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Precision Low Drift 2.048 V/2.500 V SOT-23 Voltage References with Shutdown ADR390/ADR391
PIN CONFIGURATION 5-Lead SOT-23 (RT Suffix)
SHDN 1 VIN 2 VOUT(SENSE) 3
5
FEATURES Initial Accuracy: 6 mV Max Low TCVO: 25 ppm/ C Max Load Regulation: 60 ppm/mA Line Regulation: 25 ppm/V Wide Operating Range: 2.4 V-18 V for ADR390 2.8 V-18 V for ADR391 Low Power: 120 A Max Shutdown to Less than 3 A Max High Output Current: 5 mA Min Wide Temperature Range: 40 C to +85 C Tiny SOT-23-5 Package APPLICATIONS Battery-Powered Instrumentation Portable Medical Instruments Data Acquisition Systems Industrial and Process Control Systems Hard Disk Drives Automotive GENERAL DESCRIPTION
GND
ADR390/ ADR391
4
VOUT(FORCE)
Table I.
Part Number ADR390 ADR391
Nominal Output Voltage (V) 2.048 2.500
The ADR390 and ADR391 are precision 2.048 V and 2.5 V bandgap voltage references featuring high accuracy and stability and low power consumption in a tiny footprint. Patented temperature drift curvature correction techniques minimize nonlinearity of the voltage change with temperature. The wide operating range and low power consumption with additional shutdown capability make them ideal for 3 V to 5 V battery-powered applications. The VOUT Sense Pin enables greater accuracy by supporting full Kelvin operation in systems using very fine or long circuit traces. The ADR390 and ADR391 are micropower, Low Dropout Voltage (LDV) devices that provide a stable output voltage from supplies as low as 300 mV above the output voltage. They are specified over the industrial (-40C to +85C) temperature range. Each is available in the tiny 5-lead SOT-23 package. The combination of VOUT sense and shutdown functions also enables a number of unique applications combining precision reference/regulation with fault decision and over-current protection. Details are provided in the applications section.
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 2000
ADR390/ADR391
ADR390 SPECIFICATIONS
ELECTRICAL CHARACTERISTICS (@ V
Parameter Initial Accuracy Initial Accuracy Error Temperature Coefficient Minimum Supply Voltage Headroom Line Regulation Load Regulation Quiescent Current Voltage Noise Turn-On Settling Time Long-Term Stability1 Output Voltage Hysteresis2 Ripple Rejection Ratio Short Circuit to GND Shutdown Supply Current Shutdown Logic Input Current Shutdown Logic Low Shutdown Logic High Symbol VO VOERR TCVO/C VIN - VO VO/VIN VO/ILOAD IIN eN tR VO VOHYS RRR ISC ISHDN ILOGIC VINL VINH
IN
= 5 V, TA = 25 C unless otherwise noted)
Conditions Min 2.042 0.29 -40C < TA < +85C 300 VIN = 2.5 V to 15 V -40C < TA < +85C VIN = 3 V, ILOAD = 0 mA to 5 mA -40C < TA < +85C No Load -40C < TA < +85C 0.1 Hz to 10 Hz 1,000 Hours fIN = 60 Hz 10 25 60 120 140 Typ Max Unit V % ppm/C mV ppm/V ppm/mA A A V p-p s ppm ppm dB mA A nA V V 2.048 2.054 0.29 5 25
100 5 20 50 40 85 30
3 500 0.8 2.4
NOTES 1 Long-term stability, typical shift in value of output voltage at 25C on a sample of parts subjected to operation life test of 1000 hours at 125 C. VO = VO (t0) -VO (t1000); VO (t0) = VO at 25C at time 0; V O (t1000) = VO at 25C after 1000 hours at 125C; VO = (VO (t0) - VO (t1000))/VO (t0) x 106 (in ppm). 2 Output Voltage Hysteresis, is defined as the change in 25C output voltage before and after the device is cycled through temperature. +25 C to -40C to +85C to +25C. This is a typical value from a sample of parts put through such a cycle. Refer to Figures 11 and 12. V OHYS = VO -VOTC; VO = VO at 25C at time 0; V OTC = VO at 25C after temperature cycle at +25C to -40C to +85C to +25C; VOHYS = ((V O-VOTC)/VO) x 106 (in ppm). Specifications subject to change without notice.
