C
mVPP
0.29
%/mA
0.7
1.1
%/V
All limits are specified by testing or statistical analysis.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not specified on shipped
production material.
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6.7 Typical Characteristics
VDD = 3.3 V and TA = 25°C unless otherwise noted.
300
SUPPLY CURRENT (éA)
OUTPUT VOLTAGE (V)
-0.19
-0.20
-0.21
IOUT=10 mA
IOUT = 20 mA
-0.22
-0.23
250
200
125°C
85°C
150
25°C
100
50
0
-0.24
IOUT = 5 mA
IOUT = 0 mA
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
-40°C
5.0
3.0
3.5
4.0
4.5
5.0
SUPPLY VOLTAGE (V)
Figure 1. Output Voltage vs. Supply Voltage
Figure 2. Supply Current vs. Supply Voltage
125°C
-0.20
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
-0.20
85°C
-0.21
-0.22
-40°C
-0.23
-0.24
25°C
-0.25
125°C
-0.21
-0.22
85°C
25°C
-0.23
-40°C
-0.24
-0.25
SUPPLY VOLTAGE = 5.0V
SUPPLY VOLTAGE = 3.3V
0
5
10
15
20
25
30
0
OUTPUT CURRENT (mA)
40
50
60
15
SUPPLY VOLTAGE = 3.3V
12
CRES = CFILTER = 10 éF
9
6
CRES = CFILTER = 22 éF
3
0
40
80
SUPPLY VOLTAGE = 5.0V
OUTPUT VOLTAGE RIPPLE (mVPP)
OUTPUT VOLTAGE RIPPLE (mVPP)
30
Figure 4. Output Voltage vs. Output Current
15
12
CRES = CFILTER = 10 éF
9
6
CRES = CFILTER = 22 éF
3
0
-40
120
TEMPERATURE (°C)
0
40
80
120
TEMPERATURE (°C)
Figure 5. Output Voltage Ripple vs. Temperature
6
20
OUTPUT CURRENT (mA)
Figure 3. Output Voltage vs. Output Current
0
-40
10
Figure 6. Output Voltage Ripple vs. Temperature
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Typical Characteristics (continued)
VDD = 3.3 V and TA = 25°C unless otherwise noted.
SUPPLY VOLTAGE = 5.0V
SUPPLY VOLTAGE = 3.3V
20
SUPPLY CURRENT (mA)
16
-40°C
12
25°C
8
85°C
4
0
16
-40°C
12
25°C
8
0
125°C
0
4
85°C
4
8
12
16
20
125°C
0
4
SUPPLY VOLTAGE = 3.3V
110
85°C
25°C
100
95
125°C
90
85
80
0
4
8
12
16
100
95
90
85°C
25°C
125°C
85
80
0
4
8
12
16
20
OUTPUT CURRENT (mA)
Figure 10. Current Conversion Efficiency vs. Output Current
SUPPLY VOLTAGE = 5.0V
0V
OUTPUT VOLTAGE (0.2V/DIV)
ENABLE PULSE
ENABLE VOLTAGE
OUTPUT VOLTAGE (0.2V/DIV)
-40°C
105
SUPPLY VOLTAGE = 3.3V
0 mA
20
SUPPLY VOLTAGE = 5.0V
20
Figure 9. Current Conversion Efficiency vs. Output Current
10 mA 20 mA
16
110
OUTPUT CURRENT (mA)
0V
12
Figure 8. Supply Current vs. Output Current
CURRENT CONVERSION EFFICIECY (%)
CURRENT CONVERSION EFFICIECY (%)
Figure 7. Supply Current vs. Output Current
105 -40°C
8
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
ENABLE PULSE
0V
0 mA
0V
5 mA
10 mA
20 mA
ENABLE VOLTAGE
SUPPLY CURRENT (mA)
20
5 mA
TURN ON TIME (200 és/DIV)
TURN ON TIME (100 és/DIV)
Figure 11. Turnon Time
Figure 12. Turnon Time
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Typical Characteristics (continued)
VDD = 3.3 V and TA = 25°C unless otherwise noted.
