SYNCHRONOUS-ittyStep-Down Converter MIC2159 high efficiency, simp

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SYNCHRONOUS-ittyStep-Down Converter MIC2159 high efficiency, simp

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MIC2159
SYNCHRONOUS-ittyStep-Down Converter
MIC2159 high efficiency, simple synchronous buck controller housed 10-pin MSOP ePAD package. MIC2159 switches 400kHz, allowing smallest possible external components designed drive loads 30A. devices feature high output driver capability, combined with nchannel synchronous architecture. MIC2159 operates from 14.5V input configured generate output voltages 0.8V. Efficiencies over achieved within smallest possible printed circuit board space area. MIC2159 available thermally capable 10-pin ePAD MSOP package, with junction operating range from -40°C +125°C.
Features
Small footprint 10-lead ePAD MSOP 14.5V input voltage range Adjustable output voltage down 0.8V 400KHz operation Drives n-channel MOSFETs Built-in drivers Simple control: voltage-mode Fast transient response Transient Hysteretic control Externally compensated Over-current protection Hiccup mode short-circuit protection Dual function COMP 50µA Short minimum time 30ns Very duty cycle possible
Applications
Point-of-Load DC/DC conversion High Current Power Supplies Telecom/datacom Networking Power Supplies Servers Workstations Graphics Cards other Peripherals
Typical Application
MIC2159 Adjustable Output 400kHz Converter
MicroLead Frame registered trademark Amkor Technologies Micrel Inc. 2180 Fortune Drive Jose, 95131 (408) 944-0800 (408) 474-1000 http://www.micrel.com
October 2006
M9999-101206
Micrel
MIC2159
Ordering Information
Part Number MIC2159YMME Output Voltage Frequency 400KHz Juction Temperature Range -40°C +125°C Package 10-lead ePAD MSOP
Configuration
10-lead e-PAD MSOP (MME)
Description
Number (MSOP-10) Name COMP/EN Function Supply voltage (Input): 14.5V Internal Linear Regulator (Output): external MOSFET gate drive supply voltage internal supply When should connected Vin. Current Sense. Current-limit comparator non-inverting input. current limit sensed across during time. current resistor series with pin. Compensation (Input): Dual function pin. external compensation. this pulled below 0.2V, with reference fully device shuts down (50µA typical current draw) Feedback (Input): Input error amplifier. Regulates error amplifier 0.8V. Ground (Return) Low-Side Drive (Output): High-current driver output external synchronous MOSFET. Switch (Return): High-side MOSFET driver return High-Side Drive (Output): High current output-driver high-side MOSFET. When below 2.5v threshold FETs should used. 4.5V threshold FETs should used. Boost (Input): Provides drive voltage high-side MOSFET driver. gate drive voltage higher than source voltage minus diode drop.
October 2006
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Micrel
MIC2159
Absolute Maximum Rating(1)
Supply Voltage (VIN) .15.5V Booststrapped Voltage (VBST) Junction Temperature Range.-40°C +125°C Ambient Storage Temp .-65°C +150°C
Operating Ratings(2)
Supply Voltage. +14.5V Output Voltage Range .0.8V VIN*DMAX Package Thermal Resistance 10-lead ePAD MSOP. 63°C/W
Electrical Characteristics
bold values indicate unless otherwise specified
Parameter Regulation Feedback Voltage Reference Feedback Voltage Reference Feedback Bias Current Output Voltage Line Regulation Output Voltage Load Regulation Output Voltage Total Regulation Oscillator Section Oscillator Frequency Maximum Duty Cycle Minimum On-Time Input Supply mode supply current Shutdown Quiescent Current (MSOP-10) VCOMP Shutdown Threshold (MSOP-10) VCOMP Shutdown Blanking Period (MSOP10) Digital Supply Voltage (VDD) Error Amplifier Gain Transconductance Soft Start Soft Start Current Current Sense Current Sense Over Current Trip Point After time internal timer VCS/EN =Vin- 0.25V; 0.7V (output switching excluding external MOSFET gate current.) VCOMP 0.25 0.35 Note 14.5V; IOUT (VOUT 2.5V) Note (+/- (+/- over temp) 0.792 0.784 0.03 0.808 0.816 Condition Units
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Temperature Coefficient Parameter Upper Threshold, VFB_OVT Lower Threshold, VFB_UVT Gate Drivers Rise/Fall Time Into 3000pF Source, VIN= 4.5V Sink, 4.5V Output Driver Impedance Source, VIN= Sink, Driver Non-overlap Time Note Condition Output Fault Correction Thresholds -3.5 2300
MIC2159
ppm/°C
Units
Exceeding absolute maximum rating damage device. device guaranteed function outside operating rating. Guaranteed design Devices sensitive. Handling precautions recommended. Human body model, 1.5k series with 100pF. Specification packaged product only.
