Designers have been looking for ready-to-use, drop-in parts to provide low voltage DC output rails in the moderate current range of 1 amp to 10 amps which meet basic performance objectives, as well as efficiency and regulatory mandates. While vendors now offer many suitable and tiny DC/DC converters/regulators to address this need, it’s not prudent to assume that they are simply “drop-in” parts that can be forgotten about.
Why so? Despite their apparent simplicity, these regulators are still power sources delivering moderate levels of current to loads. As modules, all designers need to do is add a few external, non-critical passives. However, this ease of use can lull the designer into complacency, leading them to ignore the basics that affect all power sources and their rails.
This article will identify and discuss these essential basics. It will then introduce a selection of modular power solutions and show how to apply these core principles to get the most out of each.
Identifying the “gotchas” of power design
First, some good news. The operating efficiency of these devices is relatively high, typically between 80% and 95%, depending on specific model and operating point. While the output currents are modest, designers still need to do basic thermal and dissipation analysis to ensure the devices stay within their temperature rating and do not add excessively to the system cooling burden.
There are five primary areas of concern: 1) IR drop, 2) isolation, 3) output adjustability, 4) switching noise, and 5) low impedance return paths. As a first step, even before choosing a specific DC/DC regulator, a designer should verify that the unregulated DC source can provide sufficient current, while taking into account that there may be other DC/DC units also relying on this source. Also, be sure the dynamic performance of the source is adequate to handle higher current transient load demands, especially as these regulators do not have large bulk output capacitors.
IR drop: A load too far?
Designers often must face varied and conflicting pc board layout requirements for locating components, I/O ports, and possible heat sources. The power regulator can be a challenging device in this regard. Ideally, it would be placed close to the load to minimize IR drop, noise pickup, and the need for larger, space-wasting pc board tracks for the current flow.
IR drop is the easiest to overlook, yet easiest to calculate. Even a few milliohms of resistance between the DC/DC regulator output and its load(s) can result in a drop of ten or more millivolts at the current these units provide. While that may seem like a small amount, it can be significant when the nominal DC rail is just a few volts.
Therefore, the pc board tracks must be sized appropriately, or perhaps mounted on a separate pc board. Thin bus bars should be considered. Bus bars may seem an archaic solution, but they are very effective for two reasons. First, they dramatically reduce IR drop. Second, for a minor additional BOM cost, a dual layer bus bar can be used, resulting in a superior DC current ground return path.
Doing so is as important as the high-side DC rail itself in order to minimize IR drop; establish a better, lower resistance system ground; and minimize parasitics and non-DC impedance in the ground structure which can affect higher frequency performance. Of course, regardless of the physical DC rail and ground, it’s important to have low impedance, small value bypass capacitors located as close to the IC voltage supply pins or leads as possible, also to minimize noise related issues on the supply rail.
In some cases, the IR drop is still unacceptable, so a specialized regulator architecture which includes remote sensing is useful. Here, the regulator has the two conventional terminals for current supply and return, but also has two sense terminals which extend to the load for measurement of actual voltage at that load. The regulator uses this sensed value as feedback to adjust its output voltage to compensate for the voltage drop (Figure 1).
Figure 1: Remote sensing allows the DC source to directly measure the actual rail voltage at the load, and dynamically compensate as needed for any IR drop or other variations. (Image source: Analog Devices)
For example, the LTM4601 µModule® from Linear Technology Corp/Analog Devices, can deliver substantial current of up to 12 amps between 0.6 and 5.0 volts from a 4.5 to 20 volt DC input. At these higher currents, IR loss may compromise system performance and consistent behavior. Using remote sensing, the module can correct for pc board IR drop voltage losses between VOUT and VLOAD, as well as the ground return path. As a result, the LTM4601 guarantees voltage accuracy of ±2.0% or better at the load, despite line, load, and temperature variations.
Note, however, that remote sensing is not a panacea. In effect, it puts a large feedback loop between the source and the load. If you think of a power regulator as a type of power op amp – which it is – then this feedback loop exposes the source to noise and EMI/RFI, which can affect the closed-loop performance. It is even possible for the presence of this loop to result in regulator instability and oscillation. Therefore, remote sensing must be implemented with careful layout consideration.
Another approach to minimizing the effect of IR is to use multiple, smaller regulators placed close to their respective loads rather than one larger unit set in a single, centralized location. This brings a classic tradeoff of tangible “costs” of using two or more smaller, less expensive units versus one larger, more costly one. While that BOM cost difference is quantifiable, the technical impact of choosing one larger device versus multiple smaller ones is harder to assess, requiring analysis as well as judgement and experience.
For example, the LMZM33602 power module from Texas Instruments combines a step-down converter with power MOSFETs, shielded inductor, and passives, all in a relatively small, low-profile package that can deliver between 1 and 18 volts at up to 2 amps (Figure 1). It requires just four or five non-critical external passive components and eliminates the loop compensation and magnetics aspects of regulator design.