ELECTRICAL CHARACTERISTICS (@ V
Parameter Initial Accuracy Initial Accuracy Error Temperature Coefficient Minimum Supply Voltage Headroom Line Regulation Load Regulation Quiescent Current Voltage Noise Turn-On Settling Time Long-Term Stability1 Output Voltage Hysteresis2 Ripple Rejection Ratio Short Circuit to GND Shutdown Supply Current Shutdown Logic Input Current Shutdown Logic Low Shutdown Logic High Symbol
IN
= 15 V, TA = 25 C unless otherwise noted)
Conditions Min 2.042 0.29 -40C < TA < +85C 300 VIN = 2.5 V to 15 V -40C < TA < +85C VIN = 3 V, ILOAD = 0 mA to 5 mA -40C < TA < +85C No Load -40C < TA < +85C 0.1 Hz to 10 Hz 1,000 Hours fIN = 60 Hz 10 25 60 120 140 Typ Max Unit V % ppm/C mV ppm/V ppm/mA A A V p-p s ppm ppm dB mA A nA V V 2.048 2.054 0.29 5 25
VO VOERR TCVO/C VIN - VO VO/VIN VO/ILOAD IIN eN tR VO VOHYS RRR ISC ISHDN ILOGIC VINL VINH
100 5 20 50 40 85 30
3 500 0.8 VIN - 1
NOTES 1 Long-term stability, typical shift in value of output voltage at 25C on a sample of parts subjected to operation life test of 1000 hours at 125 C. VO = VO (t0) -VO (t1000); VO (t0) = VO at 25C at time 0; V O (t1000) = VO at 25C after 1000 hours at 125C; VO = (VO (t0) - VO (t1000))/VO (t0) x 106 (in ppm). 2 Output Voltage Hysteresis, is defined as the change in 25C output voltage before and after the device is cycled through temperature. +25 C to -40C to +85C to +25C. This is a typical value from a sample of parts put through such a cycle. Refer to Figures 11 and 12. V OHYS = VO -VOTC; VO = VO at 25C at time 0; V OTC = VO at 25C after temperature cycle at +25C to -40C to +85C to +25C; VOHYS = ((V O-VOTC)/VO) x 106 (in ppm). Specifications subject to change without notice.
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ADR390/ADR391
ADR391 SPECIFICATIONS
ELECTRICAL CHARACTERISTICS (@ V
Parameter Initial Accuracy Initial Accuracy Error Temperature Coefficient Minimum Supply Voltage Headroom Line Regulation Load Regulation Quiescent Current Voltage Noise Turn-On Settling Time Long-Term Stability1 Output Voltage Hysteresis2 Ripple Rejection Ratio Short Circuit to GND Shutdown Supply Current Shutdown Logic Input Current Shutdown Logic Low Shutdown Logic High
IN
= 5 V, TA = 25 C unless otherwise noted)
Conditions Min 2.494 0.24 300 VIN = 2.8 V to 15 V -40C < TA < +85C VIN = 3.5 V, ILOAD = 0 mA to 5 mA -40C < TA < +85C No Load -40C < TA < +85C 0.1 Hz to 10 Hz 1,000 Hours fIN = 60 Hz 10 25 60 120 140 Typ 2.5 5 Max 2.506 0.24 25 Unit V % ppm/C mV ppm/V ppm/mA A A V p-p s ppm ppm dB mA A nA V V
Symbol VO VOERR TCVO/C VIN - VO VO/VIN VO/ILOAD IIN eN tR VO VOHYS RRR ISC ISHDN ILOGIC VINL VINH
-40C < TA < +85C
100 5 20 50 75 85 25
3 500 0.8 2.4
NOTES 1 Long-term stability, typical shift in value of output voltage at 25C on a sample of parts subjected to operation life test of 1000 hours at 125 C. VO = VO (t0) -VO (t1000); VO (t0) = VO at 25C at time 0; V O (t1000) = VO at 25C after 1000 hours at 125C; VO = (VO (t0) - VO (t1000))/VO (t0) x 106 (in ppm). 2 Output Voltage Hysteresis, is defined as the change in 25C output voltage before and after the device is cycled through temperature. +25 C to -40C to +85C to +25C. This is a typical value from a sample of parts put through such a cycle. Refer to Figures 11 and 12. V OHYS = VO -VOTC; VO = VO at 25C at time 0; V OTC = VO at 25C after temperature cycle at +25C to -40C to +85C to +25C; VOHYS = ((VO-VOTC)/VO) x 106 (in ppm). Specifications subject to change without notice.
