Application Note

VISHAY SILICONIX
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Power IC
Application Note
Creating a Negative Output Voltage Using a Buck Converter
By Owain Bryant
Vishay constant on-time (COT) converters combine high-efficiency regulation with extremely small transient response time and
simple designs. The COT converters can also be configured in a buck-boost topology, allowing for a negative output voltage.
This application note looks at the SiP12116 configured as a negative output buck converter.
INTRODUCTION
The buck topology is conventionally used to convert a larger bus or system voltage into a smaller voltage. The advantage of
using a buck converter is that efficiency is very high when compared to a linear regulator performing the same conversion.
In order to generate a negative output voltage from a positive input voltage, the designer would usually opt for the buck-boost
topology or possibly a SEPIC converter, both of which offer reasonable efficiency that is much higher than a linear regulator.
However, the same outcome can be reached with a buck converter. With a slight alteration to the nodal references of a
synchronous buck converter, we can create a negative boost converter, as shown in Fig. 1.
Vin
Vin
D
D
Vout
Control
Control
(1-D)
(1-D)
-Vout
Synchronous Buck
Negative Output Buck
Fig. 1
This will suit applications that need to generate complimentary output voltages, such as audio, or industrial applications
requiring negative voltage levels, such as IGBT gate drive turn-off. Other uses have been observed in LCD displays and
embedded applications, where some application-specific ICs require a negative supply. This circuit offers the advantages of
the positive output buck converter in the sparsely supported negative output switching regulator application.
The circuitry is built around the SiP12116 synchronous buck converter, which has a fixed frequency of 600 kHz and offers a
simple design with outstanding efficiency. The SiP12116 comes in a DFN 3 x 3 package, which offers the designer a compact
footprint. The use of COT topology allows the user to develop a very straightforward power supply with no compensation
requirements. The SiP12116 develops the current ramp feedback from the internal low-side MOSFET so the external
components required are the power LC filter, input capacitive decoupling, and bootstrap capacitor.
The circuit uses the same design equations that can be found in the SiP12116 user guide. In fact, the circuitry uses the same
parts; the input capacitors are rated to 25 V, so these are suitable. Care must be taken to ensure the voltage rating of the part
is followed. For example, if the input voltage of the circuit is 12 V and the output voltage is 5 V, the differential voltage across
the part is 17 V, which does not exceed the recommended 18 V. A Zener diode will also be used to clamp the enable pin to
4.7 V, which should safeguard the part at switch on and off while allowing for easy enable.
Revision: 25-Feb-16
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APPLICATION NOTE
CIRCUITRY
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Creating a Negative Output Voltage Using a Buck Converter
OPERATION OF CIRCUIT
Vin
IM1
D
VgM1
0V
VLX
Control
(1-D)
IM2
VgM2
-Vout
Fig. 2 - Negative Output Buck Topology
The control of the circuit will be identical to that of the standard buck converter; however, there is a key difference in that the
change in nodal connection of the Inductor from Vout to 0 V will cause a change in circuit current flow. This in turn allows the
negative output voltage to be generated; the IC’s 0 V now becomes the negative output voltage.
V(vgatem1)
18 V
VgM1
6V
-6 V
2.0 V
V(vgatem2)
VgM2
-0.8 V
-3.5 V
14 V
4V
V(vlx)
Vlx
-6 V
-3.318 V
V(-v)
-3.357 V
-Vout
-3.395 V
1.0 A
-0.1 A
I(L1)
I Inductor
-1.2 A
1.0 A
Is(M1)
IM1
0.0 A
-1.0 A
1.2 A
Is(M2)
IM2
0.1 A
0.4 μs
0.8 μs
1.2 μs
1.6 μs
2.0 μs
2.4 μs
2.8 μs
3.2 μs
3.6 μs
4.0 μs
4.4 μs
Fig. 3 - Simulation of Nodal Waveforms from Fig. 2

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APPLICATION NOTE
-1.0 A
0.0 μs
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Creating a Negative Output Voltage Using a Buck Converter
The MOSFET drive waveforms can be seen in Fig. 3, which are similar to a standard buck converter. The LX voltage is also
shown. LX waveforms range from -3.3 V to +12 V, and the majority of the magnitude is from -3.3 V to 0 V when the low-side
MOSFET is on. The next trace represents the output voltage -3.3 V.
The inductor current can be seen next, which is centered around 0 A; there is no load in the simulation. The key waveforms
appear next - IM1 and IM2 - which indicate the current flow in the circuit. Note that these waveforms are referenced to 0 V.
Current flows from +V to 0 V through the high-side MOSFET; however, the current is flowing from positive to negative, so it is
decreasing, as can be seen in the IM1 trace. When M1 is switched off and M2 switched on, the current flows from -V to 0 V.
This is seen in the increasing current, while MOSFET M2 shows a decreasing current due to the reference point of 0 V.
In order to determine the duty cycle, the similarity with the buck converter is maintained. However, the voltage across the
inductor will now be Vin + |Vout|,
V out
D = ----------------------------V in + V out
The remaining calculations are similar to a standard buck converter.
DESIGN CALCULATIONS
The overall design specifications for the circuit are as follows:
Vin = 12 V, Vout = -3.3 V, fsw = 600 kHz, Iout = 3 A, Vripple = 150 mV, and Vin_ripple = 100 mV.
The SiP12116 senses the current across the low-side MOSFET, so this signal needs to be reasonably large in order to stand
out from any system noise that may be present. The method for this is to use a large ripple current, set to 40 % of the load
current. This will also allow the user to downscale the size of the inductor. It is worth noting at this point that the calculations
for the controller are relatively straightforward as the system runs with a COT topology, while also controlling current internally,
derived through the low-side MOSFET, leaving few external parts that need design calculations.
