MIC2185 Voltage Synchronous Boost Control Final Information

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MIC2185 Voltage Synchronous Boost Control Final Information

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MIC2185
MIC2185
Voltage Synchronous Boost Control
Final Information
General Description
Micrel's MIC2185 high efficiency synchronous boost control With wide input voltage range 2.9V 14V, MIC2185 used efficiently boost voltages 2-cell battery powered applications, well fixed 3.3V systems. powerful output drivers allow MIC2185 supply large output currents with selection proper external MOSFETs. With it's fixed frequency architecture, easily synchronized drive, MIC2185 ideal noise-sensitive telecommunications applications. nominal 400kHz operating frequency MIC2185 divided two, allowing device externally synchronized frequencies below 400kHz. MIC2185 also features current shutdown mode programmable undervoltage lockout. skipped pulse mode operation manually achieve higher efficiencies light load conditions. MIC2185 available SOIC package with ambient temperature operating range from -40°C 85°C.
Features
Input voltage range: 2.9V efficiency Oscillator frequency 200kHz/400kHz Frequency sync 600kHz 0.5µA shutdown current output drivers Front edge blanking Current Mode Control Cycle-by-Cycle current limiting Frequency foldback protection Adjustable under-voltage lockout Precision 1.245V reference output SOIC narrow body package 3.3V conversion telecom systems Satellite Phones Cable Modems 1-and 2-cell battery operated equipment
Applications
Ordering Information
Part Number MIC2185BM Frequency Voltage 200/400kHz Junction Temp. Range -40°C +125°C Package 16-lead
Typical Application
3.3V
2.4µH
Si9803DY (x2)
VOUT
VINA EN/UVLO FREQ/2 COMP VREF SYNC SKIP
OUTP VINP MIC2185
COUT
Output Efficiency
OUTN
Si4884DY (x2)
EFFICIENCY
3.3V 200kHz OUTPUT CURRENT
PGND SGND
Adjustable Output Synchronous Boost Converter
Micrel, Inc. 1849 Fortune Drive Jose, 95131 (408) 944-0800 (408) 944-0970 http://www.micrel.com
January 2002
MIC2185
MIC2185
Configuration
VINA SKIP COMP SGND EN/UVLO VREF VINP FREQ/2 OUTP OUTN PGND SYNC
Description
Number Name VINA SKIP
16-pin Narrow Body
Function Input voltage control circuitry (2.9V 14V). Skip (Input): Regulator operates mode pulse skipping) when pulled low, skip mode when raised VDD. There automatic switching between skip mode available this device. Soft Start (External Component) Reduces inrush current delays slows output voltage rise time. current source will charge capacitor VDD. Compensation (Output): Internal error amplifier output. Connect capacitor series network compensate regulator's control loop. Small Signal Ground (Return) Must routed separately from other grounds terminal COUT. Feedback (Input) Regulates 1.245V. Enable/Undervoltaqe Lockout (Input): level this will power down device, reducing quiescent current under 0.5µA. This separate thresholds, below 1.5V (typical) output switching disabled, below 0.9V (typical) device forced into complete micropower shutdown. 1.5V threshold functions accurate undervoltage lockout (UVLO) with 140mV hysteresis. Voltage Reference (Output) 1.245V reference available this pin. 0.1µF capacitor should connected form this SGnd. Current Sense (Input) input current limit comparator. built offset 100mV (typical) between SGnd conjunction with current sense resistor sets current limit threshold level. This also input current amplifier. Internal Linear-Regulator (Output) also supply voltage chip. Bypass SGND with 1µF. Maximum source current 0.5mA. Frequency Synchronization (Input): Connect external clock signal synchronize oscillator. Leading edge signal above 1.4V (typical) starts switching cycle. Connect SGND used. MOSFET Driver Power Ground (Return) Connects bottom current sense resistor terminal CIN. N-Channel Drive (Output) High current drive n-channel MOSFET. Voltage swing from ground VINP. On-resistance typically P-Channel Drive (Output) High current drive synchronous p-channel MOSFET. Voltage swing from ground VINP. On-resistance typically Frequency Divider (Input) When this low, oscillator frequency 400KHz. When this raised VDD, oscillator frequency 200KHz. Gate Drive Voltage (Input) This power input gate drive circuitry (2.9V 14V). This typically connected output voltage enhance gate drive.