ELECTRICAL CHARACTERISTICS (@ V
Parameter Initial Accuracy Initial Accuracy Error Temperature Coefficient Minimum Supply Voltage Headroom Line Regulation Load Regulation Quiescent Current Voltage Noise Turn-On Settling Time Long-Term Stability1 Output Voltage Hysteresis2 Ripple Rejection Ratio Short Circuit to GND Shutdown Supply Current Shutdown Logic Input Current Shutdown Logic Low Shutdown Logic High Symbol
IN
= 15 V, TA = 25 C unless otherwise noted)
Conditions Min 2.494 0.24 -40C < TA < +85C 300 VIN = 2.8 V to 15 V -40C < TA < +85C VIN = 3.5 V, ILOAD = 0 mA to 5 mA -40C < TA < +85C No Load -40C < TA < +85C 0.1 Hz to 10 Hz 1,000 Hours fIN = 60 Hz 10 25 60 120 140 Typ 2.5 5 Max 2.506 0.24 25 Unit V % ppm/C mV ppm/V ppm/mA A A V p-p s ppm ppm dB mA A nA V V
VO VOERR TCVO/C VIN - VO VO/VIN VO/ILOAD IIN eN tR VO VOHYS RRR ISC ISHDN ILOGIC VINL VINH
100 5 20 50 75 85 30
3 500 0.8 VIN - 1
NOTES 1 Long-term stability, typical shift in value of output voltage at 25C on a sample of parts subjected to operation life test of 1000 hours at 125 C. VO = VO (t0) -VO (t1000); VO (t0) = VO at 25C at time 0; V O (t1000) = VO at 25C after 1000 hours at 125C; VO = (VO (t0) - VO (t1000))/VO (t0) x 106 (in ppm). 2 Output Voltage Hysteresis, is defined as the change in 25C output voltage before and after the device is cycled through temperature. +25 C to -40C to +85C to +25C. This is a typical value from a sample of parts put through such a cycle. Refer to Figures 11 and 12. V OHYS = VO -VOTC; VO = VO at 25C at time 0; V OTC = VO at 25C after temperature cycle at +25C to -40C to +85C to +25C; VOHYS = ((VO-VOTC)/VO) x 106 (in ppm). Specifications subject to change without notice.
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ADR390/ADR391
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 V Shutdown Logic Level . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 V Or Supply Voltage, Whichever is Lower . . . . . . . . . . . . 18 V Output Short-Circuit Duration to GND Observe Derating Curves Storage Temperature Range RT Package . . . . . . . . . . . . . . . . . . . . . . . -65C to +150C Operating Temperature Range ADR390/ADR391 . . . . . . . . . . . . . . . . . . . -40C to +85C Junction Temperature Range RT Package . . . . . . . . . . . . . . . . . . . . . . . -65C to +150C Lead Temperature Range (Soldering, 60 sec) . . . . . . . . 300C
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Package Type 5-Lead SOT-23 (RT)
JA*
JC
Unit C/W
230
-
*JA is specified for worst-case conditions, i.e., JA is specified for device in socket for SOT packages.
