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Texas Instruments Negative Input to Positive Output made SIMPLE Application notes
Application Report
SNVA740 – July 2015
Negative Input to Positive Output made SIMPLE
Akshay Mehta, Frank De Stasi
ABSTRACT
There can be quite a few applications that require a conversion from a negative input voltage to a positive
output voltage. This design space along with being limited is not well explored. There are a few ways to go
about doing it. In this application note we will go over the use of an integrated boost regulator to convert a
negative input voltage to a positive output voltage.
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2
3
4
5
Contents
Introduction ...................................................................................................................
Application Details ...........................................................................................................
Test Results ..................................................................................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
1
2
4
8
8
List of Figures
1
-12VIN 5VOUT 1A IOUT .......................................................................................................... 2
2
Design Schematic
3
Line Regulation at IOUT = 1A ................................................................................................ 4
4
Efficiency Vs. IOUT ............................................................................................................ 5
5
Load Regulation.............................................................................................................. 5
6
Maximum Output Current Vs. Input Voltage
7
8
9
10
...........................................................................................................
.............................................................................
Load Transient VIN = -12V, VOUT = 5V, IOUT = 200mA to 2A .............................................................
Startup VIN = -12V, VOUT = 5V, IOUT = 1A...................................................................................
Top Layer .....................................................................................................................
Bottom Layer (Flipped) ......................................................................................................
2
6
6
7
7
8
List of Tables
1
1
Design BOM .................................................................................................................. 8
Introduction
The LM2587 is part of the LM258x family of SIMPLE SWITCHER® boost regulators from Texas
Instruments. The internal NPN is capable of handling a voltage of 65V and has a current limit of 6.5A. The
maximum input voltage that the device can handle is 40V. Thus this device makes a good candidate for
Wide VIN solutions. The design shown here is created for a typical input of -12V and an output of +5V at
2A load current, with a common ground between input and output. But it can handle an input voltage
range of -6V to -40V. The following sections will talk about the operation. The intent of this application
note is to investigate the method used to level shift the output voltage without going into details about
BOM calculations. For details of calculating the BOM for a buck-boost topology, see application note
Understanding Buck-Boost Power Stages in Switchmode Power Supplies.
SIMPLE SWITCHER is a registered trademark of Texas Instruments.
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1
Application Details
2
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Application Details
The basic operation of this circuit is that of a buck-boost topology. When the NPN is on, the current
through the inductor ramps up and when the NPN switches off the inductor current now flows towards the
load and the output capacitors. The voltage across the NPN is –VIN and +VOUT. Therefore the device
chosen has to be able to sustain a total voltage of VIN+VOUT across it. Throughout this application note VIN
denotes the absolute value of the input voltage. Figure 1 shows the steady state waveform. Figure 2
shows the design schematic.
5µs/div
VSW
20V/div
Vout
200mV/div
Iind
1A/div
Figure 1. -12VIN 5VOUT 1A IOUT
Figure 2. Design Schematic
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Application Details
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In this design, the ground of the IC is referenced to the negative input voltage. To obtain a positive output,
a level shifter is implemented using two PNP transistors. The advantage of using two transistors in this
design is that the base-emitter diode voltage can be mostly nullified and the output can be more accurate.
The base of the two transistors is pulled low to turn the transistors on. Since the emitter of Q1 is
connected to ground, the base of both the transistors will be one diode drop below ground, i.e. –VBE.
Applying KCL at the FB node, we get:
VOUT VBE1 VBE2
I1
RFBT
I2
VREF
RFBB
(1)
The reference voltage VREF for the LM2587 is 1.2V. Setting RFBB to be 1kΩ and equating I1 and I2, we can
obtain the value for RFBT. When two transistors are matched as closely as possible, their VBEs will be alike
and cancel out. That way the output voltage can be made accurate. It is even better to find a package with
two transistors in it. That way due to any change in temperature the two VBEs will be still close. While this
method gives more accurate output voltage it is still not completely robust. It is known that the VBE of the
transistor will depend on the collector current (IC) as shown:
VBE
§I ·
VT ˜ ln¨¨ C ¸¸
© IS ¹
(2)
With change in the collector current the VBE will change accordingly. The bottom feedback resistor RFBB is
set to 1kΩ which sets a current of 1.2mA in the collector of the transistor Q2. Resistor RB sets the collector
current of transistor Q1. Its value is set to give a matching current at max input voltage. Therefore:
VIN
RB #
0.0012
(3)
At 36VIN, RB is chosen to be 30kΩ. Now when the input voltage is reduced, because of the fixed resistor
the collector current of Q1 reduces linearly. If Q2 collector current is twice as much as Q1 collector current
then we can say the following:
§ 2 ˜ IC IS ·
˜ ¸¸ VT ˜ ln 2 18mV
VBE1 VBE 2 'VBE VT ˜ ln¨¨
© IS IC ¹
(4)
At room temperature VT is 26mV. This means that ∆VBE changes 18mV every time the collector current of
Q1 is halved. This can be seen in Figure 3 which shows the line regulation.
