Power Management for all of Tomorrow’s Innovations

Even with advances in battery technology and electromechanics, original equipment manufacturers (OEMs) struggle to meet the mix of expectations on ultralow emissions performance, vehicle range, and consumer affordability. Innovations in isolation, power management, magnetics sensing, and battery management systems (BMS) can help OEMs meet ultralow emissions, range, and vehicle cost expectations.

Technology Innovations Will Lift the Last Hurdles to Electric Vehicle (EV) Mass Adoption

There are two significant disruptions currently affecting the future of vehicular transport and semiconductor technology. The first is shifting from the internal combustion engine (ICE) to the electric motor drive. The second is the emergence of power switches employed in motor drive systems based on wide bandgap material. These offer figures of merit in the order of ten times better than the incumbent solution based on silicon. However, with the battery accounting for more than twenty-five percent (25%) of the final vehicle cost, optimization of energy use is one of the keys to achieving mass electric vehicle (EV) adoption. Reaching this goal means recognizing that every watt (W) spent is critical, and prioritizing subsystem efficiency is a primary selection criterion in automotive system design.

Recent advancements in power management for the powertrain—including isolated gate drivers, sensing, and BMS—allow designers to be creative to improve system efficiency while keeping the system cost under control (Figure 1). This article will hone in on power management and flyback converters.

Power Management

On the road toward the highest performance, every watt matters whether the EV is in “on,” “standby,” or “sleep” mode. Cutting-edge power management solutions can further increase vehicle efficiency, which correlates to extra miles while not compromising the best electromagnetic compatibility (EMC) performance from low current/low voltage to high current/high voltage applications.

Design Challenges in High Voltage Flyback Circuits

In functionally safe systems, continuity of voltage supply is critical. Generating a local low voltage rail from the high voltage battery plays a key role. In traditional isolated high voltage flyback converters, tight regulation gets achieved using optocouplers to transfer regulation information from the secondary-side reference circuitry to the primary side. The problem is that optocouplers add significant complexity to isolated designs. Optocouplers introduce propagation delay, aging, and gain variation. All these factors complicate power supply loop compensation and can reduce reliability.

Moreover, during start-up, either a bleeder resistor or high voltage start-up circuit is required to power up the IC initially. Unless an additional high voltage MOSFET gets added to the start-up components, the bleeder resistor is a source of unwelcome power loss. Let’s look at one specific example, Analog Devices LT8316 Micropower Isolated Flyback Converter.

Board-level designers are tasked with giving life to a board, monitoring its health, adjusting settings, running diagnostics, bringing it offline for inspection, troubleshooting when things are not correct, and gracefully powering down a complex board without incident. In designing and developing power supplies, power management may not only be desirable but a hard requirement. The power system manager aggregates various functions, such as power-on sequencing, detecting faults, margin testing, coordinating shutdown, measuring voltages and currents, and collecting data for analysis. This article is focused specifically on measuring supply current with LTC297x style devices.

The LTC297x DPSM Family

The focus of this article is power system managers that have a built-in current measurement, more specifically the Analog Devices LTC2971 2-Channel Power System Managers and Analog Devices LTC2975 4-Channel PMBus Power System Managers devices due to their ability to measure output current and allow the system software to read back values in units of amps using the READ_IOUT command.

Power System Managers (PSM) Basics

Power system managers (PSM) digitally view a power supply’s critical voltage and current readings. This is a powerful feature of the product family. Various current sense options will get covered in this article, including trade-offs between cost, complexity, and accuracy.

Current Sensing Options

The LTC2971/LTC2972/LTC2974/LTC2975 (Figure 1) managers accurately measure output current. When possible, engineers should select these devices because they have dedicated current sense pins and PMBus commands that provide telemetry values in amps.

A series shunt resistor is not the only way to sense current. Accuracy, cost, board space, and other factors must also be considered (Table 1).