October 2006
M9999-101206
Micrel
MIC2159
Typical Characteristics
October 2006
M9999-101206
Micrel
MIC2159
Block Diagram
MIC2159 Block Diagram
Functional Description
MIC2159 voltage mode, synchronous stepdown switching regulator controller designed high power. Current limit implemented without external sense resistor. includes internal soft-start function which reduces power supply input surge current start-up controlling output voltage rise time, generator, reference voltage, MOSFET drivers, short-circuit current limiting circuitry form complete 400kHz switching regulator. Theory Operation MIC2159 voltage mode step-down regulator. figure above illustrates block diagram voltage control loop. output voltage variation load line changes will sensed inverting input transconductance error amplifier feedback resistors compared reference voltage non-inverting input. This will cause small change voltage level output error amplifier which input comparator. other input comparator triangular waveform. comparator generates rectangular waveform whose width equal time from start clock cycle until time October 2006 triangle crosses output waveform error amplifier. illustrate control loop, assume output voltage drops sudden load turn-on, this would cause inverting input error amplifier, which divided down version VOUT, slightly less than reference voltage, causing output voltage error amplifier high. This will cause comparator increase time side MOSFET, causing output voltage bringing VOUT back regulation. this sudden load transient large enough cause change output voltage, then output Hysteretic comparator will bypass comparator drive outputs full Duty cycle order recover nominal output voltage fastest manner possible whilst fixed frequency switching maintained during normal loading. Soft-Start COMP/EN MIC2159 used following three functions: Disables part grounding this pin. External compensation stabilize voltage control loop. M9999-101206
Micrel Soft-start. better understanding soft-start feature, assume MIC2159 allowed power-up un-grounding COMP/EN pin. COMP internal 8.5µA current source that charges external compensation capacitor. soon this voltage rises 180mV Cap_COMP 0.18V/8.5µA), MIC2159 allows internal linear regulator power soon crosses under-voltage lockout 2.6V, chip's internal oscillator starts switching. this point time, COMP current source increases 40µA internal 12-bit counter starts counting, which takes approximately complete. During counting, COMP voltage clamped 0.65V. After this counting cycle COMP current source reduced 8.5µA COMP voltage rises from 0.65V 0.95V, bottom edge saw-tooth oscillator. This beginning duty cycle increases slowly causing output voltage rise slowly. MIC2159 hysteretic comparators that enabled when VOUT outside steady state. When output voltage reaches programmed output voltage then error amplifier enabled along with hysteretic comparator. From this point onwards, voltage control loop error amplifier) fully control will regulate output voltage. Soft-start time calculated approximately adding following four time frames: Cap_COMP 0.18V/8.5µA internal counter, approx Cap_COMP 0.3V/8.5µA
VOUT Cap_COMP 8.5µ.
MIC2159
hard short. circuit Figure illustrates MIC2159 current limiting circuit.