TABLE 1 - DESIGN CALCULATIONS
Duty cycle
V out
D = ----------------------------V in + V out
0.22
Inductance
V in x D
L = --------------------------- f sw x I L 
3.3 μH
 I LOAD x D 
C out = --------------------------------f sw x  V out
10 μF
Output capacitance
I RMS = I LOAD x
D x 1 - D
D
C IN = I RMS x ------------------------------- V in x f sw 
1.24 A
10 μF
The calculations can be made and carried through as seen in Table 1. Note that some of the values have been translated to the
available manufacturing values.
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APPLICATION NOTE
Input capacitance
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Creating a Negative Output Voltage Using a Buck Converter
CIRCUIT SCHEMATIC
3
R6
P1
C1
2
1
Terminal
C2
C3
NIC 10 μ 100 n
10K
U1
BOOT 8 BS
VIN
SiP12116
EN 10
EN
D1
9
BZT52C4V7 PGD
5V 2 V
CC
C4
1μ
4
3 GND
P1 GND
GND
VIN
LX
7 LX
CS
100 n L1
LX 6
VFB
R3
1 VFB
R4
1K
0V
C6 C7 C8
100 n 22 μ NIC
P2
2
1
Terminal
-V
0V
Fig. 4 - Schematic
The schematic represents the changes to the nodal reference with Vout becoming 0 V and 0 V becoming Vout. One must ensure
there is decoupling across the input to 0 V, and some decoupling across the input to -Vout. The PCB design and BOM can be
found in the appendix.
TYPICAL WAVEFORMS
Transient Response
Voltage Ripple
Fig. 6 - Green = ILoad 2 A / div,
Purple = Vout Ripple 10 mV / div
The transient response has a recovery spike of 300 mV; this
is reasonable considering the 22 μF of holdup capacitance
at the output. The current step is well controlled.
The voltage ripple is very well contained; no more than
80 mV can be observed for an output capacitance of 22 μF.
This is helped by the high switching frequency.
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APPLICATION NOTE
Fig. 5 - Green = Load Current,
Purple = Vout Ripple Voltage, +240 mV, -80 mV
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Creating a Negative Output Voltage Using a Buck Converter
Temperature Performance
Voltage Rise on Start-Up
Fig. 7 - Green = ILoad 2 A / div,
Purple = Vout 2 V / div
Fig. 8 - Temperature Measurement
It can be seen in Fig. 7 that the voltage rise time is monotonic
into a 3 A load. This is well controlled.
Note: the part is running at 3.3 V, 3 A continuously.
The ambient temperature is 24 °C.
This gives the user a temperature rise of 55.5 °C, which
works out as 5.6 °C/ or 18.5 °C/A.
Efficiency and Power Loss
Axis Title
100
2.5
5V
2nd line
Efficiency (%)
80
2.0
70
12 V
60
1.5
50
40
1.0
30
12 VIN losses
20
Power Loss (W)
2nd line
90
0.5
10
5 VIN losses
0
0
0
0.5
1.0
1.5
2.0
2.5
3.0
Fig. 9 - Efficiency Measurements
Test Conditions: VIN = 12 V, 5 V, VOUT = -3.3 V, fsw = 600 kHz, L = 3.3 μH
CONCLUSION
The SiP12116 offers an ideal way of creating a high-performance negative voltage output from a positive supply. If the designer
follows the rules, a maximum input voltage of 12 V can supply a 5 V output, or in this application a 3.3 V output. Peak efficiency
is just over 90 % and the temperature rise for the evaluation PCB at ambient is 55 °C.
A reference design including schematics, layout, and complete BOM for a negative buck regulator using the SiP12116 is
available by request by submitting a product support request here: www.vishay.com/ppg?62969.
References:
1. SiP12116 datasheet: www.vishay.com/doc?62969
Revision: 25-Feb-16
Document Number: 76946
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ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
Load Current (A)
2nd line
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Creating a Negative Output Voltage Using a Buck Converter
APPENDIX A
TABLE 2 - BILL OF MATERIALS FOR SiP12116 NEGATIVE OUTPUT VOLTAGE
(Vin = 12 V, Vout = 3.3 V, fsw = 600 kHz)
ITEM
QUANTITY
REFERENCE
PCB FOOTPRINT
VALUE
VOLTAGE
PART NUMBER
1
2
C1, C2
1210
10μ
35 V
C1210C106M6PACTU
MANUFACTURER
KEMET
2
2
C3, C6
0402
10n
50 V
GRM155R71H103KA88D
MURATA
3
1
C4
0603
1μ
10 V
C0402C105M8PACTU
KEMET
4
1
C5
0402
100n
50 V
CGA2B3X7R1V104K050BB
VISHAY
5
1
C7
0805
22μ
10 V
CL21A226MPQNNNE
SAMSUNG
6
1
R3
0402
4K53
-
CRCW04024K53FKED
VISHAY
7
1
R4
0402
1K
-
CRCW04021K00FKED
VISHAY
8
1
L1
IHLP2525
3μ3
-
IHLP2020BZER3R3M01
VISHAY
VISHAY
9
1
U1
DFN10-3x3
-
-
SiP12116
10
1
D1
SOD-123
4V7
-
BZT52C4V7
-
11
2
P1, P2
TERM2
-
-
282834-2
TE CONNECTIVITY
PCB LAYOUT
Mid Layer 1
Mid Layer 2
Bottom Layer
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APPLICATION NOTE
Revision: 25-Feb-16
Top Layer