AN715 Designing Low-Voltage DC/DC Converters with Si9145 INT
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AN715
AN715
Designing Low-Voltage DC/DC Converters with Si9145
INTRODUCTION
Siliconix Si9145 switchmode controller designed make dc-to-dc conversion smaller more efficient lowvoltage, low-power applications such portable cellular phones other battery-operated equipment. Compared with conventional bipolar BiCMOS devices, Si9145 offers extremely power consumption propagation delay times, well operation down very voltages. Built Siliconix' proprietary BiC/DMOS technology, Si9145 features operating voltage range from enabling single-cell lithium (Li+) batteries, well 4-cell NiCd NiMH batteries.
manufacturer. Compared with three NiCd batteries, however, there substantial voltage change over operational platform battery. Presently most NiCd battery designs linear regulator, with batteries, this would yield substantial drop efficiency system. battery will require high-efficiency switchmode regulator solution maintain benefits.
COMMONLY USED DC/DC TOPOLOGIES
There numerous types dc-to-dc converters, most practical designs variations Buck boost topologies. Si9145 been configured that most popular conventional topologies, including Buck, synchronous Buck, boost easily implemented.
FIGURE Buck Converter FIGURE Discharge Comparison NiCd Buck converter (Figure produces output voltages lower than input, using high-side switch (Q1). During conduction time, current flows through during time, current flows through thus maintaining continuous current. synchronous Buck converter (Figure identical Buck converter, except that MOSFET used replace rectifier. This substitution allows higher efficiencies, well continuous current, even down virtually load. Boost Converter (Figure used when higher voltage than input required output. This result obtained allowing energy stored during time allowing voltage polarity reverse during time, thus raising voltage above VIN.
OVERVIEW LITHIUM TECHNOLOGY
Lithium batteries becoming more readily available, their introduction into consumer products such camcorders minidisk players will likely make lithium technology choice foreseeable future. Lithium batteries have several advantages over their nickel-based counterparts, including higher volumetric capacity, absence memory effect, built-in protection features supplied manufacturer. single cell will produce almost linear voltage discharge curve starting ending around (Figure final discharge value varies
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FUNCTIONAL DESCRIPTION Si9145
Where extremely voltages used high efficiencies required, practical measure extremely voltages across sense resistors. Therefore, Si9145 uses voltage mode control. Si9145 (Figure configured operation high frequencies, typically between MHz, where small energy storage components (magnetic capacitive) required. Operation allows small-outline surface-mount capacitors inductors, which keep size volume minimum. Si9145's configuration allows separation noisy load switching sections from low-noise analog parts.
FIGURE Synchronous Buck Regulator
FIGURE Boost Converter
FIGURE Si9145 Block Diagram
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DESCRIPTION
This supply low-noise analog section. should well decoupled separated from power pin. Good decoupling close (pin recommended. MODE Select This allows polarity output driver changed, accommodate both p-channel drives, well enabling operation DMAX (pin When connected GND, DMAX disabled, allowing 100% duty cycle, inverted output drive, suitable p-channel MOSFETs, would case Buck regulator. When connected VDD, duty cycle programmed output driver configured low-side drive n-channel MOSFETs. This mode suitable boost regulators, flyback/forward transformer isolated types, where duty cycle limitation prevents loop instability core saturation. DMAX/SS This allows maximum duty cycle between 100%. Below duty cycle above 100%. Users program exact value using divider this pin. addition, soft start achieved placing capacitor parallel with lower divider circuits where DMAX connected GND. This adds time constant duty cycle during start-up. Pins Comp, These pins three connections error amplifier. error amplifier uses bipolar input differential stage complementary NPN/PNP output driver stage. high frequency capability error exceptional characteristics process used. unity gain bandwidth (Figure error amplifier around MHz, from VREF internal 1.5-V band reference trimmed ±1.5% internally. minimum 100-nF capacitor recommended de-coupling. This analog ground noise sensitive functions (pins through should well decoupled with VDD. Pins ROSC, COSC These pins used select oscillator frequency operation. frequency oscillator value resistor which sets value current mirror that charges timing capacitor recommended that capacitor values below used, stray capacitance packaging manufacturing tolerance will affect value selected. frequency operation calculated from following equation:
This type oscillator difficult synchronize. obtain true free running mode that lock onto available signal, spike needs superimposed triangle ramp pre-trigger circuit. (Figures Buffers generate very short spike (dependent propagation delay logic used) which drives superimpose spike onto spike duration should kept minimum, avoid dissipating power oscillator frequency shifted lower value minimize power consumption light mode changing current timing resistor with external MOSFET (Figure normal operation therefore reverse biases preventing current from flowing through When open, allows mirrow current altered, thus changing frequency. From this data (Table clearly seen that improvement efficiency light load, example sleep mode, would significant. current taken from +VIN changed significantly. which drives MOSFET output stage, significant reduction. This means that converter running normal mode, efficiency light load increase dramatically: from less than more than 75%, depending other frequency losses circuit. output ripple will change, parameters that define have changed, this case increase acceptable.