Figure MIC2159 Current Limiting Circuit
current limiting resistor calculated following equation:
DS(ON) 200µ
Equation
ILOAD Where:
IRIPPLE VOUT FSWITCHING
IRIPPLE VOUT
FSWITCHING 400kHz 200µA internal sink current program MIC2159 current limit. MOSFET RDS(ON) varies with temperature; therefore, recommended margin load current (ILOAD) above equation avoid false current limiting increased MOSFET junction temperature rise. also recommended connect resistor directly drain MOSFET resistor source accurately sense MOSFETs RDS(ON). make MIC2159 insensitive board layout noise generated switch node, resistor 1000pF capacitor recommended between switch node GND. 0.1µF capacitor parallel with should connected filter some switching noise.
Soft-Start Time(Cap_COMP=100nF) 2.1ms 3.5ms 1.8ms 10ms
Current Limit MIC2159 uses RDS(ON) power MOSFET measure output current. Since uses drain source resistance power MOSFET, very accurate. This MOSFET scheme adequate protect power supply external components during fault condition cutting back time MOSFET feedback voltage greater than 0.67V. case hard short when feedback voltage less than 0.67V, MIC2159 discharges COMP capacitor 0.65V, resets digital counter automatically shuts gate drive, error amplifier hysteretic comparators completely disabled soft-start cycles restarts. This mode operation called "hiccup mode" purpose protect down stream load case October 2006
Internal Supply MIC2159 controller internally generates self biasing provide power gate drives. This supply generated through low-dropout regulator generates from supply greater than supply voltage less than linear regulator approximately 200mV dropout. Therefore, recommended short supply input supply through resistor input supplies between M9999-101206
Micrel 2.9V
MIC2159 switch turned back CBST recharged through drive voltage derived from internal bias supply. nominal low-side gate drive voltage nominal high-side gate drive voltage approximately 4.5V voltage drop across approximate 50ns delay between low-side(off) high-side(on) driver transitions used prevent current from simultaneously flowing unimpeded through both MOSFETs (shoot-through). Adaptive gate drive implemented high-side(off) low-side(on) driver transition reduce losses flywheel diode prevent shoot-through. This operated detecting pin; once this detected reach 1.5v, high-side MOSFET assumed side driver enabled.
MOSFET Gate Drive MIC2159 high-side drive circuit designed switch N-Channel MOSFET. Functional Block Diagram shows bootstrap circuit, consisting CBST. This circuit supplies energy high-side drive circuit. Capacitor CBST circuit charged while lowside MOSFET voltage approximately When high-side MOSFET driver turned energy from CBST used turn MOSFET MOSFET turns voltage increases approximately VIN. Diode reversed biased CBST floats high while continuing keep high-side MOSFET When low-side
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MIC2159 Lower numbers translate into higher efficiency. gate-charge logic-level MOSFETs good choice with MIC2159. Parameters that important MOSFET switch selection are: Voltage rating On-resistance
Application information
MOSFET Selection MIC2159 controller works from input voltages 14.5V internal regulator provide power turn external N-Channel power MOSFETs high- low-side switches. applications where internal regulator operates dropout mode, necessary that power MOSFETs used sub-logic level full conduction mode 2.5V. applications when logiclevel MOSFETs, whose operation specified 4.5V must used. lower (<5v) applications, supply connected directly help increase driver voltage MOSFET. important note on-resistance MOSFET increases with increasing temperature. 