SUPPLY VOLTAGE = 3.3V
SUPPLY VOLTAGE = 5.0V
0.4
LOAD REGULATION (%/mA)
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0
40
80
120
-40
0
Figure 13. Load Regulation vs. Temperature
-0.210
20
+85/+125°C
10
OUTPUT VOLTAGE (V)
-0.226
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
30
-0.234
0
-0.242
30
-0.226
20
+25°C
10
0
-0.242
-10
-10
-0.250
TIME (20 us/DIV)
TIME (20 us/DIV)
Figure 15. Transient Response
Figure 16. Transient Response
40
-0.210
20
10
-0.234
+85/+125°C
-0.242
OUTPUT VOLTAGE (V)
30
OUTPUT CURRENT (mA)
SUPPLY VOLTAGE = 5V
+25°C
-0.226
-0.218
-40°C
30
+25°C
20
-0.226
10
-0.234
+85/+125°C
0
-0.242
-10
-0.250
OUTPUT CURRENT
-0.250
40
-0.210
SUPPLY VOLTAGE = 3.3V
OUTPUT VOLTAGE (V)
+85/+125°C
-0.234
OUTPUT CURRENT
-0.250
0
OUTPUT CURRENT
TIME (20 us/DIV)
-10
TIME (20 us/DIV)
Figure 17. Transient Response
8
SUPPLY VOLTAGE = 5V
-0.218
OUTPUT CURRENT
-40°C
40
-40°C
-0.218
-0.218
120
-0.210
SUPPLY VOLTAGE = 3.3V
+25°C
80
Figure 14. Load Regulation vs. Temperature
40
-40°C
40
TEMPERATURE (°C)
TEMPERATURE (°C)
OUTPUT CURRENT (mA)
-40
OUTPUT CURRENT (mA)
LOAD REGULATION (%/mA)
0.4
Figure 18. Transient Response
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Typical Characteristics (continued)
VDD = 3.3 V and TA = 25°C unless otherwise noted.
300
SUPPLY VOLTAGE = 5V
0
-0.05
SUPPLY CURRENT (éA)
OUTPUT VOLTAGE (V)
250
SUPPLY
VOLTAGE = 3.3V
-0.10
-0.15
SUPPLY
VOLTAGE = 5V
-0.20
200
150
SUPPLY VOLTAGE = 3.3V
100
50
-0.25
0
1
2
3
4
0
0
5
SHUTDOWN VOLTAGE (V)
1
2
3
4
5
SHUTDOWN VOLTAGE (V)
Figure 19. Output voltage vs. Shutdown Voltage
Figure 20. Supply Current vs. Shutdown Voltage
100
OSCILLATOR FREQUENCY (kHz)
SUPPLY VOLTAGE = 3.3V
95
90
85
80
SUPPLY VOLTAGE = 5V
75
70
-40
0
40
80
120
TEMPERATURE (°C)
Figure 21. Oscillator Frequency vs. Temperature
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7 Detailed Description
7.1 Overview
The LM7705 is a switched capacitor voltage inverter with a low-noise, −0.23-V fixed negative bias output. The
part will operate over a supply voltage range of 3 V to 5.25 V. Applying a logical low level to the SD input will
activate the part, and generate a fixed −0.23-V output voltage. The part can be disabled; the output is switched
to ground level, by applying a logical high level to the SD input of the part.
7.2 Functional Block Diagram
CFLY
VCP,OUT
VCP,IN
VDD
CHARGE
PUMP
INVERTOR
PRE
REGULATOR
VSS
VOUT
CRESERVE
POST
REGULATOR
Cout
VSS
fosc
VREF1
VREF2
7.3 Feature Description
7.3.1 Supply Voltage
The LM7705 will operate over a supply voltage range of 3 V to 5.25 V, and meet the specifications given in the
3.3-V Electrical Characteristics Table. Supply voltage lower than 3.3 V will decrease performance (The output
voltage will shift towards zero, and the current sink capabilities will decrease) A voltage higher than 5.25 V will
exceed the Absolute Maximum Ratings ratings and therefore damage the part.
7.3.2 Output Voltage and Line Regulation
The fixed and regulated output voltage of −0.23 V has tight limits, as indicated in the 3.3-V Electrical
Characteristics table, to ensure a stable voltage level. The usage of the pre- and post regulator in combination
with the charge pump inverter ensures good line regulation of 0.29%/V
7.3.3 Output Current and Load Regulation
The LM7705 can sink currents more than 26 mA, causing an output voltage shift to −200 mV. A specified loadregulation of 0.14% mA/V ensures a minor voltage deviation for load current up to 20 mA.