COMP SGND EN/UVLO
VREF
SYNC
PGND OUTN OUTP
FREQ/2 VINP
MIC2185
January 2002
MIC2185
Absolute Maximum Ratings (Note
Supply Voltage (VINA, VINP) Digital Supply Voltage (VDD) Skip Voltage (VSKIP) -0.3V Comp Voltage (VCOMP) -0.3V Feedback Voltage (VFB) -0.3V Enable Voltage (VEN/UVLO) -0.3V Current Sense Voltage (VCSH) -0.3V Sync Voltage (VSYNC) -0.3V Freq/2 Voltage (VFREQ/2) -0.3V Power Dissipation (PD) 400mW 85°C Ambient Storage Temp -65°C +150°C
Operating Ratings (Note
Supply Voltage (VINA, VINP) +2.9V +14V Operating Ambient Temperature. -40°C +85°C Junction Temperature -40°C +125°C PackageThermal Resistance 16-lead 100°C/W Rating, Note
Electrical Characteristics
VINA= VinP= VOUT=12V, VEN/UVLO VSKIP VFREQ/2 VCSH 25°C, unless otherwise specified. Bold values indicate -40°C +125°C. Parameter Regulation Feedback Voltage Reference (±1%) (±2%) VINA 75mV; (±3%) Feedback Bias Current Output Voltage Line Regulation Output Voltage Load Regulation Input Supply VINA Input Current, mode VINP Input Current, mode VINA Input Current, SKIP mode Shutdown Quiescent Current Digital Supply Voltage (VDD) Digital Supply Load Regulation Undervoltage Lockout VSKIP VSKIP (excluding external MOSFET gate current) VSKIP VEN/UVLO (IVINA IVINP) 0.5mA upper threshold (turn threshold) lower threshold (turn threshold) Reference Output (VREF) Reference Voltage (±1.5%) (±2.5%) Reference Voltage Line Regulation Reference Voltage Load Regulation Enable/UVLO Enable Input Threshold UVLO Threshold UVLO Hysteresis Enable Input Current VEN/UVLO VinA IREF 100µA 1.226 1.213 1.245 1.264 1.276 0.03 2.75 2.65 VINA 75mV 1.233 1.220 1.208 1.245 +0.08 -1.2 1.245 1.258 1.270 1.282 Condition Units
January 2002
MIC2185
MIC2185
Parameter Soft Start Soft Start Current Current Limit Current Limit Threshold Voltage Error Amplifier Error Amplifier Gain Current Amplifier Current Amplifier Gain SKIP Input SKIP Threshold SKIP Input Current Oscillator Section Oscillator Frequency (fS) Maximum Duty Cycle Minimum Time FREQ/2 frequency (fS) Frequency Foldback Threshold Frequency Foldback Frequency SYNC Threshold Level SYNC Input Current SYNC Minimum Pulse Width SYNC Capture Range Gate Drivers (OUTN OUTP) Rise/Fall Time Driver Non-overlap Time 3300pF VINP VINP Output Driver Impedance Source; VINP Sink; VINP Source; VINP Sink; VINP
Note
Condition Unit
Voltage trip current limit
VSKIP 1.0V 1.5V VFREQ/2=3V Measured VFREQ/2=0V
Note fO+15
Absolute maximum ratings indicate limits beyond which damage component occur. Electrical specifications apply when operating device outside operating ratings. maximum allowable power dissipation function maximum junction temperature, TJ(max), junction-to-ambient thermal resistance, ambient temperature, device guaranteed function outside operating rating. Devices sensitive. Handling precautions recommended. application information limitations maximum operating frequency.