ORDERING GUIDE
Model ADR390ART-REEL7 ADR390ART-REEL ADR391ART-REEL7 ADR391ART-REEL
Temperature Range -40 C to +85 C -40 C to +85 C -40 C to +85 C -40 C to +85 C
Package Description 5-Lead SOT 5-Lead SOT 5-Lead SOT 5-Lead SOT
Package Option RT-5 RT-5 RT-5 RT-5
Top Mark R0A R0A R1A R1A
Output Voltage 2.048 2.048 2.500 2.500
Number of Parts 3,000 10,000 3,000 10,000
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the ADR390/ADR391 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
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Typical Performance Characteristics- ADR390/ADR391
2.054 SAMPLE 1 2.052
A
140
120
SUPPLY CURRENT -
2.050
VOUT - V
+85 C 100 +25 C 40 C 80
2.048
SAMPLE 2
2.046
2.044
SAMPLE 3
60
2.042 40 15 10 35 TEMPERATURE - C 60 85
40 2.5
5.0
7.5 10.0 INPUT VOLTAGE - V
12.5
15.0
Figure 1. ADR390 Output Voltage vs. Temperature
Figure 4. ADR391 Supply Current vs. Input Voltage
2.506
40 IL= 0mA TO 5mA
LOAD REGULATION - ppm/mA
2.504 SAMPLE 1 2.502
35
30 VIN = 3.0V 25 VIN = 5.0V
VOUT - V
SAMPLE 2 2.500
2.498 SAMPLE 3 2.496
20
15
2.494 40 15 10 35 TEMPERATURE - C 60 85
10 40 15 10 35 TEMPERATURE - C 60 85
Figure 2. ADR391 Output Voltage vs. Temperature
Figure 5. ADR390 Load Regulation vs. Temperature
140
40 IL= 0mA TO 5mA
A
+85 C
LOAD REGULATION - ppm/mA
120
SUPPLY CURRENT -
35
30 VIN = 3.5V 25 VIN = 5.0V 20
100 +25 C 40 C
80
60
15
40 2.5
5.0
7.5 10.0 INPUT VOLTAGE - V
12.5
15.0
10 40 15 10 35 TEMPERATURE - C 60 85
Figure 3. ADR390 Supply Current vs. Input Voltage
Figure 6. ADR391 Load Regulation vs. Temperature
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ADR390/ADR391
5 VIN = 2.5V TO 15V 4
0.8
DIFFERENTIAL VOLTAGE - V
LINE REGULATION - ppm/V
0.6 +85 C +25 C 0.4 40 C 0.2
3
2
1
0 40 15 10 35 TEMPERATURE - C 60 85
0
0
1
2 3 LOAD CURRENT - mA
4
5
Figure 7. ADR390 Line Regulation vs. Temperature
Figure 10. ADR391 Minimum Input-Output Voltage Differential vs. Load Current
5 VIN = 2.8V TO 15V
LINE REGULATION - ppm/V
60 TEMPERATURE: +25 C 50 40 C +85 C +25 C
4
40
FREQUENCY
3
30
2
20
1
10
0 40 15 10 35 TEMPERATURE - C 60 85
0 0.24
0.18
0.12
0.06 0 0.06 0.12 VOUT DEVIATION - mV
0.18
0.24
0.30
Figure 8. ADR391 Line Regulation vs. Temperature
Figure 11. ADR390 VOUT Hysteresis Distribution
0.8
70 60
TEMPERATURE: +25 C
40 C
+85 C
+25 C
DIFFERENTIAL VOLTAGE - V
0.6
50
FREQUENCY
40 C 0.4 +85 C +25 C 0.2
40 30 20
10
0
0
1
2 3 LOAD CURRENT - mA
4
5
0 0.56
0.41
0.26 0.11 0.04 VOUT DEVIATION - mV
0.19
0.34
Figure 9. ADR390 Minimum Input-Output Voltage Differential vs. Load Current
Figure 12. ADR391 VOUT Hysteresis Distribution
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ADR390/ADR391
1k
0
VIN = 5V
0 0 0 0
CBYPASS = 0 F
VOLTAGE NOISE DENSITY - nV/ Hz
LINE INTERRUPTION
0.5V/DIV
ADR391
VOLTAGE
ADR390
0 0
VOUT
1V/DIV
100
0
10
100 1k FREQUENCY - Hz
10k
TIME - 10 s/DIV
Figure 13. Voltage Noise Density vs. Frequency
Figure 16. ADR391 Line Transient Response
0 0 0 0 0 0 0 0 0 TIME - 10ms/DIV
0 CBYPASS = 0.1 F 0 0 0 0 0 VOUT 0 0 0 TIME - 10 s/DIV 1V/DIV LINE INTERRUPTION
VOLTAGE - 100 V/DIV
0.5V/DIV
Figure 14. ADR390 Voltage Noise 10 Hz to 10 kHz
VOLTAGE
Figure 17. ADR391 Line Transient Response
0 0 0 0 0 0 0 0 0 TIME - 10ms/DIV
VOLTAGE - 1V/DIV
0 CL = 0nF 0 0 0 0 VLOAD ON 0 0 0 0 TIME - 200 s/DIV LOAD OFF VOUT
Figure 15. ADR391 Voltage Noise 10 Hz to 10 kHz
VOLTAGE - 100 V/DIV
Figure 18. ADR391 Load Transient Response