7.3.4 Quiescent Current
The LM7705 consumes a quiescent current less than 100 µA. Sinking a load current, will result in a current
conversion efficiency better than 90%, even for load currents of 1 mA, increasing to 98% for a current of 5mA.
7.4 Device Functional Modes
7.4.1 General Amplifier Application
This section will discuss a general DC coupled amplifier application. First, one of the limitations of a DC coupled
amplifier is discussed. This is illustrated with two application examples. A solution is a given for solving this
limitation by using the LM7705.
Due to the architecture of the output stage of general amplifiers, the output transistors will saturate. As a result,
the output of a general purpose op amp can only swing to a few 100 mV of the supply rails. Amplifiers using
CMOS technology do have a lower output saturation voltage. This is illustrated in Figure 22. For example, Texas
Instruments' LM7332 can swing to 200 mV to the negative rail, for a 10-kΩ load, over all temperatures.
10
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Device Functional Modes (continued)
OUTPUT VOLTAGE (V)
V+
OUTPUT SATURATION
VDSAT
0
0
V+
INPUT VOLTAGE (V)
Figure 22. Limitation of the Output of an Amplifier
The introduction of operational amplifiers with output rail-to-rail drive capabilities is a strong improvement and the
(output) performance of op amps is for many applications no longer a limiting factor. For example, Texas
Instruments' LMP7701 (a typical rail-to-rail op amp), has an output drive capability of only 50 mV over all
temperatures for a 10-kΩ load resistance. This is close to the lower supply voltage rail.
However, for true zero output applications with a single supply, the saturation voltage of the output stage is still a
limiting factor. This limitation has a negative impact on the functionality of true zero output applications. This is
illustrated in Figure 23.
+V
VIN
VOUT
+
VDSAT
-
0V
0V
Figure 23. Output Limitation for Single-Supply True Zero Output Application
In the One-Stage, Single-Supply True Zero Amplifier section, two applications will be discussed, showing the
limitations of the output stage of an op amp in a single supply configuration:
• A single stage true zero amplifier, with a 12-bit ADC back end.
• A dual stage true zero amplifier, with a 12-bit ADC back end.
7.4.1.1 One-Stage, Single-Supply True Zero Amplifier
This application shows a sensor with a DC output signal, amplified by a single supply op amp. The output voltage
of the op amp is converted to the digital domain using an Analog to Digital Converter (ADC). Figure 24 shows the
basic set-up of this application.
+V
LMP7701
VREF
ADC122S021
+
ADC
-
SENSOR
RG1
RF1
GAIN = 50x
Figure 24. Sensor With DC Output and a Single-Supply Op Amp
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Device Functional Modes (continued)
The sensor has a DC output signal that is amplified by the op amp. For an optimal signal-to-noise ratio, the
output voltage swing of the op amp must be matched to the input voltage range of the Analog to Digital
Converter (ADC). For the high side of the range this can be done by adjusting the gain of the op amp. However,
the low side of the range cannot be adjusted and is affected by the output swing of the op amp.
Example:
Assume the output voltage range of the sensor is 0 to 90 mV. The available op amp is a LMP7701, using a 0/+5V supply voltage, having an output drive of 50 mV from both rails. This results in an output range of 50 mV to
4.95V.
Select two resistors values for RG1 and RF1 that result in a gain of 50x. The output of the LMP7701 must swing
from 0 mV to 4.5 V. The higher value is no problem, however the lower swing is limited by the output of the
LM7701 and won’t go below 50 mV instead of the desired 0 V, causing a non-linearity in the sensor reading.
When using a 12-bit ADC, and a reference voltage of 5 V (having an ADC step size of approximate 1.2 mV), the
output saturation results in a loss of the lower 40 quantization levels of the ADCs dynamic range.
7.4.1.2 Two-Stage, Single-Supply True Zero Amplifier
This sensor application produces a DC signal, amplified by a two cascaded op amps, having a single supply. The
output voltage of the second op amp is converted to the digital domain. Figure 25 shows the basic setup of this
application.