Note Note Note
MIC2185
January 2002
MIC2185
Typical Characteristics
Quiescent Current Temperature (SKIP Mode)
VINA 5VDC VINP 12VDC IQVINA+IQVINP
IQ(PWM) (mA)
Quiescent Current Temperature (PWM Mode)
4.75 4.70 4.65 4.60 4.55 4.50 4.45 VINA 5VDC 12VDC 4.40 400kHz 4.35 QVINA+IQVINP 4.30
(mA)
Quiescent Current Input Voltage(PWM Mode)
=12V IQVINA+IQVINP INPUT VOLTAGE (VINA) 200kHz 400kHz
IQ(SKIP) (mA)
100120140 TEMPERATURE (°C)
TEMPERATURE (°C)
Quiescent Current Input Voltage (PWM Mode)
IQ(PWM) (mA)
(mA)
Quiescent Current Input Voltage (SKIP Mode)
1.2475
REFERENCE VOLTAGE
Reference Voltage Input Voltage
VINP 12VDC
VINP
0.75 VINP 12VDC
1.247 1.2465 1.246 1.2455 1.245
VINP
0.65 VINP 5VDC 0.55 INPUT VOLTAGE (VINA) VINP 9VDC
VINP
IQVINA IQVINP 400kHz INPUT VOLTAGE (VINA)
INPUT VOLTAGE (VINA)
Reference Voltage Reference Current
1.2470 1.257 REFERENCE VOLTAGE 1.255 1.253 1.251 1.249 1.247 1.245 1.243 1.241 1.239
Reference Voltage Temperature
VINP 12VDC VINA 5VDC 3.15 3.10 3.05 3.00 2.95 2.90 2.85 2.80
Input Voltage
REFERENCE VOLTAGE
1.2469 1.2468 1.2467 1.2466 1.2465 1.2464 1.2463 1.2462 1.2461 1.2460 90100 REFERENCE CURRENT (µA)
VINP 12VDC INPUT VOLTAGE (VINA)
1.237 100120140 TEMPERATURE (°C)
Load Current
3.040 VINP 12VDC 3.035 3.030 VINA 5VDC 3.08 3.07
Temperature
VINP 12VDC 3.06 VINA 5VDC 3.05 3.04 3.03 3.02 3.01 3.00 2.99 2.98
IENABLE VENABLE
VINP 12VDC -40°C
VINA 5VDC IENABLE (µA) VENABLE 20°C 85°C
3.025 3.020 3.015 3.010 0.30.4 0.60.7 IVDD (mA)
TEMPERATURE (°C)
January 2002
MIC2185
MIC2185
Oscillator Frequency Input Voltage
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
Oscillator Frequency Temperature
SOFT START CURRENT (µA)
Soft Start Current Temperature
5.15 5.10 5.05 5.00 4.95 4.90 4.85
12VDC INPUT VOLTAGE (VINA)
VINP 12VDC VINA 5VDC
VINP 12VDC VINA 5VDC
4.80
TEMPERATURE (°C)
TEMPERATURE (°C)
OVERCURRENT THRESHOLD (mV)
Overcurrent Threshold VINA
VINP 12VDC VINA
MIC2185
January 2002
MIC2185
Functional Diagram
CDECOUP
VINA VINP
EN/UVLO
VREF Bias
OUTP
VOUT
SKIP
Control fs/4
Overcurrent Reset
COUT PGND
Reset
FREQ/2
OUTN
SYNC
PGND Comparator
Correction Ramp
0.1V
Overcurrent Comparator
Gain
Error Amplifier COMP
RSENSE PGND
VREF
0.0002 Gain
PGND
100k
0.3V
fs/4 VREF
VREF Frequency Foldback SGND
SGND
Figure MIC2185 Mode Block Diagram
Functional Description
MIC2185 BiCMOS, switched mode, synchronous boost (step control synchronous switched, high side P-channel MOSFET, placed parallel with output diode, improves efficiency boost converter. lower voltage drop across MOSFET reduces power dissipation increases efficiency. Current mode control used achieve superior transient line load regulation. internal corrective ramp provides slope compensation stable operation above duty cycle. controller optimized high efficiency, high performance DC-DC converter applications. Figure block diagram MIC2185 configured synchronous boost converter. switching cycle January 2002
starts when OutN goes high turns side, Nchannel MOSFET, MOSFET equal VinP. This forces current ramp inductor. inductor current flows through current sense resistor, Rsense. voltage across resistor amplified combined with internal ramp stability. This signal compared with comp output signal error amplifier. When current signal equals error voltage signal, side MOSFET turned off. inductor current then flows through diode, output. delay between turn-off side MOSFET turn-on high side MOSFET prevents both MOSFETs from being same time, which would short output ground. non-overlap time, OutP pulls gate MOSFET ground, turning high side, P-channel MIC2185