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ADR390/ADR391
0 CL = 1nF 0 0
VOLTAGE - 1V/DIV
0 VIN = 15V 0 VIN 0 0 0 0
VLOAD ON VOUT
5V/DIV
0 LOAD OFF 0 0 0 0 TIME - 200 s/DIV
VOLTAGE
0
VOUT
2V/DIV
0 0 0 TIME - 40 s/DIV
Figure 19. ADR391 Load Transient Response
Figure 22. ADR391 Turn-Off Response at 15 V
0 CL = 100nF 0 0 VOUT
0 CBYPASS = 0.1 F 0 0 VOUT 0 0 0 VLOAD ON VIN 0 0 0 TIME - 200 s/DIV TIME - 200 s/DIV 2V/DIV
VOLTAGE - 1V/DIV
0 LOAD OFF 0 0 0 0
VOLTAGE
0
5V/DIV
Figure 20. ADR391 Load Transient Response
Figure 23. ADR391 Turn-On/Turn-Off Response at 5 V
0 VIN = 15V 0 5V/DIV 0 0 VIN 0 0 VOUT 0 0 0 TIME - 20 s/DIV 2V/DIV
0 RL = 500 0 0 VOUT 0 0 0 VIN 0 0 0 TIME - 200 s/DIV 5V/DIV 2V/DIV
VOLTAGE
Figure 21. ADR391 Turn-On Response Time at 15 V
Figure 24. ADR391 Turn-On/Turn-Off Response at 5 V
VOLTAGE
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ADR390/ADR391
0 0 0
VOLTAGE - 5V/DIV
THEORY OF OPERATION
RL = 500 CL = 100nF 2V/DIV VOUT
0 0 0 VIN 0 0 0 TIME - 200 s/DIV 5V/DIV
Bandgap references are the high-performance solution for low supply voltage and low power voltage reference applications, and the ADR390/ADR391 is no exception. But the uniqueness of this product lies in its architecture. By observing Figure 28, the zero TC bandgap voltage is referenced to the output, not to ground. The bandgap cell consists of the pnp pair Q51 and Q52, running at unequal current densities. The difference in VBE results in a voltage with a positive TC which is amplified up by the ratio of 2 x R58 . This PTAT voltage, combined with VBE's R54 of Q51 and Q52 produce the stable bandgap voltage. Reduction in the bandgap curvature is performed by the ratio of the two resistors R44 and R59. Precision laser trimming and other patented circuit techniques are used to further enhance the drift performance.
VIN
Figure 25. ADR391 Turn-On/Turn-Off Response at 5 V
80
SHDN
60 40
RIPPLE REJECTION - dB
FORCE
20
SENSE
0 20 40 60 80 100 120 10
R59
R44 TESTPAD R58 R49
R54 R53 Q51 Q52 TESTPAD
TESTPAD
100 1k 10k FREQUENCY - Hz 100k 1M
R48 R60 R61 GROUND
2RS
Figure 26. Ripple Rejection vs. Frequency Figure 28. Simplified Schematic
Device Power Dissipation Considerations
100 90 80
OUTPUT IMPEDANCE -
70 60 CL = 0 F 50 40 30 20 10 0 10 100 CL = 1 F CL = 0.1 F
The ADR390/ADR391 is capable of delivering load currents to 5 mA with an input voltage that ranges from 2.8 V (ADR391 only) to 15 V. When this device is used in applications with large input voltages, care should be taken to avoid exceeding the specified maximum power dissipation or junction temperature that could result in premature device failure. The following formula should be used to calculate a device's maximum junction temperature or dissipation:
PD = TJ - TA JA
1k 10k FREQUENCY - Hz
100k
1M
In this equation, TJ and TA are, respectively, the junction and ambient temperatures, PD is the device power dissipation, and JA is the device package thermal resistance.
Shutdown Mode Operation
Figure 27. Output Impedance vs. Frequency
The ADR390/ADR391 includes a shutdown feature that is TTL/ CMOS level compatible. A logic LOW or a zero volt condition on the SHDN pin is required to turn the device off. During shutdown, the output of the reference becomes a high impedance state where its potential would then be determined by external circuitry. If the shutdown feature is not used, the SHDN pin should be connected to VIN (Pin 2).
