Voltage RegulatorsVoltage Regulators

Used to regulate input voltage from a Used to regulate input voltage from a power source power source

Maintains power level to within set Maintains power level to within set tolerancetolerance

Prevents damage to components by Prevents damage to components by acting as a bufferacting as a buffer

Two types: Linear and SwitchingTwo types: Linear and Switching

Linear RegulatorsLinear Regulators Acts like a voltage dividerActs like a voltage divider Uses FET in ohmic regionUses FET in ohmic region

Switching RegulatorSwitching Regulator Switches on and off rapidly to alter Switches on and off rapidly to alter

outputoutput Requires Control Oscillator and Requires Control Oscillator and

charge storage componentscharge storage components

Types of Switching Types of Switching RegulatorsRegulators

Buck (step down) - lowers input Buck (step down) - lowers input Boost (step up) - raises inputBoost (step up) - raises input Buck / Boost - Buck / Boost -

lowers/raises/inverts input lowers/raises/inverts input depending on needs and depending on needs and controllercontroller

Charge Pump – provides multiples Charge Pump – provides multiples of input without using any of input without using any inductorsinductors

Linear AdvantagesLinear Advantages

Simple     Simple     Low output ripple voltage     Low output ripple voltage     Excellent line and load regulation     Excellent line and load regulation     Fast response time to load or line Fast response time to load or line

changes     changes     Low electromagnetic interference Low electromagnetic interference

(less noise)(less noise)

  

Linear DisadvantagesLinear Disadvantages

Low efficiency     Low efficiency     Large space requirement if Large space requirement if

heatsink is needed heatsink is needed  Can not increase voltage above Can not increase voltage above

the inputthe input

Switching AdvantagesSwitching Advantages

High Efficiency     High Efficiency     Capable of handling higher Capable of handling higher

power densities     power densities     Structures can provide output Structures can provide output

that is greater than, less than or that is greater than, less than or spanning the input voltage spanning the input voltage 

Switching DisadvantagesSwitching Disadvantages

Higher output ripple voltage     Higher output ripple voltage     Slower transient recovery time     Slower transient recovery time     EMI is produced  (Very noisy)EMI is produced  (Very noisy) Generally more costlyGenerally more costly

Dropout= 2Vbe + Vsat; Typically 1.5-Dropout= 2Vbe + Vsat; Typically 1.5-2.5Vdc2.5Vdc

Linear RegulatorsLinear Regulators

Darlington NPN RegulatorDarlington NPN Regulator

Dropout= Vsat; Typically <500mV.Dropout= Vsat; Typically <500mV. At light loads, this falls to 10-20mV.At light loads, this falls to 10-20mV.

PNP LDO RegulatorPNP LDO Regulator

Dropout= Vbe + Vsat.Dropout= Vbe + Vsat.

Quasi-LDO RegulatorQuasi-LDO Regulator

Two Considerations Ground Pin Current (Ignd), Two Considerations Ground Pin Current (Ignd), Vdo (Dropout Voltage At light loads, this falls to Vdo (Dropout Voltage At light loads, this falls to 10-20mV.10-20mV.

Here, Ignd= IL/β (Pass Transistor Gain)Here, Ignd= IL/β (Pass Transistor Gain) Darlington’s high gain allow Ignd= a few mA Darlington’s high gain allow Ignd= a few mA

(typically)(typically) Quasi-LDO, good performance (source 3A @ Quasi-LDO, good performance (source 3A @

<10mA)<10mA) LDO, Ignd= 10-20mA, which can be up to 7% of ILLDO, Ignd= 10-20mA, which can be up to 7% of IL All are unconditionally stable (requires no All are unconditionally stable (requires no

external capacitors)external capacitors)

Linear RegulatorsLinear Regulators

Advantageous because the amount of Advantageous because the amount of PWR dissipated is Vin * Ignd, because PWR dissipated is Vin * Ignd, because of the Pass FET’s low “on” voltage of the Pass FET’s low “on” voltage (~.7-.8V), only a small current is (~.7-.8V), only a small current is required to maintain regulation.required to maintain regulation.

P-FET LDO RegulatorP-FET LDO Regulator

Switching Regulators – Overview

Operation relies on controlled transfer of charge from input to output.

Output node charges while switch is closed and discharges while switch is open.

Requires multi-part circuitry.• Storage of the charge to be

transferred.• Control of switching scheme.• Output-stage filtering

Constant duty cycleConstant duty cycle Varying frequencyVarying frequency Noise spectrum imposed by PRM Noise spectrum imposed by PRM

varies and is more difficult to filter out.varies and is more difficult to filter out.

Switching Regulators – Switching Regulators – Pulse Rate ModulationPulse Rate Modulation

Switching Regulators – Switching Regulators – Pulse Width ModulationPulse Width Modulation

Constant frequencyConstant frequency Varying duty cycleVarying duty cycle Preferred – Efficient and easy to filter Preferred – Efficient and easy to filter

out noise.out noise.

