High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX18

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High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX18

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19-2091; 8/01
High-Efficiency, Current-Mode, Inverting Controller
MAX1846/MAX1847 high-efficiency inverting controllers allow designers implement compact, lownoise, negative-output DC-DC converters telecom networking applications. Both devices operate from +16.5V input generate -200V output. minimize switching noise, both devices feature current-mode, constant-frequency control scheme. operating frequency from 100kHz 500kHz through resistor. MAX1846 available ultra-compact 10-pin µMAX package. Operation high frequency, compatibility with ceramic capacitors, inverting topology without transformers allow compact design. Compatibility with electrolytic capacitors flexibility operate down 100kHz allow users minimize cost external components. high-current output drivers designed drive P-channel MOSFET allow converter deliver 30W. MAX1847 features clock synchronization shutdown functions. MAX1847 also configured operate inverting flyback controller with Nchannel MOSFET transformer deliver 70W. MAX1847 available 16-pin QSOP package. Current-mode control simplifies compensation provides good transient response. Accurate current-mode control over current protection achieved through low-side current sensing. Efficiency +3.0V +16.5V Input Range -200V Output Drives High-Side P-Channel MOSFET 100kHz 500kHz Switching Frequency Current-Mode, Control Internal Soft-Start Electrolytic Ceramic Output Capacitor MAX1847 also offers: Synchronization External Clock Shutdown N-Channel Inverting Flyback Option
Features
MAX1846/MAX1847
Ordering Information
PART MAX1846EUB MAX1847EEE TEMP. RANGE -40°C +85°C -40°C +85°C PIN-PACKAGE µMAX QSOP
Typical Operating Circuit
POSITIVE
Applications
Cellular Base Stations Networking Equipment Optical Networking Equipment SLIC Supplies Line Driver Supplies Industrial Power Supplies Automotive Electronic Power Supplies Servers
FREQ PGND NEGATIVE VOUT
MAX1846 MAX1847
COMP
Configurations appear data sheet.
Maxim Integrated Products
pricing, delivery, ordering information, please contact Maxim/Dallas Direct! 1-888-629-4642, visit Maxim's website www.maxim-ic.com.
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
ABSOLUTE MAXIMUM RATINGS
SHDN .-0.3V +20V PGND .-0.3V +0.3V PGND 5.7V.-0.3V (VIN 0.3V) PGND 5.7V .-0.3V PGND .-0.3V (VIN 0.3V) REF, COMP GND.-0.3V 0.3V) FREQ, POL, SYNC .-0.3V Continuous Power Dissipation +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) .444mW 16-Pin QSOP (derate 8.3mW/°C above +70°C).696mW Operating Temperature Range .-40°C +85°C Junction Temperature .+150°C Storage Temperature Range .-65°C +150°C Lead Temperature (soldering, 10s) .+300°C
Stresses beyond those listed under "Absolute Maximum Ratings" cause permanent damage device. These stress ratings only, functional operation device these other conditions beyond those indicated operational sections specifications implied. Exposure absolute maximum rating conditions extended periods affect device reliability.
ELECTRICAL CHARACTERISTICS
SHDN +12V, SYNC GND, PGND GND, RFREQ 147k ±1%, 0.47µF, CREF 0.1µF, +85°C, unless otherwise noted.)
PARAMETER CONTROLLER Operating Input Voltage Range UVLO Threshold UVLO Hysteresis Threshold Input Current Load Regulation Line Regulation Current-Limit Threshold Input Current Supply Current Shutdown Supply Current Output Voltage Load Regulation Output Voltage Load Regulation -0.1V, +3.0V +16.5V SHDN GND, +3.0V +16.5V IREF 50µA IREF 500µA 100µA 0.1mA 2.0mA 3.85 1.236 load -0.1V CCOMP 0.068µF, VOUT -48V, IOUT 20mA 200mA (Note CCOMP 0.068µF, VOUT -48V, +16.5V, IOUT 100mA 0.04 0.75 1.25 4.25 1.264 4.65 rising falling 2.74 16.5 2.95 CONDITIONS UNITS
REFERENCE REGULATOR
High-Efficiency, Current-Mode, Inverting Controller
ELECTRICAL CHARACTERISTICS (continued)
SHDN +12V, SYNC GND, PGND GND, RFREQ 147k ±1%, 0.47µF, CREF 0.1µF, +85°C, unless otherwise noted.)