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ADR390/ADR391
APPLICATIONS Membrane Switch Controlled Power Supply
The ADR390/ADR391 can operate as a low dropout power supply in hand-held instrumentation. In the following circuit, a membrane ON/OFF switch is used to control the operation of the reference. During an initial power-on condition, the SHDN pin is held to GND. Recall that this condition disables the output (read: three-state). When the membrane ON switch is pressed, the SHDN pin assumes and remains at the same potential as VIN, via the 10 k resistor thus enabling the output. When the membrane OFF switch is pressed, the SHDN pin is momentarily connected to GND which disables the ADR390/ADR391 output once again.
U2, either the external load of U1 or R1 must provide a path for this current. If the U1 minimum load is not well defined, the resistor R1 should be used, set to a value that will conservatively pass 600 A of current with the applicable VOUT1 across it. Note that the two U1 and U2 reference circuits are locally treated as macrocells, each having its own bypasses at input and output for best stability. Both U1 and U2 in this circuit can source dc currents up to their full rating. The minimum input voltage, VS, is determined by the sum of the outputs, VOUT2, plus the dropout voltage of U2. A related variation on stacking two three-terminal references is shown in the following figure where U1, an ADR391, is stacked with a two-terminal reference diode such as the AD589. Similar to the all three-terminal stacked references mentioned earlier, the two individual terminal voltage outputs of D1 and U1 are 1.235 V and 2.5 V, respectively. Thus VOUT2 is the sum of D1 and U1, or 3.735 V. When using two-terminal reference diodes such as D1, the rated minimum and maximum device currents must be observed, and the maximum load current from VOUT1 can be no greater than the current set up by R1 and VO(U1).
VIN VIN > VOUT2 +0.15V 5V 1 C1 0.1 F 2
VIN 10k ON
ADR39x
VOUT 1F TANT
OFF
U1 4 ADR39x
5 VO (U1)
Figure 29. Membrane Switch Controlled Power Supply
Stacking Reference ICs for Arbitrary Outputs
C2 1F
R1 4.99k
VOUT2 3.735V
(SEE TEXT)
Some applications may require two reference voltage sources which are a combined sum of standard outputs. The following circuit shows how this "stacked output" reference can be implemented:
OUTPUT TABLE U1/U2 ADR390/ADR390 ADR391/ADR391 VIN VIN > VOUT2 +0.15V C1 0.1 F 2 VOUT1 (V) VOUT2 (V) 2.048 2.5 4.096 5.0
D1 AD589 VIN COMMON
VO (D1)
C3 1F
VOUT1 1.235V
VOUT COMMON
Figure 31. Stacking Voltage References with the ADR390/ ADR391
A Negative Precision Reference without Precision Resistors
U2 1 ADR39x 4
(SEE TABLE)
VOUT2 VO (U2) C2 1F
5
C3 0.1 F
2 1 ADR39x 4
(SEE TABLE)
U1
5
VOUT1 VO (U1) C4 1F R1 3.9k
(SEE TEXT)
VIN COMMON
VOUT COMMON
Figure 30. Stacking Voltage References with the ADR390/ ADR391
Two reference ICs are used, fed from a common unregulated input, VIN. The outputs of the individual ICs are simply connected in series which provides two output voltages VOUT1 and VOUT2. VOUT1 is the terminal voltage of U1, while VOUT2 is the sum of this voltage and the terminal voltage of U2. U1 and U2 are simply chosen for the two voltages that supply the required outputs (see Output Table). For example, if both U1 and U2 are ADR391's, VOUT1 is 2.5 V and VOUT2 is 5.0 V. While this concept is simple, a precaution is in order. Since the lower reference circuit must sink a small bias current from U2, plus the base current from the series PNP output transistor in
In many current-output CMOS DAC applications where the output signal voltage must be of the same polarity as the reference voltage, it is often required to reconfigure a current-switching DAC into a voltage-switching DAC through the use of a 1.25 V reference, an op amp, and a pair of resistors. Using a currentswitching DAC directly requires the need for an additional operational amplifier at the output to reinvert the signal. A negative voltage reference is then desirable from the point that an additional operational amplifier is not required for either reinversion (currentswitching mode) or amplification (voltage switching mode) of the DAC output voltage. In general, any positive voltage reference can be converted into a negative voltage reference through the use of an operational amplifier and a pair of matched resistors in an inverting configuration. The disadvantage to this approach is that the largest single source of error in the circuit is the relative matching of the resistors used. The following circuit avoids the need for tightly matched resistors with the use of an active integrator circuit. In this circuit, the output of the voltage reference provides the input drive for the integrator. The integrator, to maintain circuit equilibrium, adjusts its output to establish the proper relationship between the reference's VOUT and GND. Thus, any negative output voltage desired can be chosen by simply substituting for the appropriate reference IC. The shutdown feature is maintained in the circuit with the simple addition of a PNP transistor and REV. 0
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ADR390/ADR391
a 10 k resistor. A precaution should be noted with this approach: although rail-to-rail output amplifiers work best in the application, these operational amplifiers require a finite amount (mV) of headroom when required to provide any load current. The choice for the circuit's negative supply should take this issue into account.
