Understanding Power Supply Basics and Terminology DC/DC Converters 101 Understanding Power Supply Basics and Terminology

Agenda Lecture Overview Linear Regulators Switching Power Supplies Topologies Synchronous vs. Non-synchronous Controller vs. Converter Selecting the Best Power Solution

Why should I care about power? 1. Every electronic system uses power. 2. Your power source never matches your system needs. Power Source DC/DC Supply gets you from here to there What you need Typically 5V,12V or 24V 6.0Vdc-16Vdc 40Vdc Surge 1.2V Core @ 2A 2.5V I/O @ 1.2A 3.3V 5V +/-12V 3.0Vdc-4.2Vdc

Linear Regulators vs. Switching Supplies Pass element operates in the linear region Down conversion only Switching Power Supply Pass elements switch, turning fully on/off each cycle Filtering includes an inductor Multiple topologies (Buck, Boost, Buck-boost…)

Linear Regulator ADVANTAGES: DISADVANTAGES: APPLICATIONS: Low O/P ripple & noise Fast transient response Low cost (for low power, at least) Easy to design No EMI to worry about DISADVANTAGES: Low efficiency at VIN>>VOUT High dissipation (needs large heat-sink) VOUT<VIN – always! APPLICATIONS: Extremely low ripple & noise apps Low input to output voltage difference Tight regulation

Dropout Voltage Dropout (headroom): The minimum required voltage across an LDO to maintain regulation Example: Vin = 3.1V to 4.2V Vout = 2.5V @ 100mA Need at least 600mV headroom + Vdo -

Linear Regulator vs LDO Linear Regulator has Higher Dropout Voltage. Transistor or Darlington pair pass element LM317 (1.5A linear regulator) 1.5V to 2.5V dropout voltage Good for larger Vin to Vout ratios, 12V to 5V output CHEAP!!! LDO = Low Dropout Regulator Typically higher performance PSRR, regulation tolerance, transient response, etc MOSFET pass element TPS72501 (1A LDO) 170mV dropout voltage Good for 3.3V to 3.0V output

Linear Regulator Power Dissipation Input Current = Output Current Power Loss = Iout * (Vin – Vout) Power loss is usually a limiting factor!

Linear Regulator vs Switcher 2.5W LDO + ground plane as heat sink 6W Switcher

Switcher DC DC ADVANTAGES: High efficiency VOUT>=<VIN Wide input voltage range Low power dissipation (small heatsink) High Watt/cm2 Isolation possible (with transformer) Multiple O/Ps possible (with transformer) DISADVANTAGES: EMI Slower transient response More difficult to design Higher output ripple & noise APPLICATIONS: High efficiency power supplies High ambient temperatures Large input to output voltage difference Space constraints High output power DC VIN VOUT DC

Basic Topologies Buck VIN VOUT Boost VIN VOUT Buck/Boost VIN VOUT

Synchronous vs Non Sync Non-Synchronous Buck Non-synchronous Diode voltage drop is fairly constant with output current Less efficient Less expensive Used with higher output voltages Synchronous MOSFET has lower voltage drop More efficient Requires additional control circuitry Costs more Synchronous Buck

Synchronous vs Non Sync Vin=5V Vout=1V Rdson_sync=0.12ohm Vf_diode=0.5V Iout=1A 1V Output Synchronous 1V Output Non-Synchronous Sync vs Non-sync is less of an issue with higher Vout Higher duty cycles = less power dissipation in Sync FET or Catch Diode

Synchronous vs Non Sync Power FET Synchronous FET

Synchronous vs Non Sync Integrated Power FETs Rectifier Diodes Integrated Power FET and synchronous FET

Controller vs Converter Discrete MOSFETs Provides the “brains” to control the power stage More complicated to design Full control over FET selection, switching frequency, overcurrent, compensation, softstart Can tailor the power supply to meet your specific needs Converter (Fully integrated) Integrated switches “plug and play” design Limited range of output filter components Limited control over functionality Converter (Partially integrated) May offer full or partial feature set , internal or external compensation Internal Power FET, external sync-FET or catch diode Limited control over frequency, overcurrent, softstart, etc Allows wider range of output filter components

Converter (Fully Integrated) TPS62293 2.3V to 6V input 1A Output Current 2.25MHz Everything is integrated, minimum external components

Converter (Partially Integrated) TPS54620 4.5V to 17V input 6A Output Current Internal FETs, external SoftStart, Compensation, Frequency set… more flexibility Set frequency Compensation

Controller TPS40303/4/5 3V to 20V input 10A Output Current 300kHz to 1.2MHz External FETs Compensation Softstart Current limit

Size vs. Cost vs. Efficiency Synchronous Non-synchronous Linear Regulator Power Density Cost Converter (Fully Integrated) Converter (Partially Integrated) Controller

Efficiency vs Vout Efficiency depends on output voltage? Why isn’t MY supply 95% efficient? The datasheet says:

Efficiency vs Vout 3.3V Output 1V Output Power FET Conduction Losses Simplified power dissipation equations assuming no inductor current ripple 3.3V Output 1V Output Power FET Conduction Losses Sync FET Conduction Losses Total FET Losses (does not include other circuit losses) 0.173 W 0.136 W

Efficiency vs Vout 3.3V Output 1V Output

PWM vs PFM Pulse Width Modulation Pulse Frequency Modulation Constant frequency Low output voltage ripple Used with high output currents Pulse Frequency Modulation Varying frequency with Vin and load Very high efficiency at very light loads Higher output voltage ripple Potential operation in audio range

PWM vs PFM PFM mode PWM mode

Startup - Softstart Slowly turning on the power supply Controlled rise of output voltage Minimizes inrush currents Minimizes system level voltage drops Pulling high currents out of input bus High impedance batteries Internal vs SS capacitor Larger SS capacitor = longer softstart time

Startup - Sequencing Sequencing Sequential sequencing Controlling the order that different power supplies are turned on Important for uP loads Minimizing overall inrush current Sequential sequencing

Startup - Sequencing Ratiometric Sequencing Simultaneous Sequencing

Easy Answers – Power Quick Search Provides a list of possible linear regulators, controllers and converters based on inputs Great starting point for selecting a device

Easy Answers – Power Quick Search

More Answers – Browse The Product Tree

Easy (Simulated) Answers – WEBench Provides a complete design based on inputs Best for customers with little or no power background

Easy (Real) Answers – TI Designs/PowerLAB Searches reference designs based on input

THANKS!! Questions??? ufseniordesignanaloghelp@list.ti.com