Application Note No.101 Frequency Step-Down Switching Regulator C

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Application Note No.101 Frequency Step-Down Switching Regulator C

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Application Note No.101
Frequency Step-Down Switching Regulator Controllers S-8520/8521 Series Product Overview S-8520/8521 series consists CMOS step-down switching regulator controllers control (S-8520 series) PWM/PFM switchable control (S-8521 series). These devices contains reference voltage source, oscillation circuit, error amplifier other components. These controllers available three types having oscillation frequencies kHz. This report focuses type, providing introduction important points terms features design from viewpoint compact size light weight systems which used. Development Background Dramatic reductions have recently been made size weight portable devices. case battery-powered products particular, increased efficiency power supply circuit become important aspect ensure longterm portable use. result, number cases which switching regulators used increasing. case using switching regulators portable devices, component size issue that frequently causes problems designer terms attempting reduce size. case attempting reduced component mounting height particular, there many cases which coil height presents difficult problem. Recently, development thin coils proceeded actively, numerous products have been released offering mounting height less than limitations physical size thin coils, however, there many that have inductance value order several order such coil, essential raise switching frequency. addition lowering inductance value coil increasing switching frequency, since peak current during switching operation decreases, becomes possible smaller coils having lower mounting height. Amidst this background, have developed S-8520/8521 series high-frequency step-down switching regulator controllers based development concepts described below.
Accommodation compact (thin) coils using oscillation frequency kHz. current consumption because CMOS process high conversion efficiency (90% more). (S-8521 holds high efficiency even when output current less.) Seiko Instruments Inc. Space-savings through ultracompact package (mounting surface area: externally connected components.
Features features S-8520/8521 series shown Table
current consumption During operation: max. During power max. Input voltage range 2.5-16 Output voltage range steps from Duty cycle 0-100% (PWM control: S-8520) 25-100% (PWM/PFM switchable control: S-8521) Externally connected components Switching transistor, coil, diode, capacitor only Built-in soft start function With power function SOT-23-5 package Table major features S-8520/8521 current consumption taking advantage CMOS process, SOT-23-5 package, smallest package world step-down switching regulator controller. addition, externally connected components that compose step-down switching regulator consist only power MOSFET transistor, coil, capacitor diode, enabling power supply unit portable devices designed while saving space. Fig. shows block diagram illustrating configuration step-down switching regulator with S-8520/8521 series.
Oscillation circuit VOUT
Reference voltage source with soft start function
PWM, PWM/PFM
Cout
switchable control circuit
Note: diode inside parasitic diode.
Fig.
Important Design Aspects When designing step-down switching regulator using members S-8520/8521 series, most important factor selection externally connected components. coil
Frequency Step-Down Switching Regulator Controllers S-8520/8521 Series switching transistor particular have significant effect maximum output current value efficiency. Therefore, following provides description important points that should considered when selecting coil switching transistor. Coil Selection Important parameters when selecting coil include inductance value, maximum allowable current direct current resistance value. inductance value direct current resistance value have particularly large effect efficiency. lower direct current resistance value, more advantageous efficiency. Since there tradeoff between efficiency coil size, there many cases which coil selected while placing priority component size portable devices other applications. Next, with respect inductance value, inductance value becomes smaller, peak current coil increases, output current value reaches maximum certain inductance value. Moreover, inductance value decreases, switching transistor loses capacity drive current resulting decrease maximum output current value. Conversely, since peak current becomes smaller when inductance value increased, loss switching transistor decreases, efficiency reaches maximum certain inductance value. Moreover, inductance value increases, loss series resistance coil increases resulting poorer efficiency. optimum inductance value changes depending what output current value which efficiency emphasized. case S-8520/8521 series (300 type), since inductance value around results most suitably balanced characteristics, select optimum inductance value vicinity this value. When selecting coil, also necessary attention allowable current value. Application current excess this allowable current coil causes magnetic saturation coil resulting significant decrease efficiency. Consequently, necessary select inductance value that peak current does exceed allowable current coil. Caution required particularly with respect thin coils since there some types that susceptible magnetic saturation. peak current indicated with equation continuous mode. (Vout (Vin Vout) Fosc (Vin .