MIC2185
MOSFET, Current flows through MOSFET because voltage drop less than diode MOSFET remains until switching cycle. There another nonoverlap time delay between turn-off high side MOSFET turn-on side MOSFET beginning next switching cycle. description MIC2185 controller broken down into basic functions. Control Loop Operation SKIP Mode Operation Current Limit MOSFET gate drive Reference, Enable UVLO Oscillator Sync Soft Start
CDECOUP
VINA
Control Loop SKIP modes operation MIC2185 operate either (pulse width modulated) skip mode. efficiency boost converter improved lower output loads manually selecting skip mode operation. potential disadvantage skip mode variable switching frequency that accompanies this mode operation. occurrence switching pulses depends component values well line load conditions. mode best choice operation higher output loads. skip mode, current through inductor settle zero, causing voltage ringing across inductor. mode advantages lower output ripple voltage higher efficiencies higher output loads. Another advantage synchronous mode operation that inductor current always continuous, even
VINP EN/UVLO
VREF Bias
VOUT
OUTP SKIP
Control PGND
F/2=H, off-time F/2=L, off-time
COUT
OUTN
50mV
FREQ/2
PGND
SYNC
Current Reset
Skip Current Limit Comparator
RSENSE PGND
COMP
PGND VREF 1.245V
VREF
VREF Hysteresis Comparator SGND
SGND
Figure MIC2185 Skip Mode Block Diagram MIC2185 January 2002
MIC2185
zero output current. This reduces parasitic ringing that occurs during discontinuous mode operation found lightly loaded, non-synchronous boost converters. Pulling SKIP (pin will force controller operate mode load conditions. Pulling SKIP high will force controller operate SKIP mode. Skip Mode Operation This control method used improve efficiency output loads. block diagram MIC2185 skip mode shown Figure power drawn MIC2185 control (IVINA VVINA) (IVINP VVINP). power dissipated significant portion total output power during periods output current, which lowers efficiency power supply. skip mode MIC2185 lowers supply current disabling high side drive running lower than switching frequency. also turns portions control drive circuitry when switching. disadvantage this method greater output ripple variable switching frequency. Soft Start Sync pins have effect when operating skip mode. skip mode, switching starts when feedback voltage drops below lower threshold level hysteresis comparator. OutN goes high, turning N-channel MOSFET, Current ramps inductor until either SKIP mode current limit comparator hysteretic voltage comparator turns Q1's gate drive. feedback voltage exceeds upper hysteretic threshold, Q1's gate drive terminated. voltage exceeds skip mode current limit threshold, terminates gate drive that switching cycle. gate drive remains constant period each switching cycle. This time period typically when when high. Figure shows some typical switching waveforms SKIP mode.