FIGURE Si9145 Unity Gain Bandwidth Phase
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AN715
FIGURE Synchronization External Source TABLE FOSC
+VIN Current (mA) Current (mA) Total Current (mA) Total Power from Input (mW) Percentage Output Power* Output Ripple (mV) 1.28 34.2 35.48 35.5
FOSC
0.94 3.94 4.88 24.4 16.3
*Taken with output power sleep mode mW).
This used indicate internal overtemperature shutdown. internal integrated sensor detects excessive temperature latches this event overtemperature. Normally this connected enable allow shutdown event overtemperature. Overtemperature will most likely encountered event short circuit failure both devices being driven from output driver. Enable enable should pulled high normal operation. Pulling this down stops operation chip allows reduce consumption mode. This normally configured with (see 11). UVLOset UVLO used determine circuit's cut-off point operation event low-voltage input. This function prevents excessive discharging damage batteries. internal 1.2-V reference compared with this pin, built 200-mV hysteresis prevents oscillations close threshold operation. This might encountered with highimpedance sources, such battery end-of-charge.
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FIGURE Oscillator Synchronization
FIGURE Frequency Shifting Oscillator
AN715
Pins PGND, Output, These pins three connections output driver stage, supplying output buffer. output buffer complementary type, with very fast transition times extremely output impedance. also capable generating noise area close chip. Access pins allows proper decoupling minimize supply return current paths load being driven. output driver transition time fast enough drive pair complementary MOSFETs directly with common gate connections, while maintaining very shoot-through current. Significant improvements efficiency achieved using external break-before-make circuit, (see Figure 10). Using this circuit, efficiencies better than obtained with proper MOSFETs. important oversize MOSFET(s) being driven application required. Using larger, lower on-resistance MOSFET will necessarily produce better result. input output voltage ripple that appears input output capacitors will determined quality capacitor used. experimental tests, large variations were encountered from manufacturer manufacturer from different series. Good, low-ESR -ESL (Equivalent Series Resistance Inductance) devices should selected. boost converters, discontinuous nature energy transfer, even higher peak currents will encountered, which will turn generate higher ripple without lower resistances. Board layout critical performance. Design methodology should include minimization switching current paths well separated signal power grounds, with single point connections. following design examples illustrate types converter that easily designed with Si9145.
DESIGN EXAMPLE
Buck Converter Assume following design specification:
VOUT (MHz) IOUT POUT (mA)
First, select correct inductor value: (see Appendix
LOUT
FIGURE output driver Si9145 been optimized driving on-resistance MOSFETs Siliconix LITTLE FOOT® series. recently introduced LITTLE FOOT TSSOP-8 TSOP-6 devices offer similar better performance classic LITTLE FOOT SO-8 while using smaller outline. Changes introduced lead connections these packages have allowed creation low-voltage, low-gate threshold devices that used low-profile, small-surface area power converters. design highly efficient dc-to-dc converter, several parameters need considered: Required minimum efficiency. Usually achievable, with higher efficiency occurring with lowest switching frequency, lowest input output voltage differential. Conduction losses caused current switching through on-resistance MOSFETs. Gate drive losses caused turn turn gate charge (Qg) both devices Si9145. Output capacitance losses caused each MOSFET conducting shorting output capacitance. highest efficiency, select synchronous Buck topology, using channel MOSFETs. choosing MOSFETs, remember that device will driven only from rail rail. device selected needs have gate thresholds specified over input operating voltage suitably sized avoid excessive power loss gate drive switching losses. small Schottky diode used prevent current flow through body diode n-channel MOSFET avoid recovery time through this device. only conducts current during crossover time. therefore dissipates virtually power, n-channel device shunts when fully enhanced. synchronous Buck regulator, conduction duty cycles p-channel devices are:
Pchannel
Nchannel
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conduction losses worst possible case, temperature coefficient MOSFET operating 100°C will approximately 1.4. required on-resistance, based power dissipated each device will approximately
LOSS
total gate charge losses MOSFET need also considered. gate charge losses both devices were equal half full load conduction losses, then these would approximately required gate charges would determined from:
QGTOT QGTOT
n-channel device, then
LOSS QGTOT
Assuming that conduction power losses each MOSFET will equal total losses full load, then losses each device efficient converter will
1.06 0.85 1.06 0.16 LOSS 0.25 0.16 0.04
Ideally, p-channel devices with each should selected. complete schematic converter shown Figure this circuit example, Si9145's power consumption measured
LOOP STABILITY
optimize stability loop, POWER456[2] software utilized. data parameters Buck converter stage were entered resulting loop compensation components were extracted. manual detailed analysis voltage mode loop stability should review Siliconix application note AN710[3]. Figure shows converter switching waveforms. middle trace shows input voltage node choke, which also common drain both MOSFETs. bottom trace shows common gate connection both MOSFET. Figure shows same waveforms greatly expanded, showing rise fall times output driver. effect noise clearly apparent timing capacitor waveform.