Switching Regulator – Switching Regulator – Continuous ModeContinuous Mode

Current through inductor never Current through inductor never drops to zero.drops to zero.

Allows for highest output power Allows for highest output power for a given input voltage, for a given input voltage, switching scheme and switch switching scheme and switch current rating.current rating.

Overall performance is better.Overall performance is better.

Switching Regulator – Switching Regulator – Discontinuous ModeDiscontinuous Mode

Current through inductor drops Current through inductor drops to zero during each cycle.to zero during each cycle.

Allows for smaller inductor and Allows for smaller inductor and thus smaller overall circuit size.thus smaller overall circuit size.

Better when output current is Better when output current is low.low.

Switching TopologiesSwitching Topologies

Dielectric isolation vs. non-Dielectric isolation vs. non-isolatingisolating

NI: Uses – small change in NI: Uses – small change in Vout/Vin Vout/Vin

DI: Uses – radiation-intense DI: Uses – radiation-intense environmentsenvironments

Ideally Power in = Power outIdeally Power in = Power out

Non-Isolating formsNon-Isolating forms

Examples include: Examples include:

Buck, Boost, Buck, Boost, Buck/Boost, Cuk, Buck/Boost, Cuk, Charge PumpCharge Pump

Buck, Boost, and Buck, Boost, and Buck/BoostBuck/Boost

CircuitsCircuits

Isolated types Isolated types

Examples include: Flyback converters and Examples include: Flyback converters and Forward ConvertersForward Converters

Other Considerations for Other Considerations for Switchers Switchers

MOSFETs and DiodesMOSFETs and Diodes Synchronous Rectification – Synchronous Rectification –

Getting more EfficiencyGetting more Efficiency Operating FrequenciesOperating Frequencies

Ideal InductorsIdeal Inductors

Purely Inductive

Real InductorsReal Inductors

Inductive

Resistive

Capacitive

SRF

Frequency-Dependence Frequency-Dependence for Switching Regulatorsfor Switching Regulators

The frequency-dependent effects The frequency-dependent effects limit the inductor to a useful limit the inductor to a useful range of frequencies.range of frequencies.

Usually the SRF (Self-Resonant Usually the SRF (Self-Resonant Frequency) is specified by the Frequency) is specified by the manufacturer.manufacturer.

This sets a hard upper limit. The This sets a hard upper limit. The useful limit is generally lower.useful limit is generally lower.

High Frequencies are High Frequencies are GoodGood

As a general rule of thumb, doubling the As a general rule of thumb, doubling the switching frequency allows halving the switching frequency allows halving the inductance.inductance.

An old DC-DC converter at 30kHz might An old DC-DC converter at 30kHz might need an inductor measuring 1” in diameter.need an inductor measuring 1” in diameter.

A modern converter operating at 1MHz can A modern converter operating at 1MHz can deliver the same current with an inductor deliver the same current with an inductor measuring less than .25” in diameter.measuring less than .25” in diameter.

A smaller inductor will have a smaller A smaller inductor will have a smaller series resistance.series resistance.

Series ResistanceSeries Resistance Series resistance increases losses.Series resistance increases losses. It can also lead to instability in It can also lead to instability in

boost configurations.boost configurations.

0 0.2 0.4 0.6 0.8 10

1

2

3

4

5

0 0.2 0.4 0.6 0.8 10

1

2

3

4

5

in

out

V

V

in

out

V

V

0Load

Inductor

R

R01.0

Load

Inductor

R

R

Duty CycleDuty Cycle

Series Resistance - Series Resistance - SolutionsSolutions

Current sensing / overcurrent Current sensing / overcurrent protectionprotection

Duty cycle limitingDuty cycle limiting

0 0.2 0.4 0.6 0.8 10

1

2

3

4

5

in

out

V

V01.0

Load

Inductor

R

R

Duty Cycle

Cores and SaturationCores and Saturation

Magnetic cores increase the magnetic flux density when an external field is applied, increasing energy storage Magnetic cores increase the magnetic flux density when an external field is applied, increasing energy storage capacity and inductance.capacity and inductance.

When a core is saturated, increasing the external magnetic field results in a negligible increase in flux density.When a core is saturated, increasing the external magnetic field results in a negligible increase in flux density.

No External Field In an External Field

1μm

Cores and SaturationCores and Saturation

The onset of saturation can occur very rapidly. The onset of saturation can occur very rapidly. At saturation, the effective inductance decreases dramatically.At saturation, the effective inductance decreases dramatically. This decrease can lead to instability (increasing duty cycle, decreasing inductance).This decrease can lead to instability (increasing duty cycle, decreasing inductance). Without current or duty cycle limiting, this can result in catastrophic failure.Without current or duty cycle limiting, this can result in catastrophic failure. The manufacturer will usually specify this as the DC saturation current, the point where the inductance reaches 10% of nominal.The manufacturer will usually specify this as the DC saturation current, the point where the inductance reaches 10% of nominal.