SKIP Mode Waveform
Switch Node Voltage (Low Side Drain) 5V/div Side Gate Drive 5V/div VOUT Ripple Voltage 200mV/div Inductor Current 5A/div 3.3V VOUT IOUT 0.55A TIME (50µs/div)
decrease until feedback voltage drops below lower threshold voltage limit. switching converter then turns gate drive back While gate drive disabled, MIC2185 draws less supply current then while switching, thereby improving efficiency output loads. Figure shows improvement efficiency that SKIP mode makes when lower output currents.
MIC2185 Skip Mode Efficiency
400kHz SKIP
EFFICIENCY
VIN=3.3V VOUT=5V
0.02 0.04
0.06
0.08
0.12 0.14
0.16 0.18
OUTPUT CURRENT
Figure maximum peak inductor current depends skip current limit threshold value current sense resistor, RSENSE. typical 50mV current limit threshold skip mode, peak inductor current
IINDUCTOR_pk
50mV RSENSE
maximum output current skip mode depends input conditions, output conditions circuit component values. Assuming discontinuous mode where inductor current starts from zero each cycle, maximum output current calculated below:
IO(max)=
RSENSE
where: IO(max) maximum output current output voltage input voltage value boost inductor switching frequency efficiency boost converter RSENSE value current sense resistor constant based skip mode current threshold (50mV)2
Figure SKIP mode waveforms skip mode current threshold limits peak inductor current cycle. Depending input, output circuit parameters, many switching cycles occur before feedback voltage exceeds upper hysteretic threshold. Once voltage feedback exceeds upper hysteretic threshold gate drive disabled. output load discharges output capacitance causing Vout
January 2002
0.20
MIC2185
MIC2185
Operation
Mode Waveform
3.3V Switch Node Voltage (Low Side Drain) 5V/div High Side Gate Drive 5V/div Side Gate Drive 5V/div Inductor Current offset; 0.5A/div VOUT Ripple Voltage 200mV/div VOUT IOUT 0.75A
required current mode control prevent unstable operation duty cycles greater than 50%. transconductance amplifier used error amplifier, which compares attenuated output voltage with reference voltage. output error amplifier compared current sense waveform block. When current signal rises above error voltage, comparator turns side drive. error signal brought COMP (pin allowing external components stabilize voltage loop. Current Sensing Overcurrent Protection inductor current sensed during switch time current sense resistor located between source MOSFET, ground (RSENSE Figure Exceeding current limit threshold will immediately terminate gate drive N-channel MOSFET. This forces operate reduced duty cycle, which reduces output voltage. boost converter, overcurrent limit will protect power supply load during severe overcurrent condition short circuit condition. output short-circuited ground, current will flow from input, through inductor output diode,D1, ground. Only impedance source components limits current. minimum input voltage, maximum output power minimum value current limit threshold determine value current sense resistor. switch, synchronous operation MIC2185 forces converter always operate continuous mode because current flow both ways through high side P-channel MOSFET. equations below will help determine current sense resistor value. Maximum Peak Current peak inductor current equal average inductor current plus half peak peak inductor current. peak inductor current
IIND(pk)= IIND(ave)
I_inductor
TIME (1µs/div)
Figure mode waveforms Figure shows typical waveforms mode operation. gate drive signal turns external side MOSFET, allowing inductor current ramp When side MOSFET turns high side MOSFET, turns current flowing inductor forces MOSFET drain voltage rise until clamped approximately output voltage. MIC2185 uses current mode control improve output regulation simplify compensation control loop. Current mode control senses both output voltage (outer loop) inductor current (inner loop). uses inductor current output voltage determine duty cycle buck converter. Sampling inductor current effectively removes inductor from control loop, which simplifies compensation. simplified current mode control diagram shown figure
I_inductor
Voltage Divider
I_inductor
Gate Driver VREF
IIND(pk)=
I_inductor
(VIN
IIND(pp)
VCOMP
Gate Drive OUTN
TPER
Figure Control Loop block diagram MIC2185 current mode control loop shown Figure inductor current sensed measuring voltage across resistor, Rsense. current sense amplifier buffers amplifies this signal. ramp added this signal provide slope compensation, which MIC2185