(Equation Peak current Fosc Oscillation frequency Coil inductance value :Diode forward voltage Iout Switching Transistor Selection members S-8520/8521 series allows direct driving power MOSFET having gate capacitance about 1000 case using power MOSFET, since switching speed greater than bipolar transistor there power loss caused base current, efficiency increased roughly 2-3%. Important parameters when selecting power MOSFET include threshold voltage, gate-source breakdown voltage, drain-source breakdown voltage, total gate capacitance, resistance current rating. Total gate capacitance resistance particular have effect efficiency. power loss charging discharging gate capacitance switching operation increases total gate capacitance increases input voltage increases, thereby having effect efficiency regions load current. Especially oscillation frequency increases, since proportion power loss charging discharging gate capacitance becomes larger, total gate capacitance MOSFET important parameter. Conversely, regions large load current, power loss caused resistance MOSFET effect efficiency. Since there typically tradeoff between total gate capacitance resistance, important select MOSFET according what load current range which efficiency emphasized actual application.
Applications following provides introduction several examples efficiency characteristics actual applications. Furthermore, those externally connected components used acquire data listed Table
Seiko Instruments Inc.
Frequency Step-Down Switching Regulator Controllers S-8520/8521 Series
Component Coil Product name CDH113 D62F Diode MA720 MA737 Capacitor MOSFET TM6201 IRF7606 Manufacturer Sumida Electric Toko Matsushita Electronics Nichicon Matsushita Electronics Toyoda Automatic max. max. -0.7V min. Ciss 320pF typ. Loom Works 0.25 max.(Vgs=-4.5V) ,SOT-89-3 International Rectifier max. -2.4A max. min. Ciss 470pF typ. 0.15 max.(Vgs=-4.5V) ,Micro value 22uH Direct current resistance 0.09 0.25 Max. allowable current 1.44A 0.70A Diameter 11.0mm 6.0mm Height 3.7mm 2.7mm
Forward current 0.55 Forward current
characteristics values above table were excerpted from October 1998 catalog. Please inquire respective component manufacturer further details their characteristics. Table
S-8520F33MC Output current efficiency characteristics (D62F/22uH,TM6201) Vin=9V Efficiency 0.01 Vin=6V Vin=4V
S-8520F33MC
Output current efficiency characteristics (CDH113/22uH,IRF7606)
Vin=14V Efficiency 0.01 Vin=9V Vin=6V Vin=4V
1000
1000
Output current (mA)
Output current (mA)
Graph Graph illustrates application which output voltage input voltage less, load current roughly This example attempting combine smallest externally connected components possible. Toko D62F coil having diameter height used coil, while Toyoda Automatic Loom Works TM6201 SOT-89-3 package used switching transistor. With this configuration, possible obtain maximum efficiency case input voltage Graph indicates application which output voltage input voltage less, load current about Sumida Electric CDH113 coil having diameter 11.0 height selected coil based relationship with maximum allowable current value. addition, International Rectifier IRF7606 used switching transistor
Graph consideration relationship with withstand voltage. comparison with characteristics Graph since direct current resistance coil resistance switching transistor lower, efficiency good regions high output current (approximately more). Conversely, increase gate capacitance switching transistor, efficiency poor regions output current (approximately less). Graph illustrates example characteristics case using S-8521 PWM/PFM switchable control. externally connected components same Graph Sumida Electric CDH113 coil International Rectifier IRF7606 switching transistor used.
Seiko Instruments Inc.
Frequency Step-Down Switching Regulator Controllers S-8520/8521 Series
S-8521F33MC Output current efficiency characteristics (CDH113/22uH,IRF7606) Vin=14V Vin=9V Efficiency 0.01 Vin=6V Vin=4V
1000
Output current (mA)
Graph S-8521 PWM/PFM switchable control normally operates from 100% duty cycle control, during light loads, automatically switches control duty cycle 25%. reason increase efficiency vicinity output current S-8521 switching control. S-8521 employing PWM/PFM switchable control makes possible realize high efficiency over wide range applications which load current varies between device standby operation.
Direction Future power supply circuits portable devices expected become increasingly small thin future. Although oscillation frequency member S-8520/8521 series introduced this report kHz, consideration increasingly reduced size ripple externally connected components, higher oscillation frequencies will necessary. However, this does mean that oscillation frequency should increased limitlessly battery-powered portable devices. Since higher oscillation frequencies result greater selfcurrent consumption this somewhat disadvantageous terms efficiency. planning developing products offering even higher frequencies (e.g., MHz) future while taking advantage merit CMOS, namely current consumption.
Seiko Instruments Inc.

Application Note No.101 Frequency Step-Down Switching Regulator C

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