OSCILLATOR RFREQ 500k Oscillator Frequency RFREQ 147k RFREQ 76.8k RFREQ 500k Maximum Duty Cycle SYNC Input Signal Duty-Cycle Range Minimum SYNC Input Logic Pulse Width SYNC Input Rise/Fall Time SYNC Input Frequency Range DIGITAL INPUTS POL, SYNC, SHDN Input High Voltage POL, SYNC, SHDN Input Voltage POL, SYNC Input Current SHDN Input Current SOFT-START Soft-Start Clock Cycles Soft-Start Levels OUTPUT Sink/Source Current On-Resistance +5V, VEXT forced +2.5V high low, tested with 100mA load, high low, tested with 100mA load, 1024 Cycles POL, SYNC VSHDN VSHDN +16.5V 0.45 (Note RFREQ 147k RFREQ 76.8kz
MAX1846/MAX1847
Note Production test correlates operating conditions. Note Guaranteed design characterization.
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
ELECTRICAL CHARACTERISTICS
SHDN +12V, SYNC GND, PGND GND, RFREQ 147k ±1%, 0.47µF, CREF 0.1µF, -40°C +85°C, unless otherwise noted.) (Note
PARAMETER CONTROLLER Operating Input Voltage Range UVLO Threshold Threshold Input Current Load Regulation Current Limit Threshold Input Current Supply Current Shutdown Supply Current Output Voltage Load Regulation Output Voltage Load Regulation OSCILLATOR Oscillator Frequency Maximum Duty Cycle SYNC Input Signal Duty-Cycle Range Minimum SYNC Input Logic Pulse Width SYNC Input Rise/Fall Time SYNC Input Frequency Range DIGITAL INPUTS POL, SYNC, SHDN Input High Voltage POL, SYNC, SHDN Input Voltage 0.45 (Note RFREQ 500k RFREQ 147k RFREQ 500k RFREQ 147k -0.1V, +3.0V +16.5V SHDN GND, +3.0V +16.5V IREF 50µA IREF 500µA 100µA 0.1mA 2.0mA 3.85 1.225 rising falling load -0.1V CCOMP 0.068µF, VOUT -48V, IOUT= 20mA 200mA (Note 1.275 4.65 16.5 2.95 CONDITIONS UNITS
REFERENCE REGULATOR
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
ELECTRICAL CHARACTERISTICS (continued)
SHDN +12V, SYNC GND, PGND GND, RFREQ 147k ±1%, 0.47µF, CREF 0.1µF, -40°C +85°C, unless otherwise noted.) (Note
PARAMETER POL, SYNC Input Current SHDN Input Current OUTPUT On-Resistance high low, IEXT 100mA, high low, IEXT 100mA, CONDITIONS POL, SYNC SHDN SHDN +16.5V UNITS
Note Parameters -40°C guaranteed design characterization.
Typical Operating Characteristics
(Circuit references from Table Main Application Circuits section, 0.47µF, CREF 0.1µF, +25°C, unless otherwise noted.)