where: maximum output current output voltage minimum input voltage value boost inductor switching frequency efficiency boost converter voltage across inductor approximated higher input voltage. However, voltage drop across inductor winding resistance side MOSFET on-resistance must accounted lower input voltages that MIC2185 operate January 2002
MIC2185
WINDING RDS(ON)
where: RWINDING winding resistance inductor RDS(ON) resistance side switching MOSFET maximum value current sense resistor RSENSE VSENSE IIND(pk)
where: VSENSE minimum current sense threshold current sense pin, CSH, noise sensitive signal level. current sense voltage measurement referenced signal ground MIC2185. current sense resistor ground should located close ground. Make sure there high currents flowing this trace. trace between high side current sense resistor should also short routed close ground connection. input internal current sense amplifier 30nS dead time beginning each switching cycle. This dead time prevents leading edge current spikes from prematurely terminating switching cycle. small filter between current sense current sense resistor help attenuate larger switching spikes high frequency switching noise. Adding filter slows down current sense signal, which effect slightly raising overcurrent limit threshold. MOSFET Gate Drive MIC2185 synchronous boost converter drives both high side side MOSFET. side drive, OUTN, drives n-channel MOSFET. high-side drive, OUTP, designed switch p-channel MOSFET (the p-channel MOSFET doesn't require bootstrap circuit which would needed drive n-channel MOSFET). VINP must connected output, which provides power drive high side MOSFETs. skip mode, high side MOSFET disabled forcing OUTP high (equal VOUT). MOSFET Selection boost converter, MOSFET, approximately equal output voltage. maximum rating MOSFET must high enough allow ringing spikes. MIC2185 input voltage range 2.9V 14V. MOSFETs with ratings ideal with this part. n-channel gate drive voltage supplied OUTN output. startup boost converter, output voltage equals input voltage. threshold voltage n-channel MOSFET must enough operate minimum input voltage guarantee boost converter will start p-channel MOSFET must have minimum threshold voltage equal lower than output voltage. Five volt threshold (logic level) MOSFETs recommended p-channel MOSFET. Ringing gate drive signal January 2002
cause MOSFETs with lower gate thresholds erroneously turn There limit maximum amount gate charge MIC2185 will drive. Higher gate charge will slow down turn-on turn-off times MOSFETs. MOSFET's must able completely turn within driver non-overlap time shoot-through will occur. MOSFET gate charge also limited power dissipation MIC2186. power dissipated gate drive circuitry calculated below: PGATE_DRIVE =QGATE VINP where: QGATE total gate charge both external p-channel MOSFETs. graph Figure shows total gate charge which driven MIC2185 over input voltage range, different values switching frequency.
Frequency Maximum Gate Charge
TOTAL GATE CHARGE (nC) 500kHz INPUT VOLTAGE 600kHz 400kHz 200kHz 300kHz
Figure MIC2185 Frequency Max. Gate Charge External Schottky Diode external boost diode parallel with high side MOSFET used keep inductor current flow continuous during non-overlap time when both MOSFETs turned off. Although average current through this diode small, diode must able handle currents equal peak inductor current. This peak current calculated Current Limit section this specification reverse voltage requirement diode
VDIODE_RRM= VOUT
MIC2185, Schottky diodes with rating recommended. Schottky diodes with lower reverse voltage ratings have higher reverse leakage current which will cause ringing excessive power dissipation diode side MOSFET. external Schottky diode necessary circuit operation since high side MOSFET contains parasitic body diode. However, body diode relatively slow reverse recovery time relatively high forward voltage drop. lower forward voltage drop Schottky diode both prevents parasitic diode from turning improves efficiency. lack reverse recovery mechanism Schottky diode causes less ringing than MOSFET's parasitic diode. Depending circuit components operating conditions, external Schottky diode will improve converter efficiency 1/2% MIC2185
MIC2185