Table 1. Summary of Current Sense Options
	Shunt Resistor	Inductor DCR	IMON
Accuracy	Very good	Good	Good, however light load accuracy is generally not spec’d
Output Path	Lossy (IR drop)	No additional losses	Lossless
Filter	1-pole filter per pin	2-pole filter per pin	Single RC
Other			Virtually no common-mode limitations, offset voltage on IMON pin on some devices

Shunt Resistor Sensing

The most common sensing method uses a shunt resistor, sometimes called a current shunt. The shunt resistor is placed in series with the output, whether the DC-to-DC converter is a switching regulator or a linear regulator. The feedback resistor divider gets connected to the output node. The shunt is inside the feedback loop, which allows the regulator to compensate for the shunt resistor’s IR drop when the load current is applied, significantly improving the load regulation (Figure 2).

The data sheet specifications list the maximum differential sense voltage developed across the ISENSE pins. Most of the LTC297x devices are limited to ±170mV of differential voltage. This provides more than enough range for most applications. The max sense voltage gets calculated as follows: V_SENSE = R_SNS × I_OUT(MAX). Generally, the max sense voltage is determined first, and the RSNS current sense resistor gets calculated as follows: R_SNS = V_SENSE / I_OUT(MAX). 

A related method adds a ground-referenced current sense amplifier (CSA) to provide a single-ended output fed into the manager’s current sense pins. This approach gets typically employed for level translating a rail that is higher than the 6V limit of most LTC297x managers. The CSA should have good high-side common-mode performance. It is typical to power such a device from the sensed rail and GND (Figure 3).

Inductor DCR Sensing

DCR sensing is the method that senses current through a buck regulator’s output inductor. An inductor can get modeled as an ideal inductance and a series resistance called DCR (Figure 4). This is typically the preferred method for high current (>20 A) rails. The addition of a resistive shunt is an extra component that dissipates power and generates heat.

One must have access to both ends of the inductor to sense across it, and a filter network must get inserted between the sense points and the LTC297x sense pins. The filter network is a two-stage differential RC low-pass filter. A 4-element resistor array can get employed for convenience and a small footprint. An alternative filtering scheme uses only two resistors and two capacitors. This reduces the component count from eight to four; however, the filter performance is not as good (Figure 5).

Conclusion

This article has introduced engineers to digital power system managers (DPSM) and some primary current sensing methods. Analog Devices Digital Power System Management (DPSM) devices help designers simplify and accelerate power system characterization, optimization, and data mining during prototyping, deployment, and field operation.

Learn more about Analog Devices Power System Management »  

This article investigates the effect that power designs have on the phase noise of RF amplifiers.

What Is Phase Noise?

Phase noise is the noise in a signal that comes from an unexpected lead or lag when the signal arrives at the receive side of a system. Just as amplitude noise is a shift or deviation from the signal’s nominal amplitude, phase noise is a shift or deviation from the signal’s nominal phase.

Cause and Contribution of Phase Noise

A critical and often-overlooked cause of phase noise is the DC power solution of the signal chain. Any noise or ripple on the power rails that supply the signal chain can couple internally. This can lead to an increase in phase noise, which may hide critical frequency components in the bandwidth transmitted or could induce spurs offset from the carrier.

Power Solutions

Ensuring proper biasing and power supply to amplifiers in an RF signal chain can be challenging, especially with drain voltages also used as the output port. There are numerous types of power solutions and topologies on the market. Which power solution you may need will depend on your application and system requirements. For this experiment, the data gets obtained using low dropout (LDO) linear regulators and step-down or buck switching regulators (Figure 1).

Setup

Analog Devices experimented by utilizing three different Analog Devices’ power products: LTM8063 40VIN, 2A Silent Switcher µModule Regulator, LTM4626 µModule Regulator, and LT3045 Power Supply Rejection Ratio (PSRR) Linear Regulator (Table 1).

Table 1. Data Sheet Specifications of Power Solutions Used
	LTM8063	LTM4626	LT3045
Topology	Buck µModule®	Buck µModule	LDO regulator
Input Voltage Range	3.2V to 40V	3.1V to 20V	1.8V to 20V
Output Voltage Range	0.8V to 15V	0.6V to 5.5V	0V to 15V
Output Current	2A	12A	500mA
Noise	~15mV ripple	~35mV ripple	1μV rms
Switching Frequency	200kHz to 2MHz	600kHz to 2MHz	—

The input signal swept the frequency range of 100MHz, 200MHz, 500MHz, and 1GHz to 10GHz. Analog Devices analyzed phase noise with a frequency offset of 10Hz to 30MHz (Table 2).