EFFICIENCY LOAD CURRENT
MAX1846/7 toc01
EFFICIENCY LOAD CURRENT
MAX1846/7 toc02
EFFICIENCY LOAD CURRENT
EFFICIENCY 16.5V
MAX1846/7 toc03
EFFICIENCY APPLICATION CIRCUIT VOUT 1000 16.5V
EFFICIENCY APPLICATION CIRCUIT
3.3V
VOUT -12V 1000 10,000
APPLICATION CIRCUIT
VOUT -48V 1000
10,000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
OUTPUT VOLTAGE LOAD REGULATION
MAX1846/7 toc04
SUPPLY CURRENT SUPPLY VOLTAGE
(mA) 1.242 -0.1V 1.238
MAX1846/7 toc05
REFERENCE VOLTAGE TEMPERATURE
MAX1846/7 toc06
-11.90 -11.92 -11.94 OUTPUT VOLTAGE -11.96 -11.98 -12.00 -12.02 -12.04 -12.06 -12.08 -12.10 APPLICATION CIRCUIT
1.262 1.258 1.254 VREF 1.250 1.246
LOAD CURRENT (mA)
TEMPERATURE (°C)
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
Typical Operating Characteristics (continued)
(Circuit references from Table Main Application Circuits section, 0.47µF, CREF 0.1µF, +25°C, unless otherwise noted.)
VOLTAGE TEMPERATURE
MAX1846/7 toc07
REFERENCE LOAD REGULATION
1.260 4.340 4.300 4.260 VREF 1.250 4.220 4.180 1.245 4.140
LOAD REGULATION
MAX1846/7 toc08 MAX1846/7 toc09
4.27
4.26
1.255
4.25
4.24
4.23
1.240 IREF (µA)
4.100 TEMPERATURE (°C)
4.22 (mA)
SHUTDOWN SUPPLY CURRENT TEMPERATURE
MAX1846/7 toc10
OPERATING CURRENT TEMPERATURE
MAX1846/7 toc11
SWITCHING FREQUENCY RFREQ
MAX1846/7 toc12
SHUTDOWN SUPPLY CURRENT (µA) TEMPERATURE (°C) 16.5V
VOUT -12V OPERATING CURRENT (mA) APPLICATION CIRCUIT VOUT 16.5V, VOUT
fOSC (kHz)
TEMPERATURE (°C)
RFREQ
SWITCHING FREQUENCY TEMPERATURE
MAX1846/7 toc13
RISE/FALL TIME CAPACITANCE
MAX1846/7 toc14
EXITING SHUTDOWN
MAX1846/7 toc15
FREQUENCY (kHz) RFREQ 147k TEMPERATURE (°C)
TIME (ns) RISE TIME 2000 4000 6000 8000 FALL TIME
5V/div SHDN VOUT 5V/div
APPLICATION CIRCUIT 1ms/div
1A/div
10,000
CAPACITANCE (pF)
High-Efficiency, Current-Mode, Inverting Controller
Typical Operating Characteristics (continued)
(Circuit references from Table Main Application Circuits section, 0.47µF, CREF 0.1µF, +25°C, unless otherwise noted.)
ENTERING SHUTDOWN
MAX1846/7 toc16
MAX1846/MAX1847
HEAVY-LOAD SWITCHING WAVEFORM
MAX1846/7 toc17
SHDN 5V/div VOUT 100mV/div
5V/div VOUT
1A/div
APPLICATION CIRCUIT 1ms/div
1A/div
10V/div
APPLICATION CIRCUIT 1µs/div ILOAD 600mA
LIGHT-LOAD SWITCHING WAVEFORM
MAX1846/7 toc18
VOUT
100mV/div
1A/div
10V/div
APPLICATION CIRCUIT 1µs/div ILOAD 50mA
LOAD-TRANSIENT RESPONSE
MAX1846/7 toc19
LOAD-TRANSIENT RESPONSE
MAX1846/7 toc20
ILOAD
ILOAD
VOUT 500mV/div
VOUT
200mV/div
1A/div
500mA/div
APPLICATION CIRCUIT 2ms/div ILOAD 10mA 400mA
APPLICATION CIRCUIT 400µs/div ILOAD 100mA
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
Description
MAX1846 MAX1847 NAME FUNCTION Sets polarity pin. Connect with external PMOS high-side FET. Connect with external NMOS lowside transformer-based applications. Low-Dropout Regulator. Connect 0.47µF ceramic capacitor from GND. Oscillator Frequency Input. Connect resistor (RFREQ) from FREQ internal oscillator frequency from 100kHz (RFREQ 500k) 500kHz (RFREQ 76.8k). RFREQ still required external clock used SYNC. Setting Operating Frequency section. Compensation Node Error Amp/Integrator. Connect series resistor/capacitor network from COMP loop compensation. Design Procedure. 