### NCP3170, NCP3020: Positive Input to Negative Output Conversion Buck-Boost (+12 Vin to -5 Vout)

```DN05004/D
Design Note – DN05004/D
Positive Input to Negative Output Conversion using a
Buck Converter
Device
Application
Input Voltage
Output Voltage
Topology
I/O Isolation
NCP3170
NCP3020
Power supplies
Positive
Negative
Buck
None
Input Voltage
Output Voltage
Nominal Current
Nominal Efficiency
+5, 8, 12 Vdc
-3.3, -5 Vdc
0.5 A or 8 A
80 - 90%
Introduction
The buck-boost is usually the topology of choice for voltage inverting applications. There are plenty of
components to choose from and most of them even come with integrated transistors or mosfets. They
can work in hysteretic mode or they can switch at a fixed frequency. Very often when more power is
required, an external transistor is used and while the circuit can deliver the needed power, the efficiency
is very poor.
This Design Note describes an alternate solution that is very simple, but little known. It provides superior
efficiency while using commonly available components.
Circuit Description
The circuit is built around a buck converter. Depending on output power, a regulator with integrated fets
or a controller with external fets can be used.
The circuit in Figure 1 uses the NCP3170A to obtain a -5 V rail from a +8 V input at up to 0.5A output
current. Efficiency reaches 85%
The circuit in Figure 2 converts a +12 V input into -3.3 V or -5V at up to 8A. 90% efficiency is achievable.
There are many more options available for buck controllers and converters as well as for mosfets. The
user needs to pay attention to the voltage rating of the part. In this application the controller and the
mosfets will need to withstand a voltage equal to input voltage plus output voltage. For example the
circuit in figure 1 needs to be able to handle 8 + 5 = 13 V.
May 2011, rev. 0
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Figure 1
Figure 2
May 2011, rev.0
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Efficiency Curves
NCP3170A Efficiency
90
85
80
Eff (%)
75
70
-5V output
65
60
55
50
0.0
0.1
0.2
0.3
0.4
0.5
NCP3020A Efficiency
92
90
Eff (%)
88
-5V output
86
-3.3V output
84
82
80
0
1
2
3
4
5
6
7
8
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Key Features



Simple and familiar topology
High efficiency
Many options available for component selection
References
 Data sheet NCP3170: 3A Switching regulator
 Data sheet NCP3020: 28V Synchronous Buck Controller
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