75°C rise junction temperature will increase channel resistance MOSFET resistance specified 25°C. This change resistance must accounted when calculating MOSFET power dissipation calculating value current-sense (CS) resistor. Total gate charge charge required turn MOSFET under specified operating conditions (VDS VGS). gate charge supplied MIC2159 gate-drive circuit. 400kHz switching frequency above, gate charge significant source power dissipation MIC2159. output load, this power dissipation noticeable reduction efficiency. average current required drive high-side MOSFET IG[high-side](avg)=QG Where: IG[high-side](avg)=Average high-side MOSFET gate current total gate charge high-side MOSFET taken from manufacturer's data sheet Switching Frequency (400kHz) low-side MOSFET turned because freewheeling diode conducting during this time. switching loss low-side MOSFET usually negligible. Also, gate-drive current low-side MOSFET more accurately calculated using CISS instead gate charge. low-side MOSFET: IG[low-side](avg) CISS Since current from gate drive comes from input voltage, power dissipated MIC2159 gate drive PGATEDRIVE VIN.(IG[high-sde](avg) IG[low-side](avg)) convenient figure merit switching MOSFETs resistance times total gate charge RDS(ON)
October 2006
Total gate charge voltage ratings bottom MOSFET essentially equal input voltage. safety factor should added VDS(max) MOSFETs account voltage spikes circuit parasitic elements. power dissipated switching transistor conduction losses during on-time (PCONDUCTION) switching losses that occur during period time when MOSFETs turn (PAC). PCONDUCTION Where: PCONDUCTION (RMS
PAC(off PAC(on
on-resistance MOSFET switch duty cyle Making assumption turn-on turn-off transition times equal; transition times approximated where: CISS COSS measured gate-drive current (1.4A MIC2159) total high-side MOSFET switching loss (VIN Where: Switching transition time (~20ns) Freewheeling diode drop (0.5v) Switching Frequency (400kHz) low-side MOSFET switching losses negligible ignored these calculations.
Inductor Selection Values inductance, peak, currents required select output inductor. input output voltages inductance value determine peak-to-peak inductor ripple current. Generally, higher
M9999-101206
Micrel inductance values used with higher input voltages. Larger peak-to-peak ripple currents will increase power dissipation inductor MOSFETs. Larger output ripple currents will also require more output capacitance smooth larger ripple current. Smaller peak-to-peak ripple currents require larger inductance value therefore larger more expensive inductor. good compromise between size, loss cost inductor ripple current equal maximum output current. inductance value calculated equation below. VOUT (max) VOUT (max) IOUT (max) where: switching frequency, ratio ripple current output current VIN(max) maximum input voltage peak-to-peak inductor current ripple current) VOUT .(VIN (max) VOUT (max).FS calculated equation below: PINDUCTORCu IINDUCTOR )2.RWINDING
MIC2159
resistance copper wire, RWINDING, increases with temperature. value winding resistance used should operating temperature. RWINDING( RWINDING( 0.0042 THOT Where: THOT temperature wire under full load T20oC ambient temperature RWINDING(20oC) room temperature winding resistance (usually specified manufacturer)
Output Capacitor Selection output capacitor values usually determined capacitors (equivalent series resistance). Voltage current capability other important factors selecting output capacitor. Recommended capacitors tantalum, low-ESR aluminium electrolytic, POSCAPS. output capacitor's usually main cause output ripple. output capacitor also affects overall voltage feedback loop from stability point view. "Feedback Loop Compensation" section more information. maximum value calculated:
RESR VOUT
peak inductor current equal average output current plus half peak-to-peak inductor ripple current. IOUT(max)+0.5 inductor current used calculate losses inductor.
