AN-1207: Programmable High Voltage Source with Boosted Output Current Using the AD5292 Digital Potentiometer, OP184 Op Amp, and MOSFETs (Rev. B) PDF

AN-1207
APPLICATION NOTE
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Programmable High Voltage Source with Boosted Output Current Using the AD5292
Digital Potentiometer, OP184 Op Amp, and MOSFETs
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
The circuit shown in Figure 1 provides a low cost, programmable,
high voltage source with boosted output current using the AD5292
digital potentiometer in conjunction with the OP184 operational
amplifier. The BSS138 PMOS transistor and Si2307CDS NMOS
transistor provide current drive capability up to 2.5 A.
Table 1. Devices Connected/Referenced
The circuit offers 1024 different voltage settings, controllable
through an SPI-compatible digital interface. This circuit offers
10-bit resolution over an output voltage range of 0 V to 30 V
and is capable of delivering up to 2.5 A output current.
This circuit employs the AD5292 digital potentiometer, in
conjunction with the OP184, the BSS138 N-MOSFET from
Diodes, Inc., and the Si2307CDS P-MOSFET from Vishay
Siliconix, providing a low cost, 10-bit resolution, high voltage
programmable source with boosted current output. The circuit
guarantees monotonicity, ±1 LSB DNL, and integral nonlinearity
of ±2 LSB typical.
The ±1% resistor tolerance of the AD5292, in conjunction with
an external resistor shown in Figure 2, increases the accuracy of
the circuit by providing 10-bit resolution over a reduced output
voltage range. This, in effect, creates a vernier DAC, which offers
higher resolution over the reduced range.
In addition, the AD5292 has an internal 20-times programmable
memory that allows a customized VOUT at power-up. The circuit
provides an accurate, low noise, low drift output voltage and high
current capabilities—and is well suited for power applications.
PMOS
Si2307CDS
20kΩ
30V
RBIAS
The OP184 is a single op amp that offers a high slew rate, low noise,
and rail-to-rail input and output. In the circuit, it is configured
in the follower mode. It guarantees that the output voltage, VOUT, is
equal to the voltage set in the digital potentiometer by driving
the BSS138 NMOS transistor. This MOSFET drives the Si2307CDS
PMOS transistor that delivers the current, IOUT, to the load.
SIGNAL
NMOS
OP184
BSS138
PMOS
LASER
DIODE
VSS
Figure 1. Programmable Voltage Source with Boosted Current Output
(Simplified Schematic: Decoupling and All Connections Not Shown)
SERIAL
INTERFACE
Si2307CDS
VOUT
VIN
30V
VDD
30V
AD5292
20kΩ
IOUT
R1
10kΩ
30V
RBIAS
OP184
BSS138
V’
25V
R2
100kΩ ± 1%
SIGNAL
NMOS
LASER
DIODE
VSS
Figure 2. Programmable Voltage Source with Increased Accuracy Over
Reduced Output Range (Simplified Schematic: Decoupling
and All Connections Not Shown)
Rev. B | Page 1 of 3
08454-002
AD5292
IOUT
R1
10kΩ
OP184
08454-001
SERIAL
INTERFACE
VDD
30V
Description
Digital potentiometer, 10 bits, 1% resistor
tolerance
Precision instrumentation amplifier
Resistor R1 ensures that the PMOS transistor is always on,
thereby eliminating latch-up or start-up problems. However,
this resistance limits the maximum settling time in the circuit.
The value chosen is a trade-off between the power dissipated in
the resistor and the maximum VOUT settling time.
VOUT
VIN
30V
Product
AD5292
AN-1207
Application Note
Equation 1 calculates the time constant of the network.
τ = R1 × CIN
end-to-end resistor tolerance. This affects the circuit accuracy
due to the mismatch between the digital potentiometer and the
external resistors. The ±1% resistor tolerance of the AD5292
helps to overcome the mismatch resistance error.
(1)
where CIN is the input capacitance in the PMOS gate (~380 pF
for the Si2307CDS). The time constant of the network is 3.8 µs.
The single-pole bandwidth of this network is approximately
42 kHz. Bandwidth can be increased by decreasing R1, but
power dissipation will increase.
Figure 6 shows the output voltage vs. digital code for the circuits
of Figure 1 (normal mode, 1 LSB = 29 mV) and Figure 2
(reduced output mode, 1 LSB = 4.9 mV).
The AD5292 has 20 times programmable memory, which
enables the user to preset the output voltage to a specific value
at power-up.
Typical integral nonlinearity (INL) and differential nonlinearity
(DNL) plots are shown in Figure 3 and Figure 4 using the
configuration in Figure 1. In this configuration, the AD5292 is
operating ratiometrically, which means that variation in the
total resistor tolerance does not affect the performance.
Excellent layout, grounding, and decoupling techniques must be
utilized in order to achieve the desired performance from the
circuits discussed in this note (see Tutorial MT-031 and
Tutorial MT-101). As a minimum, a 4-layer PCB should be
used with one ground plane layer, one power plane layer, and
two signal layers.
To improve the circuit accuracy, the voltage reference across the
AD5292 can be reduced by using an external resistor as shown
in Figure 5. This gives the full 10 bits of resolution over a
limited voltage range. Most digital potentiometers have a ±20%
COMMON VARIATIONS
The AD5291 (8 bits with 20-times programmable power-up
memory) and the AD5293 (10 bits, no power-up memory) are
both ±1% tolerance digital potentiometers that are suitable for
this application.
2
1
INL (LSB)
VIN
30V
AD5292
SERIAL
INTERFACE
0
0
500
1000
CODE (DEC)
VOUT
08454-005
V’
25V
R2
100kΩ ± 1%
08454-003
–1
20kΩ
Figure 5. Increased Accuracy Over a Reduced Output Range (Simplified
Schematic: Decoupling and All Connections Not Shown)
Figure 3. INL vs. Decimal Code
0.5
30
VOUT RANGE = 25V TO 30V
1LSB = 4.9mV
VOUT (V)
0.1
20
–0.1
10
VOUT RANGE = 0V TO 30V
1LSB = 29mV
–0.5
0
500
CODE (DEC)
0
SHUTDOWN 0
1000
08454-006
–0.3
08454-004
DNL (LSB)
0.3
256
512
768
1023
CODE (DEC)
Figure 6. Output Voltage vs. Decimal Code for Circuits of Figure 1 and Figure 2
Figure 4. DNL vs. Decimal Code
Rev. B | Page 2 of 3
Application Note
AN-1207
LEARN MORE
REVISION HISTORY
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND", Analog Devices.
4/13—Rev. A to Rev. B
Changed Document Title from CN-0015 to
AN-1207 .............................................................................. Universal
MT-032 Tutorial, Ideal Voltage Feedback (VFB) Op Amp, Analog
Devices.
MT-061 Tutorial, Instrumentation Amplifier Basics, Analog
Devices.
MT-087 Tutorial, Voltage References, Analog Devices.
3/10—Rev. 0 to Rev. A
Changes to Circuit Function and Benefits Section....................... 1
9/09—Revision 0: Initial Version
MT-091 Tutorial, Digital Potentiometers, Analog Devices.
MT-095 Tutorial, EMI, RFI, and Shielding Concepts, Analog
Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Data Sheets
AD5292 Data Sheet
AD5291 Data Sheet
AD5293 Data Sheet
OP184 Data Sheet
©2009–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08454-0-4/13(B)
Rev. B | Page 3 of 3
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