Reference, Enable UVLO Circuits output drivers enabled when following conditions satisfied: voltage (pin greater than undervoltage threshold. voltage Enable greater than Enable /UVLO threshold. internal bias circuitry generates 1.245V bandgap reference voltage error amplifier voltage internal supply bus. reference voltage MIC2185 buffered brought VREF must bypassed (pin with 0.1µF capacitor. must decoupled ground with ceramic capacitor. Enable (pin threshold levels, allowing MIC2185 shut down micro-current mode, turn output switching standby mode. Below 0.9V (typical), device forced into low-power shutdown. enable between 0.9V 1.5V (typical) output gate drive disabled internal circuitry powered soft start voltage forced low. There typically 140mV hysteresis below 1.5V threshold insure part does oscillate ripple voltage input. Raising Enable voltage above UVLO threshold 1.5V enables output drivers allows soft start capacitor charge. Enable pulled VINA. Oscillator Sync internal oscillator self-contained requires external components. allows user select from switching frequencies. level sets oscillator frequency 400kHz high level sets oscillator frequency 200kHz. maximum duty cycle both frequencies typically 85%. minimum pulse width increases does double when frequency changed from 400kHz 200kHz. This means minimum duty cycle slightly lower 200kHz. This important input voltage approaches output voltage. lower duty cycles, input voltage closer output voltage without output rising regulation. frequency foldback mode enabled voltage Feedback (pin less than 0.3V. frequency foldback oscillator frequency reduced approximately factor 400kHz setting, oscillator runs 100khz frequency foldback. 200kHz setting oscillator runs approximately 50kHz. SYNC input (pin allows MIC2185 synchronize with external CMOS clock signal. rising edge sync signal generates reset signal oscillator, which turns high side gate drive output. sidedrive then turns restarting switching cycle. sync signal inhibited when controller operates skip mode frequency foldback. sync signal frequency must greater than maximum specified free running frequency MIC2185. synchronizing frequency lower, double pulsing gate drive outputs will occur. When used, sync must connected ground.
Figure shows timing between external sync signal, side-drive high side drive when low. delay between rising edge sync signal turn side gate drive typically 900ns when high 600ns when low.
Sync Waveform
Sync Input 2V/div Switch Node Voltage (Low Side Drain) 5V/div High Side Gate Drive 5V/div Side Gate Drive 5V/div
600ns TIME (500ns/div)
Figure Sync Waveforms maximum recommended output switching frequency 600kHz. Synchronizing higher frequencies possible, however there some concerns. switching frequency increased, switching period decreases. minimum time MIC2185 becomes greater part total switching period. This prevent proper operation approaches Vout also minimize effectiveness current limit circuitry. maximum duty cycle decreases sync frequency increased. Figure shows relationship between minimum maximum duty cycle frequency.
MIC2185 Sync Frequency Duty cycle
HIGH HIGH FREQUENCY (kHz) MAX. DUTY
MIN. DUTY
Figure Table summarizes differences MIC2185 different states pin. Switching Typical Typical tOFF Level Frequency Duty Min. Duty SKIP Mode (kHz) cycle cycle MIC2185 Table
MIC2185
January 2002
MIC2185
Soft Start Soft Start reduces power supply input surge current start limiting output voltage rise time. Input surge current occurs when boost converter charges output capacitance. Slowing output rise time lowers input surge current. Soft Start also used power supply sequencing. soft start cannot control initial surge current boost converter when applied. This surge current caused output capacitance charging input voltage. current flows from input through inductor output diode output capacitors. soft start voltage applied directly comparator. internal current source used charge soft start capacitor. Either UVLO conditions will pull soft start capacitor low. When voltage drops below UVLO threshold When Enable drops below UVLO threshold part switches duty cycle when soft start voltage zero. soft start voltage rises from 0.7V, duty cycle increases from minimum duty cycle operating duty cycle. oscillator runs foldback frequency until feedback voltage rises above 0.3V. boost converter output voltage equal input voltage before MIC2185 starts switching. ratio Vout/Vin low, voltage feedback will already greater than 0.3V converter begin switching selected operating frequency. risetime output dependent soft start capacitor, output capacitance, input output voltage load current. scope photo Figure10 shows output voltage soft start voltage startup. output voltage initially input voltage less diode drop. After converter enabled output slowly rises minimum duty cycle controller. soft start voltage increases, output voltage rises controlled fashion until output voltage reaches regulated value.
