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Power Management Fundamentals

July 2011 Webcast

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Power Management FundamentalsAgendaWhat is Power Management?Types of Power Conversion ProductsThe Need for “Clean” & Stable Power SuppliesWhat is a Linear Voltage Regulator?Why use a Linear Regulator over a Switching Regulator?Common Types of Switching RegulatorDC Power DistributionThe Road to Successful Power DesignsUpcoming webcasts in the ‘Fundamentals’ Series

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What is Power Management?

All Electronic systems require a power supply & some form of power management. System power needs vary from simple 3-terminal regulators, to sophisticated IC voltage regulators providing multiple outputs. In addition, high-performance systems may require continuous monitoring of power-supply voltages & currents during operation


What is Power Management? contd.

What functions are included in Power & System ManagementPower conversion products; Low dropout regulators,

switching regulators, switching controllersPower switches or Load switchesSupply voltage monitoring Supply voltage sequencing and trackingBattery switchover devicesHot-swap controllersSupervisory products; reset controllers, watchdog

timers, multi-voltage supervisors


Power Conversion Products Nearly all electronic equipment is powered from low voltage DC

supplies. The power source is either a battery, a combination of battery & DC/DC converter, or a power supply converting AC mains into low voltage DC supplies, suitable for electronic components

Electronic components require: DC supply that is well regulated Low output noise and ripple Fast transient response to load changes

AC power supplies, & some DC/DC converters, provide isolation from the input to the output for safety, noise reduction, & transient protection

There are two types of DC/DC conversion in common use:1. The Linear Voltage Regulator 2. The Switching Regulator


Key Regulator Specs & their Significance Input Voltage (Vin) Range

The voltage range over which a regulator is specified for normal operation Output Voltage (Vout) & Output Current

The factory-set output voltage & the maximum current available from this output Output Voltage Accuracy & Drift

Accuracy of the factory-set output voltage, specified in mV or % of output at 25C. Drift is the change in DC output voltage over the operating temperature range

Dropout Voltage Dropout voltage is a term specific to LDOs and is the minimum Vin-Vout differential

voltage necessary for correct LDO operation at a specified load current Output Noise

Noise generated by the regulator itself is specified as broadband or peak-peak Power Supply Rejection Ratio (PSRR)

The ability of a regulator to reject a large voltage change at it’s input and produce a small change at it’s output. Usually specified in dB for DC input changes

Quiescent Current & Shutdown Current Quiescent current is the current drawn by the regulator when there is no load current.

Shutdown current is the current drawn when the regulator is disabled, or powered off


The Need for “Clean” & Stable Power Supplies

Regulators are available in different levels of performance from ‘commodity’ to specialized high-performance types

In low-performance systems, commodity LDOs & switching regulators may work fine. However, systems that use high-resolution (12-Bit & higher) converters may experience performance degradation if powered by commodity regulators

Commodity switching regulators tend to be; less efficient, require larger external components, & may offer less protection under fault conditions compared to high-performance switching regulators

Output Noise from a Switching Regulator, 10mV/division


The Need for “Clean” & Stable Power Supplies contd.

Which voltage regulator parameters can compromise system performance? Voltage drift, noise, and ripple on the regulator’s output are primary causes of signal distortion & measurement errors caused by the supply

How do drift, noise, & ripple get into the signal-chain?They are conductively coupled from the power supply directly to the ‘victim’ circuits, e.g. op amp, reference, ADC, etc. however, the signal is reduced by the power supply rejection ratio (PSRR) of the ICs

How do you quantify PSRR?PSRR is the ratio of the DC change in power-supply voltage to the resulting change in the load IC’s gain, offset, or other errors. PSRR can be expressed in fractions of a least significant bit (LSB) for an ADC, a percentage, or in dB (PSR = 20 × log10 (PSRR))

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What is a Linear Voltage Regulator?A Linear Voltage Regulator is any device that reduces a voltage in a dissipative manner for the purpose of regulation Includes Zener diodes (or regular diodes) and shunt references Generally a closed-loop system with a transistor operating in its

linear region Can only reduce voltage, can’t boost it Non-Isolated output Efficiency is directly proportional to Vout/Vin (neglecting bias

current loss)


Low-Noise LDOs for ADC, RF, & PLL Power

The 150mA ADP150 & 200mA ADP151 are ultra-low noise (9µV rms) LDOs suitable for powering the most sensitive circuits such as high-resolution ADCs & low-noise PLLs

ADP150


LDOs with Excellent High-Frequency PSRRThe 800mA ADP1752-53 and 1.2A ADP1754-55 are low-

voltage LDOs with high PSRR (65dB) & low-noise for post-switcher applications. Input voltage range is 1.6V to 3.6V

ADP1752 Output Noise ADP1752 PSRR, Vout = 0.75V


Micro-Power LDOs for Portable ApplicationsThe ADP160 is a micro-power LDO optimized for battery

powered & portable applications. Quiescent current is typically 560nA @ 0µA load, & only 3µA with a 100µA load. PSRR is typically 60dB @ 100Hz, a reduction of 1000:1