VIN 10k 2N3906 2 VS 1 SHDN VOUT VOUT 4 +5V A1 100 VREF 5V A1 = 1/2 OP295, 1/2 OP291 1k 1F
High-Power Performance with Current Limit
In some cases, the user may want higher output current delivered to a load and still achieve better than 0.5% accuracy out of the ADR390/ADR391. The accuracy for a reference is normally specified on the data sheet with no load. However, the output voltage changes with load current. The circuit below provides high current without compromising the accuracy of the ADR390/ADR391. The series pass transistor Q1 provides up to 1 A load current. The ADR390/ADR391 delivers only the base drive to Q1 through the force pin. The sense pin of the ADR390/ADR391 is a regulated output and is connected to the load.
R1 4.7k SHDN VIN VOUT (FORCE) VOUT (SENSE) Q2 Q2N2222 RS Q1 Q2N4921
SHDN TTL/CMOS
ADR39x
GND 5 10k 100k 1F
VIN
U1 GND
Figure 32. A Negative Precision Voltage Reference Uses No Precision Resistors
Precision Current Source
Many times in low-power applications, the need arises for a precision current source that can operate on low supply voltages. As shown in the following figure, the ADR390/ADR391 can be configured as a precision current source. The circuit configuration illustrated is a floating current source with a grounded load. The reference's output voltage is bootstrapped across RSET, which sets the output current into the load. With this configuration, circuit precision is maintained for load currents in the range from the reference's supply current, typically 90 A to approximately 5 mA.
VIN
ADR390/ADR391
RL
IL
Figure 34. ADR390/ADR391 for High-Power Performance with Current Limit
A similar circuit function can also be achieved with the Darlington transistor configuration, see Figure 35.
R1 4.7k SHDN VIN
SHDN VOUT
VIN
U1 GND Q2N2222 Q1 Q2 Q2N4921 RS
ADR39x
VS VOUT R1 R1
GND
1F ISY ADJUST P1 IOUT RL
}
VOUT (FORCE) VOUT (SENSE)
ADR390/ADR391
RSET
RL
Figure 35. ADR390/ADR391 High Output Current with Darlington Drive Configuration
Figure 33. A Precision Current Source
The transistor Q2 protects Q1 during short circuit limit faults by robbing its base drive. The maximum current is ILMAX 0.6 V/RS.
REV. 0
-11-
ADR390/ADR391
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
0.1181 (3.00) 0.1102 (2.80)
0.0669 (1.70) 0.0590 (1.50) PIN 1
5 1 2
4 3
0.1181 (3.00) 0.1024 (2.60)
0.0374 (0.95) BSC 0.0748 (1.90) BSC 0.0512 (1.30) 0.0354 (0.90) 0.0059 (0.15) 0.0019 (0.05) 0.0197 (0.50) 0.0138 (0.35) 0.0571 (1.45) 0.0374 (0.95) SEATING PLANE 10 0
0.0079 (0.20) 0.0031 (0.08)
0.0217 (0.55) 0.0138 (0.35)
-12-
REV. 0
PRINTED IN U.S.A.
C3863-8-4/00 (rev. 0) 00419
5-Lead SOT-23 (RT Suffix)

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