IINDUCTOR IOUT (max) IOUT (max)
Where: VOUT peak-to-peak output voltage ripple peak-to-peak inductor ripple current total output ripple combination output capacitance. total ripple calculated below:
VOUT
Maximizing efficiency requires proper selection core material minimizing winding resistance. high frequency operation MIC2159 requires ferrite materials most cost sensitive applications. Lower cost iron powder cores used increase core loss will reduce efficiency power supply. This especially noticeable output power. winding resistance decreases efficiency higher output current levels. winding resistance must minimized although this usually comes expense larger inductor. power dissipated inductor equal core copper losses. higher output loads, core losses usually insignificant ignored. lower output currents, core losses significant contributor. Core loss information usually available from magnetics vendor. Copper loss inductor October 2006
Where: duty cycle COUT output capacitance value switching frequency voltage rating capacitor should twice voltage tantalum greater aluminium electrolytic. output capacitor current calculated below: ICOUT power dissipated output capacitor
PDISS (COUT ICOUT RESR (COUT
M9999-101206
Micrel Note that Vout should kept within nominal limits prevent loop switching between hysteretic voltage mode control. VREF VREF
MIC2159
Input Capacitor Selection input capacitor should selected ripple current rating voltage rating. Tantalum input capacitors fail when subjected high inrush currents, caused turning input supply tantalum input capacitor's voltage rating should least times maximum input voltage maximize reliability. Aluminium electrolytic, OS-CON, multilayer polymer film capacitors handle higher inrush currents without voltage de-rating. input voltage ripple will primarily depend input capacitor's ESR. peak input current equal peak inductor current,
External Schottky Diode external freewheeling diode used keep inductor current flow continuous while both MOSFETs turned off. This dead time prevents current from flowing unimpeded through both MOSFETs typically ~50ns. diode conducts twice during each switching cycle. Although average current through this diode small, diode must able handle peak current.
ID(avg) IOUT 50ns
reverse voltage requirement diode VDIODE(rrm) power dissipated Schottky diode PDIODE ID(avg) Where: forward voltage peak diode current external Schottky diode, necessary circuit operation since low-side MOSFET contains parasitic body diode. external diode will improve efficiency decrease high frequency noise. MOSFET body diode used, must rated handle peak average current. body diode relatively slow reverse recovery time relatively high forward voltage drop. power lost diode proportional forward voltage drop diode. high-side MOSFET starts turn body diode becomes short circuit reverse recovery period, dissipating additional power. diode recovery circuit inductance will cause ringing during high-side MOSFET turn-on. external Schottky diode conducts lower forward voltage preventing body diode MOSFET from turning lower forward voltage drop dissipates less power than body diode. lack reverse recovery mechanism Schottky diode causes less ringing less power loss. Depending circuit components operating conditions, external Schottky diode will give 1/2% improvement efficiency.
IINDUCTOR( peak RESR(CIN
input capacitor must rated input current ripple. value input capacitor current determined maximum output current. Assuming peak-to-peak inductor ripple current low:
IOUT (max)
power dissipated input capacitor PDISS (CIN ICIN )2.RESR (CIN
Voltage Setting Components MIC2159 requires resistors output voltage shown Figure
Figure Voltage-Divider Configuration
output voltage determined equation:
VREF
typical value between 10k. large, allow noise introduced into voltage feedback loop. small, value, will decrease efficiency power supply, especially light loads. Once selected, calculated using:
Feedback Loop Compensation MIC2159 controller comes with internal transconductance error amplifier used compensating voltage feedback loop placing capacitor (C1) series with resistor (R1) another capacitor parallel from COMP ground. "MIC2159 Block Diagram." Power Stage power stage voltage mode controller inductor, with winding resistance (DCR)
M9999-101206
October 2006
Micrel connected output capacitor, COUT, with electrical series resistance (ESR) shown Figure transfer function G(s), such system
MIC2159
gain curve that output inductor capacitor create pole system with break frequency
COUT
Therefore, 3.6kHz looking phase curve, seen that output capacitor (0.050) cancels system poles (LCOUT) introducing zero
Figure Output Filter voltage Mode Buck Converter
fZERO=
COUT
G(s)=
s(DCR COUT+ COUT) s2(L COUT)
COUT
Plotting this transfer function with following assumed values (L=2 DCR=0.009, COUT=1000µF, ESR=0.050) gives insight needs compensate loop adding resistor capacitors COMP pin. Figures show gain curve phase curve above transfer function.
Therefore, FZERO 6.36kHz. From point view compensating voltage loop, recommended higher output capacitors since they provide phase gain power path. comparison purposes, Figure shows same phase curve with value 0.002.