ADP160 LDO Ground Current ADP160 LDO PSRR Vout = 3.3V


Cleaning the Output of a Boost Regulator in a Battery Powered Application

Single Cell Boost

Regulator

ADP162Low IQ

LDO

High Performance

Analog

1.5V to 0.9V

3.3V 2.8V

100mV/DIV

20mV/DIV

Better than 40dB Ripple Rejection

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Why choose a Linear Voltage Regulator over a Switching Regulator?Easy to Use, three terminal deviceLower parts countLittle to no compensation needed

Lower Output NoiseLower Input and Output rippleNo high dI/dt or dV/dt loops or nodes to radiate noise (EMI)

In some situations LDOs can achieve similar efficienciesLight-load efficiency is often better than a switcher’sHigh Vout/Vin ratio (near dropout) means high efficiency

Specialized types availableLower Cost


Buck or Step-down Regulator, Vin > Vout

Boost or Step-up Regulator, Vin < Vout

Buck-Boost Regulator (invert), Vout is opposite polarity to Vin

Flyback Regulator, Vin < Vout or Vin > Vout, can have multiple outputs

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Most Common Switching Regulators


Low-Noise Switching Regulators

The ADP2114 & ADP2116 are dual buck (step-down) regulators with ‘reduced EMI drive’ to enable powering of sensitive signal-chain components; Op Amp, ADC, etc.


Summary Comparison of Linear Regulators & Switch-Mode Regulators

Parameter Linear Regulator Switch-Mode Regulator Efficiency Low, <80% High, 80% to 95%+

Output Noise and Ripple

Low NoiseNo switching ripple

Higher noise than LDORipple at switching frequency+

Design Complexity Simple More Complex

PCB Area Small, 3 components Medium, 5+ components

EMI Minimal Medium (depends on switching frequency and PCB layout)

Cost Low to Medium Medium to High

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DC Power DistributionSystems frequently require multiple power supplies for analog,

digital I/O, CPU, FPGA, displays, etc.Due to fast-changing load currents (e.g. in FPGAs & CPUs), and

the need to provide good load transient response, DC/DC converters are placed adjacent to the load rather than routing a power cable from a distant supply

The requirement for local DC/DC regulation has driven the popularity of the Distributed Power Architecture (DPA)


How do DC Drift, Noise, & Ripple get into the Signal-Chain?

DC drift, noise, & ripple from the power supply is conductively coupled into the supply pins of the load ICs but the magnitude is reduced by the PSRR of these loads. Note that PSRR is highly frequency dependent so HF noise from switching regulators has to be controlled

Low-Frequency Op Amp PSRR High-Frequency Op Amp PSRR


How do DC Drift, Noise, & Ripple get into the Signal-Chain?

As switching regulators replace linear regulators (for higher efficiency) the effects of HF power-supply ripple & noise must be calculated. The graphs below show the difference in PSRR performance between a commodity LDO and a high-performance LDO

Commodity LDO PSRR, <30dB @ 100kHz ADP123 LDO PSRR, >60dB @ 100kHz


How to Minimize Ripple & Noise?Switching power-supply ripple & noise can be dramatically

reduced by using high-performance LDO post-regulators for the most sensitive Analog & Mixed-Signal circuits


The Road to Successful Power Designs Select the voltage regulator type(s) that convert the input voltage to the output

voltage(s) required, e.g. buck regulator, boost regulator, SEPIC. Ensure the outputs meet or exceed the load current requirements

Determine if a specialized regulator is required for the application, e.g. a low-noise LDO to power a high-performance 14-Bit ADC

When a switching regulator provides a high output current that also needs to have low voltage-noise, consider using an LDO post-regulator to clean up the switching noise with only a small reduction in overall efficiency

Buck Regulator with Low-Noise LDO post-regulator


The Road to Successful Power Designs contd.

For switching regulators, follow the manufacturer’s PCB layout recommendations to ensure optimum results. PCB layout is shown in the product datasheet on ADI’s switching regulators

Calculate the temperature rise for high-dissipation parts & verify that Trise + Tamb for all parts is within the safe operating range

Use High-Quality passives (L, C, & R) for best efficiency & output accuracy. High Quality means components that are accurate & stable vs. temperature & applied voltage. HQ passives ensure HQ results

1. When using multi-layer ceramic capacitors (MLCC), use types with the X5R or X7R dielectric. Y5V & Z5U dielectrics are not recommended due to their poor temperature coefficient & DC bias characteristics

2. Inductors used in switching regulators should have; good magnetic shielding, low DC resistance, low self-capacitance, low leakage inductance, high saturation current, and a “Soft” Saturation Curve

3. Resistors used for setting output voltage should be accurate to 1% or better & have a temperature coefficient of 100ppm/C or less


Fundamentals Webcasts 2011 January Introduction and Fundamentals of Sensors February The Op AmpMarch Beyond the Op AmpApril Converters, Part 1, Understanding Sampled Data SystemsMay Converters, Part 2, Digital-to-Analog Converters June Converters, Part 3, Analog-to-Digital Converters July Powering your circuitAugust RF: Making your circuit mobileSeptember Fundamentals of DSP/Embedded System designOctober Challenges in Industrial DesignNovember Designing with MEMSDecember Tips and Tricks for PC Board Layout

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