1.25V Reference Output. source 500µA. Bypass with 0.1µF ceramic capacitor from GND. Feedback Input. Connect center resistor-divider connected between output REF. threshold Connection Shutdown Control. Drive SHDN turn DC-DC controller. Drive high connect normal operation. Analog Ground. Connect PGND. Negative Rail Driver Negative Current-Sense Input. Connect GND. Positive Current-Sense Input. Connect current-sense resistor (RCS) between External MOSFET Gate-Driver Output. swings from PGND. Power-Supply Input Operating Frequency Synchronization Control. Drive SYNC connect internal oscillator frequency with RFREQ. Drive SYNC with logic-level clock input signal externally converter's operating frequency. DC-DC conversion cycles initiate rising edge input clock signal. Note that when driving SYNC with external signal, RFREQ must still connected FREQ.
FREQ
10,11
COMP N.C. SHDN PGND
SYNC
High-Efficiency, Current-Mode, Inverting Controller
Typical Application Circuit
+5.5V 22µF
MAX1846/MAX1847
FDS6375 CMSH5-40 0.47µF SHDN Sanyo 16TPB47M 0.02 95.3k 10µH DO5022P-103 47µF 47µF
VOUT -12V 400mA
SYNC
MAX1847
0.22µF 150k FREQ COMP
N.C.
PGND
10.0k 1200pF
0.1µF
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
Functional Diagram
SHDN MAX1847 ONLY STARTUP CIRCUITRY PGND DRIVER REGULATOR
UNDERVOLTAGE LOCK
CONTROL CIRCUITRY
MAX1846 MAX1847
SYNC MAX1847 ONLY FREQ
OSCILLATOR
ERROR COMPARATOR COMP
ERROR AMPLIFIER SOFT-START SLOPE COMP REFERENCE CURRENTSENSE AMPLIFIER
PGND
High-Efficiency, Current-Mode, Inverting Controller
Detailed Description
MAX1846/MAX1847 current-mode controller inverting topology that ideal generating output voltages from -200V. Features include shutdown, adjustable internal operating frequency synchronization external clock, soft-start, adjustable current limit, wide (+3V +16.5V) input range. when exiting shutdown with power already applied, when exiting undervoltage lockout.
MAX1846/MAX1847
Shutdown (MAX1847 only)
MAX1847 shuts down reduce supply current 10µA when SHDN low. this mode, internal reference, error amplifier, comparators, biasing circuitry turn off. output becomes high impedance external pullup resistor connected pulls VEXT VIN, turning P-channel MOSFET. When shutdown mode, converter's output goes
Controller
architecture MAX1846/MAX1847 currentmode controller Bi-CMOS multi-input system that simultaneously processes output-error signal, current-sense signal, slope-compensation ramp (Functional Diagram). Slope compensation prevents subharmonic oscillation, potential result current-mode regulators operating greater than duty cycle. controller uses fixed-frequency, current-mode operation where duty ratio input-to-output voltage ratio. current-mode feedback loop regulates peak inductor current function output error signal.
Frequency Synchronization (MAX1847 only)
MAX1847 capable synchronizing switching frequency with external clock source. Drive SYNC with logic-level clock input signal synchronize MAX1847. switching cycle starts rising edge signal applied SYNC. Note that frequency signal applied SYNC must higher than default frequency RFREQ. This required that internal clock does start switching cycle prematurely. SYNC inactive entire clock cycle internal oscillator, internal oscillator takes over switching operation. Choose RFREQ such that fOSC fSYNC.