+V
1/2 LMP7702
VREF
+V
+
A1
1/2 LMP7702
+
A2
ADC122S021
-
ADC
-
SENSOR
RG1
RF1
RG2
GAIN = 10x
RF2
GAIN = 5x
Figure 25. Sensor With DC Output and a 2-Stage, Single-Supply Op Amp
The sensor generates a DC output signal. In this case, a DC coupled, 2-stage amplifier is used. The output
voltage swing of the second op amp must me matched to the input voltage range of the Analog to Digital
Converter (ADC). For the high side of the range this can be done by adjusting the gain of the op amp. However,
the low side of the range can’t be adjusted and is affected by the output drive of the op amp.
Example:
Assume; the output voltage range of the sensor is 0 to 90 mV. The available op amp is a LMP7702 (Dual
LMP7701 op amp) that can be used for A1 and A2. The op amp is using a 0/+5-V supply voltage, having an
output drive of 50 mV from both rails. This results in an output range of 50 mV to 4.95 V for each individual
amplifier.
Select two resistors values for RG1 and RF1 that result in a gain of 10x for the first stage (A1) and a gain of 5x for
the second stage (A2) The output of the A2 in the LMP7702 must swing from 0V to 4.5 V. This swing is limited by
the 2 different factors:
1. The high voltage swing is no problem; however the low voltage swing is limited by the output saturation
voltage of A2 from the LM7702 and will not go below 50 mV instead of the desired 0 V.
2. Another effect has more impact. The output saturation voltage of the first stage will cause an offset for the
input of the second stage. This offset of A1 is amplified by the gain of the second stage (10x in this example),
resulting in an output offset voltage of 500mV. This is significantly more that the 50 mV (VDSAT) of A2.
When using a 12-bit ADC, and a reference voltage of 5 Volt (having an ADC step size of approximate 1.2 mV),
the output saturation results in a loss of the lower 400 quantization levels of the ADCs dynamic range. This will
cause a major non-linearity in the sensor reading.
12
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Device Functional Modes (continued)
7.4.1.3 Dual-Supply, True Zero Amplifiers
The limitations of the output stage of the op amp, as indicated in both examples, can be omitted by using a dual
supply op amp. The output stage of the used op amp can then still swing from 50 mV of the supply rails.
However, the functional output range of the op amp is now from ground level to a value near the positive supply
rail. Figure 26 shows the output drive of an amplifier in a true zero output voltage application.
+V
VIN
VOUT
+
0V
-
0V
-V
Figure 26. Amplifier Output Drive With a Dual-Supply
Disadvantages of this solution are:
• The usage of a dual-supply instead of a simple single supply is more expensive.
• A dual supply voltage for the op amps requires parts that can handle a larger operating range for the supply
voltage. If the op amps used in the current solution cannot handle this, a redesign can be required.
A better solution is to use the LM7705. This low-noise negative bias generator has some major advantages with
respect to a dual-supply solution:
• Operates with only a single positive supply, and is therefore a much cheaper solution.
• The LM7705 generates a negative supply voltage of only −0.23 V. This is more than enough to create a Truezero output for most op amps.
• In many applications, this small extension of the supply voltage range can be within the abs max rating for
many op amps, so an expensive redesign is not necessary.
In the Typical Application section, a typical amplifier application will be evaluated. The performance of an
amplifier will be measured in a single supply configuration. The results will be compared with an amplifier using a
LM7705 supplying a negative voltage to the bias pin.
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8 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.
8.1 Application Information
8.1.1 Functional Description
The LM7705, low-noise negative bias generator, can be used for many applications requiring a fixed negative
voltage. A key application for the LM7705 is an amplifier with a true zero output voltage using the original parts,
while not exceeding the maximum supply voltage ratings of the amplifier.
The voltage inversion in the LM7705 is achieved using a switched capacitor technique with two external
capacitors (CFLY and CRES). An internal oscillator and a switching network transfers charge between the two
storage capacitors. This switched capacitor technique is given in Figure 27.