Table 2. Data Sheet Specifications of RF Amplifiers Used
	HMC8411	ADPA9002
Frequency Range	10MHz to 10GHz	DC to 10GHz
VDD (typ.)	5V	12V
IDD (typ.)	56mA	385mA
Gain	15.5dB	15dB
Output P1dB Compression (typ.)	20dBm	29dBm

Results

The power amplifier (PA) is observed to have slightly lower performance past 1/f frequencies. The PA draws substantially more supply current, resulting in roughly a 2dB to 4dB increase in observed phase noise (Table 2(a)(b), 3).

This relationship shows every doubling of the input frequency results in roughly a 6dB increase in phase noise. This can be seen with a 4× increase in frequency, resulting in approximately a 12dB increase from 10Hz to 100Hz frequency offset.

Bench supply phase noise response was used as a baseline to judge the performance of specific power solutions. The LTM8063 has exceptional performance across various frequencies compared to the bench supply, with only an approximate 2dB increase in the wideband noise floor (Figure 4).

Commonly, a high current module such as the LTM4626, will be used as the main supply so that the power distribution network can get stepped down according to each circuit block’s requirements. In this example, the LTM8063 exhibits similar phase noise performance to the LTM4626 cascaded with the LT3045 ultralow noise LDO regulator (Figure 5). If the voltage and current output supplied by the LTM8063 can satisfy design requirements, this power solution can save considerable cost and board space.

The switching power supplies exhibit markedly different behavior in various frequency bands (Figure 6a). The LTM8063 and LTM4626 have similarly negligible impact on the power low noise amplifier (LNA) phase noise below 5kHz but diverge significantly above that.

The LTM4626 was designed and optimized to power high-end digital products. These devices typically demand high efficiency and fast transient response. Their power supplies may have low passive impedance, fast switching edge rates, and increased control loop gain and bandwidth. These features can create a few millivolts (mV) of perturbations in the output voltage. Though inconsequential in a digital system, these perturbations can degrade the performance of a signal chain product. Despite this, the output spectrum using the LTM4626 showed no noticeable spurs with a spurious-free dynamic range (SFDR) of 102.7dB (Figure 6b).

The LTM8063, however, was designed for low noise—both electromagnetic interference (EMI) and output—optimizing its performance in signal chain applications. It exhibits excellent low-frequency stability, small output perturbations, and much less noise at the switching fundamental and its harmonics.

Conclusion

Considering all noise sources when performing a signal chain analysis is essential. One commonly overlooked source is the DC power solution, which can couple to and severely degrade the performance of the signal chain. Our results show that proper choice of power modules is paramount and can lead to a 10dB improvement of phase noise at 10kHz offset. For this application, the LTM8063 returned the best results. While the LTM4626 cascaded with the LT3045 gave comparable phase noise performance, knowing the correct power solution to choose is critical for optimizing your RF signal chain.

The full article may be viewed at How to Select the Best Power Solution for RF Signal Chain Phase Noise Performance.  

This article presents a specific Internet of Things (IoT) power management solution for small, portable gadgets while also reviewing its shortcomings (Figure 1). We then introduce a nano-Power boost converter that overcomes these shortcomings while ‘operating on fumes’ down to minimal amounts of residual battery energy.

Typical Wearable Power Solution

A wearable heart monitoring patch (Figure 2) is typically tiny and must last a long time; hence, minimization of the size and power dissipation is crucial. This device, powered by a 100mAh alkaline button cell and consuming 100µA in operation, will last three weeks. On the other hand, in shutdown mode, the device may need to last up to three years, which requires a leakage current of 4µA or less. A typical voltage regulator, with a leakage current of 0.2µA and a total quiescent current of 10µA, will rob 1.8 months from the device’s shelf life and two days of operation.