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Test Results
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3 Test Results
The following scope plots and efficiency data were taken on the custom PCB. The layout is shown in
Figure 9 and Figure 10 and the BOM used is shown in Table 1.
5.2
5.18
5.16
Output Voltage (V)
5.14
5.12
5.1
IOUT = 0.1A
IOUT = 1A
5.08
5.06
5.04
5.02
5
0
5
10
15
20
25
30
35
40
45
Input Voltage (V)
Figure 3. Line Regulation at IOUT = 1A
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Test Results
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90
80
70
Efficiency (%)
60
50
VIN = 5V
40
VIN = 24V
VIN = 40V
30
20
10
0
0
0.5
1
1.5
2
2.5
3
3.5
Output Current (A)
Figure 4. Efficiency Vs. IOUT
5.2
5.18
5.16
Output Voltage (V)
5.14
5.12
5.1
VIN = 5V
VIN = 25V
5.08
VIN = 40V
5.06
5.04
5.02
5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Output Current (A)
Figure 5. Load Regulation
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Test Results
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6
Maximum Output Current (A)
5
4
3
2
1
0
0
5
10
15
20
25
30
35
40
45
Input Voltage (V)
Figure 6. Maximum Output Current Vs. Input Voltage
500µs/div
Vout
500mV/div
2A
IOUT
1A/div
200mA
Figure 7. Load Transient VIN = -12V, VOUT = 5V, IOUT = 200mA to 2A
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Test Results
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VIN
20V/div
10ms/div
Vout
2V/div
IIND
1A/div
Figure 8. Startup VIN = -12V, VOUT = 5V, IOUT = 1A
Figure 9. Top Layer
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Conclusion
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Figure 10. Bottom Layer (Flipped)
Table 1. Design BOM
4
DESIGNATOR
DESCRIPTION
PART NUMBER
Cc
CAP, CERM, 0.1 µF, 25 V, +/- 10%, X7R, 0805
08053C104KAT2A
Cin1
CAP, AL, 10 µF, 63 V, +/- 20%, ohm, SMD
EMVA630ADA100MF55G
Cin2
CAP, CERM, 1 µF, 100 V, +/- 20%, X7R, 1206
C3216X7R2A105M160AA
Cout1, Cout2
CAP, AL, 220 µF, 6.3 V, +/- 20%, 0.018 ohm, SMD
APXC6R3ARA221MH70G
Cout3
CAP, CERM, 1 µF, 16 V, +/- 10%, X5R, 0805
0805YD105KAT2A
D1
Diode, Schottky, 100 V, 2 A, PowerDI123
DFLS2100-7
L1
Inductor, Shielded Drum Core, Ferrite, 22 µH, 4.1 A, 0.033 ohm,
SMD
744770122
Q1, Q2
Transistor, PNP, 80 V, 0.5 A, SOT-23
MMBTA56LT1G
Rb
RES, 30.1 k, 1%, 0.125 W, 0805
CRCW080530K1FKEA
Rc
RES, 1.33 k, 1%, 0.125 W, 0805
CRCW08051K33FKEA
Rfbb
RES, 1.00 k, 1%, 0.125 W, 0805
CRCW08051K00FKEA
Rfbt
RES, 4.07 k, 0.1%, 0.125 W, 0805
RT0805BRD074K07L
Conclusion
Thus we see that just by adding a couple of external components, a SIMPLE SWITCHER® boost regulator
like the LM2587 could be used to create a positive output from a negative input using the buck-boost
topology. The showcased design has good line regulation and load transient response.
5
References
1. Understanding Buck-Boost Power Stages in Switch Mode Power Supplies
2. Converting Negative Input Voltages To Positive Output Voltages
8
Negative Input to Positive Output made SIMPLE
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