Internal Regulator
MAX1846/MAX1847 incorporate internal lowdropout regulator (LDO). This 4.25V output powers MAX1846/MAX1847 functions (excluding EXT) primary purpose stabilizing performance over wide input voltage range (+3V +16.5V). input this regulator connected dropout voltage typically 100mV, that when less than 4.35V, typically minus 100mV. When dropout, MAX1846/MAX1847 still operate with best performance, recommended connect when input supply less than 4.5V.
Polarity (MAX1847 only)
MAX1847 features option utilize N-channel MOSFET configuration, rather than typical P-channel MOSFET configuration (Figure order drive different polarities these MOSFETs, MAX1847 capable reversing phase degrees. When driving P-channel MOSFET, connect GND. When driving N-Channel MOSFET, connect These connections ensure proper polarity EXT. design guidance regard this application, refer MAX1856 data sheet.
Undervoltage Lockout
MAX1846/MAX1847 have undervoltage lockout circuit that monitors voltage falls below UVLO threshold (2.8V typ), control logic turns P-channel (EXT high impedance). rest circuitry still powered operating. When increases 60mV above UVLO threshold, resumes operation from start condition (soft-start).
Design Procedure
Initial Specifications
order start design procedure, parameters must identified: minimum input voltage expected IN(MIN) maximum input voltage expected (VIN(MAX)), desired output voltage (VOUT), expected maximum load current (ILOAD). Calculate Equivalent Load Resistance This simple calculation used shorten verification equations: RLOAD VOUT ILOAD
Soft-Start
MAX1846/MAX1847 feature "digital" soft-start that preset requires external capacitor. Upon startup, threshold decrements from reference voltage steps over 1024 cycles fOSC fSYNC. Typical Operating Characteristics scope picture soft-start operation. Soft-start implemented: when power first applied
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
+12V 12µF VP1-0190 12.2µH
CMR1U-02 0.47µF SHDN SYNC MAX1847 0.033µF 270k COMP FREQ 1800pF 10.0k N.C. PGND 0.05 0.5W 100pF 100V 383k IRLL2705
VOUT -48V 100mA 12µF 100V
150k
0.1µF
Figure Using N-Channel MOSFET (MAX1847 only)
Calculate Duty Cycle duty cycle ratio on-time MOSFET switch oscillator period. This determined ratio input voltage output voltage. Since input voltage typically range operation, minimum (DMIN) maximum (DMAX) duty cycle calculated DMIN
VOUT VIN(MAX) VLIM VOUT VOUT VIN(MIN) VLIM VOUT
1.25V regulation voltage nominally load presented reference feedback resistors must less than 500µA. This guarantee that VREF regulation (see Electrical Characteristics Table). Conversely, current through feedback resistors must large enough that leakage current input (50nA) insignificant. Therefore, select that between 50µA 250µA. VREF where VREF 1.25V. typical value 10k. Once selected, calculate with following equation: (-VOUT VREF)
DMAX
where forward drop across output diode, drop across external when current-limit threshold. begin with, assume 0.5V Schottky diode, 100mV, VLIM 100mV. Remember that VOUT negative when using this formula.
Setting Operating Frequency
MAX1846/MAX1847 capable operating switching frequencies from 100kHz 500kHz. Choice operating frequency depends number factors: Noise considerations dictate setting synchronizing) above below certain frequency band frequencies, particularly applications.