Figure Phase Curve with 0.002
Figure Gain Curve G(s)
seen from Figure that kHz, phase approximately -90° versus Figure where number -150°. This means that transconductance error amplifier provide phase boost about achieve closed loop phase margin crossover frequency 50kHz Figure versus 105° Figure simple R1,C1 compensation scheme allows maximum error amplifier phase boost about 90°. Therefore, easier stabilize MIC2169A voltage control loop using high value output capacitors.
Figure Phase Curve G(s)
seen from transfer function G(s)
October 2006
Error Amplifier undesirable have high error amplifier gain high frequencies because high frequency noise spikes would picked transmitted large amplitude output, thus, gain should permitted fall high frequencies. frequency, desired have high open-loop gain attenuate power line ripple. Thus, error amplifier gain should allowed increase rapidly frequencies.
M9999-101206
Micrel transfer function with internal error amplifier approximated following equation:
Error Amplifier(z) (C1+
MIC2159
above equation simplified assuming C2<<C1,
Error Amplifier
From above transfer function, that introduce zero pole following frequencies: Fzero= Fpole Fpole@origin Figures show gain phase curves above transfer function with 9.3k, 1000pF, 100pF, .005-1. seen that kHz, error amplifier exhibits approximately phase margin.
Figure Error Amplifier Phase Curve
Total Open-Loop Response open-loop response MIC2159 controller easily obtained adding power path error amplifier gains together, since they already scale. desirable have gain curve intersect zero tens kilohertz, this commonly called crossover frequency; phase margin crossover frequency should least 45°. Phase margins less cause power supply have substantial ringing when subjected transients, have little tolerance component environmental variations. Figures show open-loop gain phase margin seen from Figure that gain curve intersects approximately 50kHz, from Figure that 50kHz, phase shows approximately margin.
Figure Error Amplifier Gain Curve
Figure Open-Loop Gain Margin
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MIC2159
Figure Open-Loop Phase Margin
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MIC2159 bottom MOSFET gate drives. necessary, gate resistors less should used. gate charge MOSFETs should used maximize efficiency. Compensation component GND, feedback resistor ground, chip ground should together connect output capacitor ground. demo board layout, bottom layer. 10µF ceramic capacitor should placed between drain MOSFET source bottom MOSFET. 10µF ceramic capacitor should placed right without vias. source bottom MOSFET should connect directly input capacitor with thick trace. output capacitor input capacitor should connect directly plane. Place 0.01µF 0.1µF Ceramic capacitor parallel with resistor filter switching noise.
Design Example
Layout Checklist: Connect current limiting resistor directly drain MOSFET. resistor from input supply MIC2159. Also, place ceramic Capacitor from this GND, preferably thru via. feedback resistors should placed close pin. side resistor should connect directly output node. this trace away from switch node. bottom side lower resistor should connect MIC2159. compensation resistor capacitors should placed right next COMP other side should connect directly MIC2159 rather than going plane. resistor 1000pF capacitor from switch node ground pin. page Current Limiting section more detail. place holders gate resistors
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MIC2159
Package Drawing
10-Lead e-PAD MSOP (MME)
MICREL, INC. 2180 FORTUNE DRIVE JOSE, 95131
(408) 944-0800 (408) 474-1000 http:/www.micrel.com
information furnished Micrel this data sheet believed accurate reliable. However, responsibility assumed Micrel use. Micrel reserves right change circuitry specifications time without notification customer. Micrel Products designed authorized components life support appliances, devices systems where malfunction product reasonably expected result personal injury. Life support devices systems devices systems that intended surgical implant into body support sustain life, whose failure perform reasonably expected result significant injury user. Purchaser's sale Micrel Products life support appliances, devices systems Purchaser's risk Purchaser agrees fully indemnify Micrel damages resulting from such sale. 2004 Micrel, Incorporated.
October 2006
M9999-101206
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MIC2159
Revision History
Date
6/15/06
Revision
Edits
Mclean
Description Change Initial Spec Coversion Micrel Format
October 2006
M9999-101206

SYNCHRONOUS-ittyStep-Down Converter MIC2159 high efficiency, simp

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