V+
CAP+ S3
S1
CFLY
S4
S2
OUT=V-
CAP-
CRES
Ó1
Ó2
OSCILLATOR
Figure 27. Voltage Inverter
The internal oscillator generates two anti-phase clock signals. Clock 1 controls switches S1 and S2. Clock 2
controls switches S3 and S4. When Switches S1 and S2 are closed, capacitor CFLY is charged to V+. When
switches S3 and S4 are closed (S1 and S2 are open) charge from CFLY is transferred to CRES and the output
voltage OUT is equal to –V+.
Due to the switched capacitor technique, a small ripple will be present at the output voltage with a frequency of
the oscillator. The magnitude of this ripple will increase for increasing output currents. The magnitude of the
ripple can be influenced by changing the values of the used capacitors.
8.1.2 Technical Description
As indicated in Functional Description, the main function of the LM7705 is to supply a stabilized negative bias
voltage to a load, using only a positive supply voltage. A general block diagram for this charge pump inverter is
given in Figure 28. The external power supply and load are added in this diagram as well.
LM7705
POWER
SUPPLY
PRE
REGULATOR
CHARGE
PUMP
POST
REGULATOR
LOAD
Figure 28. LM7705 Architecture
The architecture given in Figure 28 shows that the LM7705 contains 3 functional blocks:
• Pre-regulator
• Charge pump inverter
• Post-regulator
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Application Information (continued)
The output voltage is stabilized by:
• Controlling the power supplied from the power supply to the charge pump input by the pre-regulator
• The power supplied from the charge pump output to the load by the post-regulator.
A more detailed block diagram of the negative bias generator is given in Figure 29. The control of the preregulator is based on measuring the output voltage of the charge pump. The goal of the post-regulator is to
provide an accurate controlled negative voltage at the output, and acts as a lowpass filter to attenuate the output
voltage ripple. The voltage ripple is a result of the switching behavior of the charge pump and is dependent of the
output current and the values of the used capacitors.
CFLY
VCP,OUT
VCP,IN
VDD
CHARGE
PUMP
INVERTOR
PRE
REGULATOR
VSS
VOUT
CRESERVE
POST
REGULATOR
Cout
VSS
fosc
VREF1
VREF2
Figure 29. Charge Pump Inverter With Input and Output Control
In Charge Pump Theory, a simple equation will be derived that shows the relation between the ripple of the
output current, the frequency of the internal clock generator and the value of the capacitor placed at the output of
the LM7705.
8.1.3 Charge Pump Theory
This section uses a simplified but realistic equivalent circuit that represents the basic function of the charge
pump. The schematic is given in Figure 30.
A B
V2
V1
CFLY
CRES
RL
Figure 30. Charge Pump
When the switch is in position A, capacitor CFLY will charge to voltage V1. The total charge on capacitor CFLY is
Q1 = CFLY × V 1. The switch then moves to position B, discharging CFLY to voltage V2. After this discharge, the
charge on CFLY will be Q2 = CFLY × V2. The charge has been transferred from the source V1 to the output V2. The
amount of charge transferred is:
Âq = q1 -q2 = CFLY (V1 ± V2)
(1)
When the switch changes between A and B at a frequency f, the charge transfer per unit time, or current is:
I = f Âq = f CFLY (V1 ± V2)
(2)
The switched capacitor network can be replaced by an equivalent resistor, as indicated in Figure 31.
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Application Information (continued)
REQ
V2
V1
CRES
RL
Figure 31. Switched Capacitor Equivalent Circuit
The value of this resistor is dependent on both the capacitor value and the switching frequency as given in
Equation 3
I=
V1 ± V2
V1 ± V2
=
REQ
1
·
§
©f CFLY ¹
(3)
The value for REQ can be calculated from Equation 3 and is given in Equation 4
REQ =
§ 1 ·
© f CFLY ¹
(4)
Equation 4 show that the value for the resistance at an increased internal switching frequency, allows a lower
value for the used capacitor.
8.2 Typical Application
This section shows the measurement results of a true zero output amplifier application with an analog to digital
converter (ADC) used as back-end. The biasing of the op amp can be done in two ways:
• A single supply configuration
• A single supply in combination with the LM7705, extending the negative supply from ground level to a fixed
–0.23 Voltage.
VREF
+V
LMP7701
VIN
ADC122S021
+
ADC
SDO
-
CRES
-V
A
B
+V
LM7705
CFLY
COUT
Figure 32. Typical True Zero Output Voltage Application With or Without LM7705
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Typical Application (continued)
8.2.1 Design Requirements
The key specifications of the used components are shown in Table 1.