Setting Output Voltage
output voltage using external resistors form resistive-divider between output (refer Figure VREF nominally
High-Efficiency, Current-Mode, Inverting Controller
Higher frequencies allow smaller value (hence smaller size) inductors capacitors. Higher frequencies consume more operating power both operate charge discharge gate external FET. This tends reduce efficiency light loads. Higher frequencies exhibit lower overall efficiency more transition losses FET; however, this shortcoming often nullified trading some inductor capacitor size benefits lower-resistance components. High-duty-cycle applications require lower frequencies accommodate controller minimum off-time 0.4µs. Calculate maximum oscillator frequency with following formula: fOSC(MAX) VIN(MIN) VLIM tions, most circuits more efficient economical operating continuous mode, which refers continuous current inductor. continuous mode there trade-off between efficiency transient response. Higher inductance means lower inductor ripple current, lower peak current, lower switching losses, and, therefore, higher efficiency. Lower inductance means higher inductor ripple current faster transient response. reasonable compromise choose ratio inductor ripple current average continuous current minimum duty cycle 0.4. Calculate inductor ripple with following formula: IRIPPLE ILOAD(MAX) VIN(MAX) VLIM VOUT
MAX1846/MAX1847
(VIN(MAX) VLIM
VIN(MIN) VLIM VOUT OFF(MIN)
Then calculate inductance value: (VIN(MAX) IRIPPLE) (DMIN fOSC) Choose closest standard value. Once again, remember that VOUT negative when using this formula.
Remember that VOUT negative when using this formula. oscillator frequency resistor, RFREQ, connected from FREQ GND. relationship between fOSC RFREQ slightly nonlinear, illustrated Typical Operating Characteristics. Choose resistor value from graph check oscillator frequency using following formula:
Determining Peak Inductor Current
peak inductor current required particular output ILPEAK ILDC (ILPP where ILDC average input current ILPP inductor peak-to-peak ripple current. ILDC ILPP terms determined follows: ILDC ILOAD VOUT VIN(MIN) VLIM
fOSC
5.21 1.92 (RFREQ FREQ 4.86
ILPP External Synchronization (MAX1847 only) SYNC input provides external-clock synchronization desired). When SYNC driven with external clock, frequency clock directly sets MAX1847's switching frequency. rising clock edge SYNC interpreted synchronization input. sync signal lost, internal oscillator takes over last cycle, frequency returned rate RFREQ. Choose RFREQ such that fOSC fSYNC.
(VIN(MIN) VLIM VOUT
fOSC VOUT
Choosing Inductance Value
inductance value determines operation current-mode regulator. Except low-current applica-
where selected inductance value. saturation rating selected inductor should meet exceed calculated value ILPEAK, although most coil types operated over their saturation rating without difficulty. addition saturation criteria, inductor should have series resistance possible. continuous inductor current, power loss inductor resistance (PLR) approximated (ILOAD VOUT VIN)2 where inductor series resistance.
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
Once peak inductor current calculated, current sense resistor, RCS, determined 85mV ILPEAK high peak inductor currents (>1A), Kelvin-sensing connections should used connect PGND Connect PGND together ground side RCS. lowpass filter between required prevent switching noise from tripping current-sense comparator heavy loads. Connect resistor between high side RCS, connect 1000pF capacitor between GND. associated with charging gate. addition, this parameter helps predict current needed drive gate selected operating frequency. power MOSFET inverting converter must have high enough voltage rating handle input voltage plus magnitude output voltage spikes induced leakage inductance. Choose RDS(ON)(MAX) specified VIN(MIN) times RCS. Verify that VIN(MAX) VGS(MAX) VDS(MAX) VIN(MAX) VOUT Choose riseand fall-times (tR, less than 50ns.
Output Capacitor Selection
output capacitor (COUT) does filtering inverting converter. output ripple created variations charge stored output capacitor with each pulse voltage drop across capacitor's equivalent series resistance (ESR) caused current into capacitor. There properties output capacitor that affect ripple voltage: capacitance value, capacitor's ESR. output ripple output capacitor's value given VRIPPLE-C (ILOAD DMAX TOSC COUT output ripple output capacitor's given VRIPPLE-R ILPP RESR These ripple voltages additive total output ripple VRIPPLE-T VRIPPLE-C VRIPPLE-R ESR-induced ripple usually dominates this last equation, typically output capacitor selection based mostly upon capacitor's ESR, voltage rating, ripple current rating. following formula determine maximum desired output ripple voltage (VRIPPLE-D): RESR VRIPPLE-D ILPP Select capacitor with rating less than RESR. value this capacitor highly dependent dielectric type, package size, voltage rating. general, when choosing capacitor, recommended low-ESR capacitor types such ceramic, organic, tantalum capacitors. Ensure that selected capacitor sufficient margin safely handle maximum ripple current (ILPP) maximum output voltage.