Figure 2: Using multiple smaller, lower current regulators like the LMZM33602 from Texas Instruments may increase the BOM, but may also simplify layout and improve performance with respect to IR drop and noise. (Image source: Texas Instruments)
Measuring just 9 mm × 7 mm × 4 mm in a QFN package, the LMZM33602 can easily be placed close to the load component or subcircuit. Doing so minimizes IR drop in two ways.
First, it is close to the load, reducing rail resistance and noise pickup. Second, the output current is only a few amps, also reducing IR drop. As a result, deploying a few of these units may offer more layout flexibility, reduced IR drop, less noise pickup, more distributed thermal dissipation, and other system level benefits compared to using a single, larger 10 amp unit.
Isolation: sometimes optional, often mandatory
The need for galvanic isolation – the absence of any ohmic path between two parts of a circuit – may range from somewhat beneficial to mandatory. It may be useful to eliminate system ground loops as it may be needed to interface with a “floating” (non-grounded) transducer, or mandated to ensure safety between higher voltage circuitry and a user of medical instrumentation. For many designers, such isolation is either unknown, or somewhat mysterious as to its need or virtue.
Regardless of the rationale, the often overlooked reality is that an isolated subcircuit also requires isolated power, generally at relatively low current levels. In the past, this need for isolated power, even at low currents, required a significant real estate footprint, and BOM costs were often disproportionately high as compared to other functions. Going the “build” rather than “buy” route was often not a viable option since an isolated design is not trivial in terms of design or assembly. Further, for many applications, the isolated design and physical implementation would need to be tested and certified to meet industry and regulatory standards, a costly, complex process.
However, the problem can be largely overcome due to the availability of tiny, yet fully compliant and approved isolated DC/DC modules such as the LTM8047 from Analog Devices (Figure 3). Using an isolated flyback topology, it provides isolation of 725 VDC.
Figure 3: Advances in components, topology, and packaging allow the LTM8047 regulator module from Analog Devices to provide galvanic isolation, meeting all relevant regulatory standards for its voltage rating, yet it appears to the user as an otherwise conventional, non-isolated device. (Image source: Analog Devices)
Within its tiny BGA package, measuring 11.25 mm × 9 mm × 4.92 mm, are the switching controller, power switches, and all support components, plus the core element of an isolation transformer (Figure 4). It can provide outputs from 2.5 to 12 volts from a wide input voltage range of 3.1 to 32 volts (always in buck mode). Although the amount of current it can supply is modest – 440 milliamps (mA) at 2.5 volts DC – this is more than adequate to power many isolated subcircuits and transducer front ends.
Figure 4: Due to the laws of physics and associated regulatory mandates, providing isolation requires a gap for physical separation between input and output; the size of the LTM8047 from Analog Devices supports isolation to 750 volts, which is sufficient for many application situations. (Image source: Analog Devices)
Adjustability: useful, but be careful
Rarely do these readily available DC/DC regulators deliver a fixed, preset voltage. Instead, the user can set the voltage using a pair of resistors in a voltage divider configuration. Doing so provides several advantages: the same regulator can be used in many locations, thus simplifying the BOM; the output voltage can be adjusted “up” a few mV to compensate for IR drop (not a recommended practice in many cases, but is often done; and the output voltage can be adjusted upward for desired settings in analog circuits, especially RF, where there is a specified performance versus dissipation tradeoff (higher voltage gives better SNR and wider bandwidth, but at the cost of increased dissipation).
However, users need to recognize that the stability and temperature coefficient (tempco) of the voltage setting resistors, along with the thermal operating environment, must be factored into any calculation of the nominal DC output voltage of the regulator. It is possible that at higher temperatures, the DC rail will drift out of specification for the load. As a result, it may be prudent or necessary to select voltage setting resistors with low tempco for this function, rather than general purpose devices that may be adequate pull-ups for other, non-critical functions.
Another setting offered by some DC/DC regulators is the selection of switching frequency (all of these regulators use switching topology; none are low-dropout regulators – LDOs – for reasons of efficiency and size). For example, the MAX17536 from Maxim Integrated can be set via a single resistor to operate anywhere in the wide range from 100 kilohertz (KHz) to 2.2 megahertz (MHz) (Figure 5). This allows it to be set to avoid the impact of its switching noise on nearby circuitry which has overlapping frequencies, such as the AM radio band from 550 to 1600 kHz, or to avoid a specific narrow band which contains a signal of interest.
Figure 5: A single resistor establishes the switching frequency of the Maxim MAX17536 regulator within a wide band from 100 kHz to 2.2 MHz, thus providing flexibility to minimize circuit or signal interference. (Image source: Maxim Integrated)
Note that the relationship between resistor and switching frequency is nonlinear and somewhat imprecise. For this and other reasons, the MAX17536 can instead be set to synchronize to an external source rather than operate at a frequency as set by its resistor. Doing so also avoids undesired frequency mixing with other clock sources in the system and resultant beat frequencies which may cause subtle and hard to diagnose problems.
These tiny, complete DC/DC converters remove much of the risk and headache which occurs when designing low voltage, moderate current sources which deliver between 1 (or less) and 10 amps. However, as with any component, there are some basic rules which must be acknowledged and generally followed for a successful installation, and to realize their full potential and avoid any “gotchas”.