Electronic equipment uses predominantly positive voltage rails for power; occasionally, some negative voltage rails are also used. For this reason, negative (or inverting) output DC-DC converter solutions are not as common as their positive output DC-DC counterparts. Nevertheless, when powering high performance devices in factory automation, building automation, and communications systems such as high-speed DACs, op amps, RF power amplifiers, AFEs, GaN FET gate drivers, and IGBT gate drivers, a negative voltage rail is needed.
Designers face a big challenge looking for a negative voltage solution as most legacy devices require external level shifter circuits with which to communicate. They are also outdated, inefficient, complex, and bulky. This article discusses in detail the drawbacks of legacy solutions, and then investigates a new breed of highly-integrated devices that addresses the deficiency, and offers a compact, easy-to-use, and highly-efficient negative output DC-DC solution.
Negative output DC-DC converter challenges
A typical power system has its lowest voltage potential as ground reference, or GND. For a positive output DC-DC output converter, the ground reference is simply the GND (0-V potential). Its input/output signals are naturally referenced to this ground. The system controller communicates with the DC-DC converter simply and directly using I/O pins.
Figure 1 This simplified system schematic uses solely positive voltage rails. Source: Maxim Integrated
Figure 1 illustrates such a system where the system microcontroller (MCU) drives the EN (enable) pin of the converter to turn it on and off. The controller also reads the status of the converter through its PGOOD (i.e., RESET) pin to know whether the converter power output is within its regulation and is ready for powering up the whole system. For simplicity, only one DC-DC converter is shown here, but the principle also applies to a system with multiple positive voltage rails.
When a negative DC-DC is used, communication to the system controller is not trivial. The converter has its I/O’s pin referenced to its lowest voltage potential, which in this case, is the negative output voltage, not the system ground (GND). When using negative voltage rails, designers need to implement level shifter circuits for the system MCU to communicate with the DC-DC converter. Figure 2 illustrates a simplified schematic of a system with two level shifters.
Figure 2 This simplified system schematic uses negative voltage rails. Source: Maxim Integrated
Again, for simplicity, only one negative output DC-DC converter is shown here, but the principle applies to systems with multiple negative voltage rails or with a mix of both positive and negative voltage rails. One level shifter is needed per I/O pin of each negative output DC-DC converter.
The level shifter circuit is large, creating a challenge for the designers. In addition, legacy negative DC-DC converter solutions are complex and inefficient, imposing yet another challenge.
Challenge 1: Level shifter
Figure 3 illustrates a typical level shifter circuit. Its purpose is to shift the ground reference of a signal to match that of the system MCU. It’s used here to translate the ON command from the system MCU to turn on/off the DC-DC converter. This level shifter consists of 9 components. Its operation is straight forward: when ON is driven high by the system controller, Q1 turns on, which in turn biases Q2 on and drives EN high to enable the DC-DC converter. When ON is driven low, both Q1 and Q2 are off, and EN is driven low to disable the converter.
Figure 3 The typical level shifter circuit translates the ON command from the system controller. Source: Maxim Integrated
Figure 4 describes a common level shifter circuit variation. It’s used here to translate the PGOOD signal from the DC-DC converter, so that the system micro-converter can read it. When PGOOD is driven high (open drained) by the DC-DC converter, Q3 turns on, which in turn, biases on Q4 and drives RESET high, taking the system MCU out of reset.
Figure 4 A level shifter translates the PGOOD signal from the DC-DC converter. Source: Maxim Integrated
These two level shifters require 18 external components, imposing a challenge to the designers trying to fit the solution into the ever-shrinking equipment and board space.
Challenge 2: Inefficiency
The legacy negative output DC-DC solution is inefficient. The extra heat generated due to inefficiency creates another challenge for designers, who now have the extra burden of removing that heat from the system. Figure 5 is a simplified circuit schematic of such a system.
Figure 5 This is a simplified schematic of a nonsynchronous, dual-inductor inverting output DC-DC converter. Source: Maxim Integrated
This topology faces two inefficiency issues. First, it employs nonsynchronous switching where the output rectifying diode, D1, dissipates more power compared to a synchronous solution. Second, it has an extra power inductor, L1, and an extra capacitor, C1, which also dissipate more power. Figure 6 shows the efficiency curve for this converter, measured at 12V input and -15V output. Its peak efficiency is only 83% while dissipating approximately 460 mW at 150 mA output current.
Figure 6 The power loss curve shows the efficiency of a nonsynchronous, dual-inductor inverting output DC-DC converter. Source: Maxim Integrated
A smaller, more efficient negative output DC-DC solution
The MAX17577 and MAX17578 synchronous inverting DC-DC step-down converters were developed to meet growing requirements for smaller and lower-heat generating devices in factory automation, building automation, and communications systems. The devices integrate level shifting circuitry to reduce component cost and count, and employ synchronous rectification for higher efficiency. Figure 7 shows their typical application circuit.
These DC-DC converters have a wide input voltage range. The devices operate from 4.5 to 60V input and can deliver up to 300 mA output current. With integrated level shifters, these devices conserve up to 72% of board space by reducing component count by half while using 35% less energy than the closest legacy solution.
Figure 8 The MAX17577 features 88.5% efficiency at -15V output. Source: Maxim Integrated
Figure 8 shows MAX17577 peak efficiency at 88.5%, measured at 16V input and -15V/150-mA output. This is 5.5 percentage points higher efficiency compared to the legacy solution shown in Figure 6. Why is efficiency important? At 88.5% efficiency, the device dissipates only 292 mW while delivering 2.25 W power to the load. And 292 mW means 37% less heat for the system to cool, compared to the 460-mW amount of the legacy solution shown earlier.
Figure 9 shows an improved version of Figure 2, eliminating the level shifters. The system MCU can communicate directly with MAX17579/MAX17580 even though they have different grounding references.
Figure 9 The figure shows the MAX17579/MAX17580 in a system using negative voltage rails. Source: Maxim Integrated
It is also worth noting that with a wide operating voltage range, these new solutions can withstand and tolerate system voltage fluctuations such as power surge event, back EMF, and cable voltage ringing, and thus increase system reliability. In addition, there are the MAX17577 and MAX17578, which are in the same family with similar performance, but can deliver up to 1 A of output current. These devices are highly suitable for powering RF power amplifiers, GaN FET gate drivers, and IGBT gate drivers.
A new breed of highly-integrated devices
Growing requirements for smaller solution size and lower heat generation in devices in factory automation, building automation, and communications systems impose big challenges for designers looking for negative voltage DC-DC converters, where most legacy solutions are outdated, inefficient, complex, and bulky.
A new breed of highly-integrated devices with on-board level shifters, synchronous rectification, and wide operating input voltage brings to the table the most compact, highly-efficient, and robust negative output DC-DC solutions.
Thong “Anthony” Huynh is principal member of the technical staff of the industrial & healthcare business unit of Maxim Integrated.