Practical implementation of negative-input, negative-output step-down switching converters

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.

Negative Buck Topology
Figure 1 shows the basic architecture of a negative buck switching converter.

Figure 1: Negative buck topology
(Click to enlarge image)

(Note: In the figure, while it is common to draw positive rails at the bottom of circuits in schematic diagrams and, based on it, negative buck topologies show ground (most positive rail) on the top in many cases. Here, ground was drawn at the bottom with the circuit's V_in and V_out at the top on purpose, to reflect the simplicity and similarity of this topology compare to the positive buck.)

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 NFET used as a high-side pass device requires a gate-drive voltage higher (more positive) than the systems input voltage (V_in) 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 have usually 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 (-V_in). In this case, since this input voltage is the most negative voltage in the system, though 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 PWM 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, particularly when larger output currents are needed, as well as when there's the need to optimize for efficiency and thermal dissipation, than can be achieved by choosing a FET with optimal R_dson, packaging, turn-on and turn-off times, and other factors for the requirements.

Figure 2 shows a simplified diagram of an LM5001 boost/flyback regulator in a negative buck topology.

Figure 2: Negative buck regulator implementation using a wide input-voltage range, 1 A monolithic boost-regulator IC
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It is a 3.1 to 75 volt input-voltage-range device from National Semiconductor that has a built-in 75 V, 1 A NFET. In a regular boost application, the LM5001 will put out a gate-drive voltage to its built-in pass NMOSFET 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 (-V_in) and will yield the results needed.

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 1 A boost IC to be used for output currents up to 1 A. 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.

Voltage Translators
The other special consideration in a negative buck architecture using an off-the-shelf boost IC is the signal conditioning needed for the feedback path. Most ICs require a voltage around 1.25 V (relative to their ground) at their feedback (FB) pin to maintain regulation. This voltage is usually obtained from the output (V_out) and simply scaled down through a voltage-divider resistor network. This technique easily allows the voltage applied to the FB pin to go up as the output goes up and to go down as the output goes down, which is needed to maintain proper regulation.

When this approach is taken in a positive buck, both the FB voltage and the output voltage are naturally referenced to the system ground and the IC ground pin, so no conditioning or translation is needed. In a negative-buck application implemented through the use of a low-side FET boost IC, the output (-V_out) and any voltage-divided sample of it are still referenced to the system ground. However, since the IC ground pin is connected to -V_in and not to the system ground, the IC will not read the FB voltage correctly (nor will keep regulation correctly) and thus this voltage needs to be translated so it is referenced to the IC ground pin.

This voltage translation is represented through the small box in Figure 1 and Figure 2 labeled "Level shift". There are multiple ways of getting this implemented in hardware.

Figure 3 shows one of the simplest, most common, and possibly less expensive ways.

Figure 3: Voltage translator circuit using a matched PNP-transistor current mirror
(Click to enlarge image)

It uses a current mirror built with a couple of inexpensive PNP transistors. For best performance and tighter regulation accuracy, a matched pair is recommended. Matched pairs can be found in single packages; a good example is the DMMT3906 from Diodes Inc. In Figure 3, R_f1 and R_f2 scale down the mirrored voltage and are thus used to set the regulator's output voltage (just like the case of any adjustable regulator). In other words, feedback gain is |_Vref/v_out| and |_vout|is V_ref x R_f1/R_f2, where V_ref is the IC feedback pin (reference) voltage.

A variation of the current-mirror circuit is shown in Figure 4.

Figure 4: Voltage translator circuit using a single PNP transistor and discrete-diode current mirror
(Click to enlarge image)

In this circuit, a single PNP transistor is used. D₁ provides output voltage temperature compensation by canceling out the effects of the temperature drift of Q₁'s PNP emitter-base voltage. D₂ & D₃ provide some pre-regulation for the biasing current needed for D₁, thus improving both line regulation and ripple rejection by a factor of two. Additional performance improvement is possible by replacing the two series-connected diodes with a voltage reference such as National's LM385-1.2 or LM4040-2.5.

To simplify the circuit, or if the input voltage is relatively constant and has very little ripple on it, D₂ and D₃ could be eliminated and the biasing resistors combined. Also, eliminating D₁ will provide a negative temperature coefficient of the output voltage.

Figure 5 shows an alternative for the voltage translating circuit using an op amp, for designers who prefer the benefits and simplicity of operational amplifiers compared to designing with discrete components.

Figure 5: Voltage translator circuit using an op amp in a differential input-amplifier configuration
(Click to enlarge image)

By connecting the op amp in a very similar configuration to the one used when sensing and amplifying differential voltages, it can be used in the negative buck configuration to jointly scale-down the output voltage. This makes it suitable for the FB pin (thus setting the regulator's output voltage) and, at the same time, shifts the reference for this voltage from the system ground to the -V_in rail.

The specific op amp used depends on application requirements but, in general, a general-purpose op amp may be adequate. Low offset voltage is important for voltage accuracy of the regulator and. Obviously. the op amp needs to have a common mode voltage range (CMVR) greater than the application's V_out magnitude.

Conclusion
A variety of boost/flyback regulators can be used for implementing negative buck converters. Regulators and controllers with wide input-voltage range were used as examples, due to their flexibility in a broad range of applications. Even though boost ICs are the most readily available, off-the-shelf solution to implement negative buck converters, it is important to reiterate we are in fact not boosting a negative voltage, Instead, we are bucking it, so all the design parameters and criteria for selecting external components (inductor, MOSFET, compensation, etc.) need to be those for a buck, not a boost design. The switch current is the converter's output current, like a buck. Inductor value should also be chosen using ripple current, like a buck. Like a regular positive buck, the topology doesn't have a right-half-plane (rhp) zero either. Actually, if a voltage translation circuit is used, like the ones in Figures 3 or 4, it's evident that compensation can get extremely flexible, since adding a pole or a zero is as easy as just adding a cap in parallel with either R_f1 (zero) or R_f2 (pole).

References
1 National Semiconductor Corp., especially LM5000, LM5001, LM5002, LM5020, LM5021, LM324, LM4040, LM385 Datasheets; LM2577 application briefs.
2. Diodes, Inc., DMMT3906 Datasheet

About the author
Hector F. Arroyo holds a BSEE degree, with a major in Electronics and Communications, from the Monterrey Institute of Technology in Mexico City. In 1993, he received a Presidential Award in Mexico for winning first place in the National Science Olympics. His major interests include analog design, particularly power management and audio, and 8-bit microcontroller hardware development, although he also enjoys antique radio restoration. Hector joined National in 1997 and since then has occupied various positions in the applications and technical marketing and sales areas. He currently handles technical marketing for National's Power Management product groups in the Americas.