Then on-resistance each MOSFETs will need
LOSS 0.529
n-channel device
LOSS 0.794
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FIGURE Complete Buck Regulator Schematic
FIGURE Synchronous Buck Switching Waveforms
FIGURE Synchronous Buck Switching Waveforms (Expanded)
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DESIGN EXAMPLE
Continuous Boost Converter Assume following Design specification:
VOUT (MHz) IOUT POUT Target Efficiency
Total losses:
0.409 0.88
power dissipated MOSFET will
LOSS
First select correct inductor value (see Appendix
Ipeak 1.296 equivalent Irms 0.842
Therefore, MOSFET will need have on-resistance
LOSS 0.164 0.231 0.842 (10)
Assume that MOSFET conduction losses will represent total losses that Schottky diode
Allowing worst case heating effect, factor must added obtain data sheet specification 231/1.4 important remember that this value needs specified operation with input voltage Si9145, this only source gate drive. this case, would advisable select device with this on-resistance rated allow other losses series (such output stage tracking losses).
FIGURE Complete Boost Regulator Schematic
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FIGURE Boost Switching Waveforms gate charge losses should also considered. case boost converter, drain MOSFET switched same voltage output (ignoring diode voltage drop). gate charge losses MOSFET were equal half full load conduction losses, then these would approximately required gate charge would determined from:
(11)
FIGURE Boost Switching Waveforms (Expanded)
REFERENCES
Mohandes, Bijan. 1993. "Designing High Frequency DC-toDC Converters with Si9114." Siliconix Application Note. Riddley, Ray. 1992. POWER Power Supply Simulation Software. [Riddley Engineering, Jacaranda Drive, Battle Creek, Michigan 49017] Tel:+1-616-962-1181, Fax:+1-616-962-1180. Blattner, Robert. 1992. "High Efficiency Buck Converter Notebook Computers." Siliconix Application Note AN710. McLymann, Colonel "Designing Magnetic Components High Frequency dc-dc Converters", Magnetics, ISBN #1-883107-00-8 Coilcraft Cary, Illinois, IL60013, USA, USA: Tel:+1-708-639-6400, Fax:+1-708-639-1469 Europe: Tel:+44-236-730595, Fax:+44-236-730627 Hong Kong: Tel:+852-770-9428, Fax:+852-770-0729
Ideally, n-channel device with less than gate charge should selected. complete schematic converter shown Figure waveforms Figure Figure show typical results obtained.
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AN715
APPENDIX BUCK CONVERTER INDUCTOR DESIGN
Assuming that ideal components, then maximum input voltage duty cycle will
this case,
Therefore,
Specification requirements:
Input Voltage (VIN) VMIN, VMAX Output Voltage (VOUT)= Switching Frequency (FSW) Output Current (IOUT) Ripple Current (DIOUT) pk-pk Ripple Voltage (DVOUT) pk-pk
Then
-DIOUT
From basic electrical circuit theory, voltage across inductor given
value capacitor required output ripple needs
0.125
where voltage across inductor. maximum input voltage voltage across inductor
Therefore
ensure that ripple voltage exceeded, (Equivalent Series Resistance capacitor) needs less than:
practice, other factors such will also have effect output ripple, well noise, capacitors 10-µF range more practical.
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APPENDIX CONTINUOUS BOOST CONVERTER INDUCTOR DESIGN
calculate minimum maximum load resistance:
minimum required inductance calculated:
OMIN
Specification requirements design example:
Input Voltage (VIN) VMIN, VMAX Output Voltage (VOUT)= Switching Frequency (FSW) Output Current (IOUT) Ripple Voltage (DVOUT) (usually VOUT) Target efficiency
From equation above, minimum inductance required maintain continuous inductor current function input voltage. Inductance versus input voltage plotted graph below.