Checking Slope-Compensation Stability
current-mode regulator, cycle-by-cycle stability dependent slope compensation prevent subharmonic oscillation duty cycles greater than 50%. MAX1846/MAX1847, internal slope compensation optimized minimum inductor value (LMIN) with respect duty cycle. duty cycles greater then 50%, check stability calculating LMIN using following equation: LMIN VIN(MIN)
DMAX DMAX
where VIN(MIN) minimum expected input voltage, Slope Compensation Ramp mV/µs) DMAX maximum expected duty cycle. LMIN larger than increase value next standard value that larger than ensure slope compensation stability.
Power MOSFET Selection
MAX1846/MAX1847 drive wide variety P-channel power MOSFETs (PFETs). best performance, especially with input voltages below achieved with low-threshold PFETs that specify on-resistance with gate-to-source voltage (VGS) 2.7V less. When selecting PFET, parameters include: Total gate charge (QG) Reverse transfer capacitance (CRSS) On-resistance (RDS(ON)) Maximum drain-to-source voltage (VDS(MAX)) Minimum threshold voltage (VTH(MIN)) high switching rates, dynamic characteristics (parameters above) that predict switching losses have more impact efficiency than DS(ON), which predicts losses. includes capacitance
Choosing Compensation Components
MAX1846/MAX1847 externally loop-compensated devices. This provides flexibility designs accommodate variety applications. Proper com-
High-Efficiency, Current-Mode, Inverting Controller
pensation control loop important prevent excessive output ripple poor efficiency caused instability. goal compensation cancel unwanted poles zeros DC-DC converter's transfer function created power-switching filter elements. More precisely, objective compensation ensure stability ensuring that DCDC converter's phase shift less than 180° safe margin, frequency where loop gain falls below unity. method ensuring adequate phase margin introduce corresponding zeros poles feedback network approximate single-pole response with -20dB/decade slope unity-gain crossover. Calculating Poles Zeros MAX1846/MAX1847 current-mode architecture takes double pole caused inductor output capacitor shifts these poles much higher frequency. This makes loop compensation easier. compensate these devices, must know center frequencies right-half plane zero (zRHP) higher frequency pole (pOUT2). Calculate zRHP frequency with following formula: VIN(MIN) VOUT RLOAD ZRHP VOUT
VIN(MIN) VOUT LOAD VIN(MIN) TOSC DMAX VIN(MIN) RLOAD
MAX1846/MAX1847
where: feedback divider attenuation factor (-VOUT VREF), error amplifier transconductance µA/V, error amplifier output resistance slope compensation factor [(1.636A RCS], selected current sense resistor, selected inductance value zRHP lower frequency than pOUT2, required dominant pole frequency given pDOM zRHP Otherwise required dominant pole frequency pDOM pOUT2 Determining Compensation Component Values Using DOM, calculate compensation capacitor required: CCOMP pDOM) Select next largest standard value capacitor then calculate compensation resistor required cancel output-capacitor-induced pole (pOUT1) determined previously. zero needed cancel output-induced pole frequency this zero must equal pOUT1. Therefore: zCOMP pOUT1 RCOMP RLOAD COUT CCOMP Choose nearest lower standard value resistor. check final values selected compensation components: pCOMP CCOMP RCOMP)] order pCOMP compensate loop, openloop gain must reach unity lower frequency than right-half-plane zero second output pole, whichever lower frequency. second output pole right-half-plane zero close together frequency, higher resulting phase shift unity gain
calculations pOUT2 very complex. most applications where VOUT does exceed -48V negative sense), pOUT2 will lower than 1/8th oscillator frequency generally higher frequency than zRHP. Therefore: pOUT2 0.125 fOSC pole created output capacitor load resistance. This pole must also compensated center frequency given formula: pOUT1 RLOAD COUT) Finally, there zero introduced output capacitor. This zero determined from following equation: zESR COUT RESR) Calculating Required Pole Frequency ensure stability MAX1846/MAX1847, introduced pole (PDOM) compensation network must roll-off error amplifier gain before POUT2 occurs. First calculate open-loop gain determine frequency pole introduce.