Soft Start Waveform
MIC2185
Voltage Amplifier VREF 1.245V
Figure output voltage determined equation below. VREF where: VREF MIC2185 nominally 1.245V. Lower values resistance preferred prevent noise from apprearing pin. typically recommended value 10k. Decoupling Capacitor Selection decoupling capacitor used stabilize internal regulator minimize noise pin. Placement this capacitor critical proper operation MIC2185. must next signal ground pins. capacitor should good quality ceramic. Incorrect placement decoupling capacitor will cause jitter and/or oscillations switching waveform well variations overcurrent limit. minimum 0.1µF ceramic capacitor required decouple pin. capacitor should placed near connected directly between (VDD) (SGND). 0.1µF capacitor required decouple VREF. should located near VREF pin. Efficiency calculation considerations Efficiency ratio output power input power. difference dissipated heat boost converter. significant contributors light output loads are: VINA supply current. VINP supply current which includes current required switch external MOSFETs Core losses inductor maximize efficiency light loads: gate charge MOSFET smallest MOSFET, which still adequate maximum output current. Allow MIC2185 skip mode lower currents. running mode, frequency 200kHz. ferrite material inductor core, which less core loss than iron power core.
VOUT 2V/div
1V/div
TIME (2ms/div)
Figure Soft Start Voltage Setting Components MIC2185 requires resistors output voltage shown Figure January 2002
MIC2185
MIC2185
significant contributors power loss higher output loads approximate order magnitude): Resistive on-time losses both MOSFETs Switching transition losses side MOSFET Inductor resistive losses Current sense resistor losses Output capacitor resistive losses (due capacitor's ESR) minimize power loss under heavy loads: logic level, on-resistance MOSFETs. Multiplying gate charge on-resistance gives figure merit, providing good balance between switching resistive power dissipation. Slow transition times oscillations voltage current waveforms dissipate more power during turn-on turn-off side MOSFET. clean layout will minimize parasitic inductance capacitance gate drive high current paths. This will allow fastest transition times waveforms without oscillations. gate charge MOSFETs will switch faster than those with higher gate charge specifications. same size inductor, lower value will have fewer turns therefore, lower winding resistance. However, using small value will increase inductor current therefore require more output capacitors filter output ripple. Lowering current sense resistor value will decrease power dissipated resistor. However, will also increase overcurrent limit require larger MOSFETs inductor components handle higher currents. output capacitors minimize power dissipated capacitor's ESR.
MIC2185
January 2002
MIC2185
Package Information
0.157 (3.99) 0.150 (3.81)
DIMENSIONS: INCHES (MM)
0.020 (0.51) 0.050 (1.27)
0.020 (0.51) 0.013 (0.33) 0.0098 (0.249) 0.0040 (0.102)
0°-8° 0.050 (1.27) 0.016 (0.40) 0.244 (6.20) 0.228 (5.79)
0.0648 (1.646) 0.0434 (1.102)
0.394 (10.00) 0.386 (9.80)
SEATING PLANE
16-Pin SOIC
MICREL INC. 1849 FORTUNE DRIVE JOSE, 95131
(408) 944-0800
(408) 944-0970
http://www.micrel.com
This information believed accurate reliable, however responsibility assumed Micrel infringement patents other rights third parties resulting from use. license granted implication otherwise under patent patent right Micrel Inc. 2002 Micrel Incorporated
January 2002
MIC2185

MIC2185 Voltage Synchronous Boost Control Final Information

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