Cores and SaturationCores and Saturation

Cores also exhibit losses due to internal “friction” and eddy currents.Cores also exhibit losses due to internal “friction” and eddy currents. In switchers, these losses are usually only a small percentage of the total losses, even at saturation.In switchers, these losses are usually only a small percentage of the total losses, even at saturation. However, saturation in a DC-DC supply can cause greater resistive losses due to the drop in inductance.However, saturation in a DC-DC supply can cause greater resistive losses due to the drop in inductance.

H

B

Important ParametersImportant Parameters

Inductance: Larger inductances Inductance: Larger inductances have lower ripple current, but are have lower ripple current, but are larger and can decrease stability. larger and can decrease stability.

Peak Current: This is obtained Peak Current: This is obtained from the switcher’s datasheet.from the switcher’s datasheet.

DC Current: Dependent on load DC Current: Dependent on load requirements.requirements.

SRF: Should be well above the SRF: Should be well above the switcher’s operating frequency.switcher’s operating frequency.

Series Resistance: Affects the Series Resistance: Affects the efficiency and can affect stability.efficiency and can affect stability.

A Variety of InductorsA Variety of Inductors

Applications and Sample Applications and Sample Circuits of Some Common Circuits of Some Common

PackagesPackages National Instruments LM337 PackageNational Instruments LM337 Package

• 3-terminal adjustable regulator3-terminal adjustable regulator• Capable of supplying 1.2V to 37V with Capable of supplying 1.2V to 37V with

1.5A1.5A• Offers overload protection that is only Offers overload protection that is only

available in ICsavailable in ICs• Can also be used as a precision current Can also be used as a precision current

regulatorregulator Texas Instruments TPS32xx PackageTexas Instruments TPS32xx Package

• Optimal for battery powered portable Optimal for battery powered portable applications.applications.

• Operates a 93% efficiency at 3MHz.Operates a 93% efficiency at 3MHz.

Simple LM117 Voltage Simple LM117 Voltage Regulator ConfigurationRegulator Configuration

1.2V to 25V output C1 is only needed if the device is more that 6”

from the filter capacitors. C2 is optional – it is used to improve the

transient response.

5V Logic Regulator with 5V Logic Regulator with Electronic ShutdownElectronic Shutdown

Uses TTL for electronic shutdown. Clamping of adjustment terminal

programs the output to 1.2V where most loads draw little current.

1.2V is the minimum output voltage.

0V to 30V Regulator0V to 30V Regulator

The downside is that the full output current is not available for high input/output voltages.

Precision Current LimiterPrecision Current Limiter

Iout = Vref/R1 0.8 Ohms < R1 < 120 Ohms

AC Voltage RegulatorAC Voltage Regulator

Regulates a 24Vp-p Input to 12Vp-p at 1A.

12V Battery Charger12V Battery Charger

Rs is used to set the output impedance of the charger:

Texas Instruments Texas Instruments TPS62300TPS62300

Used in low-power portable electronics Designed for fast start-up time

ReferencesReferences TI. Understanding Buck Power Stages in Switchmode Power TI. Understanding Buck Power Stages in Switchmode Power

Supplies. Supplies. http://focus.ti.com/lit/an/slva057/slva057.pdfhttp://focus.ti.com/lit/an/slva057/slva057.pdf. . TI. Understanding Boost Power Stages in Switchmode Power TI. Understanding Boost Power Stages in Switchmode Power

Supplies. Supplies. http://focus.ti.com/lit/an/slva061/slva061.pdfhttp://focus.ti.com/lit/an/slva061/slva061.pdf. . Erickson, R. DC-DC Power Converters. Erickson, R. DC-DC Power Converters.

http://ece-www.colorado.edu/~rwe/papers/Encyc.pdfhttp://ece-www.colorado.edu/~rwe/papers/Encyc.pdf.. Shen, L., Kong, J. Applied Electromagnetism, Third Edition.Shen, L., Kong, J. Applied Electromagnetism, Third Edition. http://www.web-ee.com/primers/files/f5.pdfhttp://www.web-ee.com/primers/files/f5.pdf http://www.maxim-ic.com/appnotes.cfm/appnote_number/7http://www.maxim-ic.com/appnotes.cfm/appnote_number/7

10/ln/en10/ln/en http://www.eetasia.com/ARTICLES/2000NOV/http://www.eetasia.com/ARTICLES/2000NOV/

2000NOV30_AMD_AN2.PDF2000NOV30_AMD_AN2.PDF