High efficiency and small size are challenging requirements. Increasing the frequency of operation will reduce the size of passives while increasing losses, thereby reducing efficiency. The proliferation of portable IoT devices creates a need for multiple customized versions of voltage regulators, especially concerning the output voltage and current specifications. Accordingly, an IoT device manufacturer may get forced to maintain a sizeable and costly inventory of different regulators and the passives required to support them.

A State-of-the-Art Solution

The ideal solution is a voltage regulator that addresses these shortcomings. Analog Devices MAX17222 nanoPower synchronous boost converter is such a device. The MAX17222 offers a 0.4V to 5.5V input range, a 0.5A peak inductor current limit, and an output voltage selectable using a single standard 1% resistor.

A novel True Shutdown™ mode yields leakage currents in the nanoampere (nA) range, making this a genuinely nanoPower device!

True Shutdown Current Advantage

The essential elements of the MAX17222 concerning shutdown and quiescent currents are shown below (Figure 3).

The True Shutdown feature disconnects the output from the input with no forward or reverse current, resulting in a very low leakage current. If you recall, the reduction in shelf life we calculated for the specific solution is 1.8 months out of three years for a 100mAh cell. With 0.5nA leakage current, the shelf life for the same cell over three years gets reduced to only three hours.

If a pull-up resistor is employed to enable/disable operation, the pull-up current in True Shutdown mode must also get accounted for. If the enable (EN) pin gets driven by a push-pull external driver powered by a different supply, then there is no pull-up current, and the shutdown current is only 0.5nA.

Quiescent Current Advantage

The input quiescent current (I_QINT) for MAX17222 is 0.5nA (Enable open after startup), and the output quiescent current (I_QINT) is 300nA (Figure 3). The additional input current needed to feed the output current (I_{>QINT_IN}) must be added to IQINT to calculate the total input quiescent current. Since the output power is related to the input power by the efficiency (P_OUT = P_IN × η), it follows that:

I_{>QINT_IN} = I_QOUT × (V_OUT/V_IN)/η

If V_IN = 1.5V, V_OUT = 3V and efficiency η = 85%, we have:

I_{>QINT_IN} = 300nA × (3/1.5)/0.85 = 705.88nA

One adds the 705.88nA to the input current of 0.5nA to yield a total input quiescent current of 706.38nA (I_QINGT). This calculation is fourteen times better than the typical case previously discussed. With 0.7µA of quiescent current, the two days of reduced operation calculated for the specific solution becomes only three and a half hours.

Enable Transient Protection Mode

The MAX17222 includes an option to enable transient protection (ETP) mode. When activated by the presence of a pull-up resistor, extra on-chip circuitry powered by the output capacitor assures that EN stays high during short transient disturbances at the input. In this case, the quiescent current calculated above increases by a few tens of nanoamps.

R_SEL Advantage

The MAX17222 trades off the traditional resistor-divider used to set the output voltage value with a single output selection resistor (RSEL) (Figure 3). The chip uses up to 200μA to read the RSEL value at startup. This occurs only during the select resistor detection time (typically 600µs), virtually eliminating the contribution of RSEL to the quiescent current. A single standard 1% resistor sets one of the thirty-three different output voltages, separated by 100mV increments between 1.8V and 5V! The result is a slight reduction in the bill of materials (BOM), as we now have one less resistor and a lower quiescent current.

Efficiency Advantage

The MAX17222 features low RDSON, on-board powertrain MOSFET transistors to yield excellent efficiency even when operating at frequencies high enough to warrant a small overall PCB size. The low quiescent current design extends the outstanding efficiency performance down to a few microamps of load current (Figure 4).

Conclusion

The IoT market has generated an explosion of small, wirelessly-connected, battery-operated devices. These devices continue to lower the power loss boundaries for operation (efficiency) and shutdown (leakage current). Analog Devices MAX17222 ultra-low quiescent current, high-efficiency synchronous buck converter significantly increases IoT devices’ shelf and operation life. It is an ideal choice for this class of applications.