First calculate period operation:
Next calculate minimum maximum output power:
calculate maximum input current:
1.136 0.88
Calculate minimum maximum duty cycle:
0.215 0.25 DSQ1 0.545 0.25 DSQ1
From graph above, clearly that minimum inductance required operate continuous conduction mode throughout entire input voltage range must greater than inductance value greater than should used provide additional margin, will used this example.
peak current determined under worst case condition during minimum input voltage with maximum load.
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From this, current calculated:
DSQ1 0.25 0.545 0.319 PEAK PEAK 0.545 1.296A 1.296A 0.977A
0.842A
Output capacitance (equivalent series resistance) calculation: Boost converter's output ripple voltage determined output capacitance ESR. Equal contribution ripple voltage from capacitance will assumed this example.
0.545 µsec 9.091 0.03 0.023 1.296 PEAK
IIN(MAX) IIN(MAX) 1.136 0.319 0.977 IPEAK IPEAK 0.977 0.319 IPEAK 1.296
minimize effects, ceramic capacitors should used.
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AN715
APPENDIX STANDARD INDUCTOR SELECTION*
Buck boost inductors used design examples this literature were selected from Coilcraft "Surface-Mount Products" catalog. These devices suggested benefit designers. further information contact Coilcraft. Part Number
DT3316-102 DT3316-152 DT3316-222 DT3316-332 DT3316-472 DT3316-682 DT3316-103 DT3316-153 DT3316-223 DT3316-333 DT3316-473 DT3316-683 DT3316-104 DT3316-154 DT3316-224 DT3316-334 DT3316-474 DT3316-684 DT3316-105 DT1608-102 DT1608-152 DT1608-222 DT1608-332 DT1608-472 DT1608-682 DT1608-103 DT1608-153 DT1608-223 DT1608-333 DT1608-473 DT1608-683 DT1608-104 DT1608-154 DT1608-224 DT1608-334 DT1608-474 DT1608-684 DT1608-105
(µH)
1000 1000
(µH)
IDCMAX (mA)
0.35 0.25 0.35 0.25
1150 1450 1100 1400 2250 2900 3600 4550 8100
This data supplied information purposes only. Siliconix does recommend endorse suppliers components. Designers must determine suitability data supplier their applications.
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AN715
APPENDIX FIGURE FIGURE COMPONENT SPECIFICATIONS
Figure Synchronous Buck Converter Specifications
0.01 2200 Vishay Vitramon VJ1206A101KXAAT Vishay Vitramon Vishay Sprague Vishay Vitramon Vishay Vitramon VJ1206Y104KXAAT 2939225X9010A2T VJ1206Y104KXAAT VJ1206Y222KXAAT
Vendor
Vishay Sprague Vishay Vitramon Vishay Vitramon
Part
293D225X9010A2T VJ1206Y104KXAAT VJ1206Y103KXAAT
3.32 9.09 6.81 6.81 Vishay Dale Vishay Dale CRCW1206103JRT1 CRCW1206103JRT1 Vishay Dale Vishay Dale Vishay Dale CRCW12062210FRT1 CRCW12066811FRT1 CRCW12066811FRT1 Vishay Dale Vishay Dale Vishay Dale CRCW12063321FRT1 CRCW1206101JRT1 CRCW12069091FRT1
Vishay Dale
ILS-1206
DISC
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AN715
Figure Boost Converter Specifications
1800 (Optional) Vishay Vitramon Vishay Vitramon VJ1206A100KXAAT VJ1206A101KXAAT
Vendor
Vishay Sprague Vishay Vitramon Vishay Vitramon Vishay Vitramon Vishay Vitramon Vishay Sprague Vishay Vitramon Vishay Vitramon
Part
293D476X9010D2T VJ1206Y103KXXAT VJ1206Y223KXXAT VJ1206Y102KXXAT VJ1206Y104KXXAT 293D106X9025D2T VJ1206Y104KXXAT VJ1206Y182KXXAT
9.09 (Optional)
Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale
CRCW1206102JRT1 CRCW1206103JRT1 CRCW12065110FRT1 CRCW12061000FRT1 CRCW12069091FRT1 CRCW1206102JRT1 CRCW1206102JRT1 CRCW1206103JRT1 CRCW1206332JRT1
Vishay Dale Vishay Dale Vishay Dale
CRCW1206103JRT1 CRCW1206103JRT1 CRCW1206153JRT1
Vishay Dale
ILS-1206
DISC
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