High-efficiency step-down switching regulators for
positive voltages are very common. However, negative step-down
switching regulators (negative voltage in, negative voltage out, common
ground) are not as well known, even though they are often needed.
Although they are not difficult to set up, literature on how to build
them is rather scarce.
This article analyzes the architecture and detailed operation of the
negative buck topology. It will also discuss actual circuit
implementations for the topology, from a system perspective down to the
building of the needed circuit blocks, and include examples on how to
build a voltage translator circuit, a key block in implementing a
negative buck regulator using readily available boost ICs.
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| Figure
1: Shown is the basic architecture of a negative buck topology. |
Negative buck topology
Figure 1 above shows the basic
architecture of a negative buckswitching converter. Like a positive
buck design, it has a high-side pass device between input and output,
an LC output filter, and a catch diode. The two big differences are the
gate drive needed in the control IC and the feedback circuitry.
In a positive buck, a typical negative-channel FET (NFET) used
as a high-side pass device requires a gate-drive voltage higher (more
positive) than the systems input voltage (Vin) in order to be turned
on. Since the input voltage is the most positive voltage in the system
already, special circuitry is needed to generate an even higher
voltage.
Positive buck ICs usually have this function built in. In a negative
buck, an NFET used as a high-side pass device also requires a
gate-drive voltage more positive than the system's input (-Vin). In
this case, since this input voltage is the most negative voltage in the
system, no special circuitry is needed.
All other voltages, including the output, are "higher" (more
positive), with the converter ground being the most positive voltage in
the system. Under these circumstances, a low-side FET pulse-width modulator control
IC (such as a boost/flyback regulator or controller) can be used to
implement the converter.
A variety of ICs may be used to implement negative buck converters,
including controllers and integrated monolithic regulators with low
side NFETs. Monolithic ICs provide simplicity, ease of implementation
and lower component count. Controllers offer greater flexibility when
larger output currents are needed, and when there's a need to optimize
for efficiency and thermal dissipation.
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| Figure
2: This negative buck topology uses a monolithic LM5001 boost/flyback
regulator. |
Figure 2 above shows a
simplified diagram of a 3.1-75V input- voltage-range boost/ flyback
regulator in a negative buck topology with a built-in
75V, 1A NFET.
In a regular boost application, it will put out a gate-drive voltage
to its built-in pass N-channel MOSFET a few volts above ground in order
to turn it on. In a negative buck application, the gate drive will
still put out a gate voltage a few volts above the IC's ground pin,
which in this case is tied to the system's input voltage (-Vin) and
will yield the needed results.
Different from a regular boost, but the same as a regular buck, peak
IC-switch current in Figure 2
is the same as peak inductor/output current, thus allowing a 1A boost
IC to be used for output currents up to 1A. Other regulators with
different ratings would be used for higher or lower switch currents. If
a controller is preferred, it would be used in a similar configuration
to the one in the figure.