Table 1. Design Parameters
PARAMETERS
EXAMPLE VALUE
SUPPLY VOLTAGE/REFERENCE VOLTAGE
Supply voltage
5V
ADC Voltage Reference
5V
LMP7701
VDSAT (typical)
18 mV
VDSAT (over temperature)
50 mV
LM7705
Output voltage ripple
4 mVPP
Output voltage noise
10 mVPP
ADC
Type
ADC122S021
Resolution
12-bit
Quantization level
5V/4096 = 1.2 mV
8.2.2 Detailed Design Procedure
8.2.2.1 Basic Setup
The basic setup of this true zero output amplifier is given in Figure 32. The LMP7701 op amp is configured as a
voltage follower to demonstrate the output limitation, due to the saturation of the output stage. The negative
power supply pin of the op amp can be connected to ground level or to the output of the negative bias generator,
to demonstrate the VDSAT effect at the output voltage range.
The output voltage of the LMP7701 is converted to the digital domain using an ADC122S021. This is an 12-bit
analog to digital converter with a serial data output. Data processing and graphical displaying is done with a
computer. The negative power supply pin of the op amp can be connected to ground level or to the output of the
negative bias generator, to demonstrate the effect at the output voltage range of the op amp.
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8.2.3 Application Curves
The output voltage range of the LMP7701 has been measured, especially the range to ground level. A small DC
signal, with a voltage swing of 50 mVPP is applied to the input. The digitized output voltage of the op amp is
measured over a given time period, when its negative supply pin is connected to ground level or connected to
the output of the LM7705.
Figure 33 shows the digitized output voltage of the op amp when its negative supply pin is connected to ground
level. The output of the amplifier saturates at a level of 14 mv (this is in line with the typical value of 18 mV given
in the datasheet) The graph shows some fluctuations (1-bit quantization error). Figure 34 show the digitized
output voltage of the op amp when its negative supply pin is connected to the output of the LM7705. Again, the
graph shows some 1-bit quantization errors caused by the voltage ripple and output noise. In this case the op
amps output level can reach the true zero output level.
Figure 33 and Figure 34 show that:
• With a single supply, the output of the amplifier is limited by the VDSAT of the output stage.
• The amplifier can be used as a true zero output using a LM7705.
• The quantization error of the digitized output voltage is caused by the noise and the voltage ripple.
• Using the LM7705 does not increase the quantization error in this set up.
0.050
DIGITIZED OUTPUT VOLTAGE (V)
DIGITIZED OUTPUT VOLTAGE (`V)
0.050
0.040
0.030
0.020
0.010
0.040
0.030
0.020
0.010
VDSAT
0.000
0
80
160
240
320
0.000
0
400
TIME (SAMPLES)
80
160
240
320
400
TIME (SAMPLES)
Figure 33. Digitized Output Voltage Without LM7705
Figure 34. Digitized Output Voltage With LM7705
9 Power Supply Recommendations
To prevent large variations at the VDD pin of the package it is recommended to add a decouple capacitor as
close to the pin as possible.
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10 Layout
10.1 Layout Guidelines
The LM7705 is a switched capacitor voltage inverter. This means that charge is transferred from different
external capacitors, to generate a negative voltage. For this reason the part is very sensitive for contact
resistance between the package and external capacitors. TI also recommends to use low ESR capacitors for
CFLY, CRES and COUT in combination with short traces.
The output voltage noise can be suppressed using a small RF capacitor, will a value of, for example, 100 nF.
10.2 Layout Examples
Figure 35 contains a layout example for the LM7705.
CBYPASS
COUT
CRES
CFLY
Figure 35. Example PCB Layout: Top layer
Figure 36. Schematics for Example PCB Layout
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11 Device and Documentation Support
11.1 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.2 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.3 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
11.4 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM7705MM/NOPB
ACTIVE
VSSOP
DGK
8
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
F26A
LM7705MME/NOPB
ACTIVE
VSSOP
DGK
8
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
F26A
LM7705MMX/NOPB
ACTIVE
VSSOP
DGK
8
3500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
F26A
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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