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
require larger compensation capacitor than calculated. might take more than couple iterations obtain suitable combination. Finally, zero introduced output capacitor's must compensated. This accomplished placing capacitor between creating pole directly feedback loop. Calculate value this capacitor using frequency zESR selected feedback resistor values with formula: RESR COUT exceed potential difference between VOUT input voltage plus leakage inductance spikes. high output voltages (-50V more), Schottky diodes practical because this voltage requirement. these cases, ultrafast recovery diode with adequate reverse-breakdown voltage.
Input Filter Capacitor
input capacitor (CIN) inverting converter designs reduces current peaks drawn from input supply reduces noise injection. source impedance input supply largely determines value CIN. High source impedance requires high input capacitance, particularly input voltage falls. Since inverting converters "constant-power" loads their input supply, input current rises input voltage falls. Consequently, low-input-voltage designs, increasing and/or lowering much conversion efficiency.
Applications Information
Maximum Output Power
maximum output power that MAX1846/MAX1847 provide depends maximum input power available circuit's efficiency: POUT(MAX) Efficiency PIN(MAX) Furthermore, efficiency input power both functions component selection. Efficiency losses divided into three categories: resistive losses across inductor, MOSFET on-resistance, currentsense resistor, input output capacitors; switching losses MOSFET's transition region, charging MOSFET's gate capacitance; inductor core losses. Typically efficiency assumed initial calculations. required input power depends inductor current limit, input voltage, output voltage, output current, inductor value, switching frequency. maximum output power approximated following formula: PMAX [VIN (VLIM ILIM RDS(ON))] ILIM (LIR [(-VOUT (VIN VLIM VOUT VD)] where peak current limit inductor current-ripple ratio calculated ILPP ILDC Again, remember that MAX1846/ MAX1847 negative.
Bypass Capacitor
addition COUT, other ceramic bypass capacitors required with MAX1846/MAX1847. Bypass with 0.1µF larger capacitor. Bypass with 0.47µF larger capacitor. bypass capacitors should located close their respective pins possible.
Board Layout Guidelines
Good board layout routing required highfrequency-switching power supplies achieve good regulation, high efficiency, stability. strongly recommended that evaluation board layouts followed closely possible. Place power components close together possible, keeping their traces short, direct, wide. Avoid interconnecting ground pins power components using vias through internal ground plane. Instead, keep power components close together route them "star" ground configuration using component-side copper, then connect star ground internal ground using multiple vias.
Main Application Circuits
MAX1846/MAX1847 extremely versatile devices. Figure shows generic schematic MAX1846. Table lists component values several typical applications. These component values also apply MAX1847. first applications featured MAX1846/MAX1847 Kit.
Diode Selection
MAX1846/MAX1847's high-switching frequency demands high-speed rectifier. Schottky diodes recommended most applications because their fast recovery time forward voltage. Ensure that diode's average current rating exceeds peak inductor current using diode manufacturer's data. Additionally, diode's reverse breakdown voltage must
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
APPLICATION ONLY
0.47µF
MAX1846
VOUT COUT
CCOMP RCOMP RFREQ
COMP FREQ
PGND
0.1µF
NOTE: APPLICATIONS CAPACITORS. APPLICATIONS ALUMINUM ELECTROLYTIC CAPACITORS.
Figure MAX1846 Main Application Circuit
Table Component List Main Application Circuits
CIRCUIT Input Output Output CCOMP (µF) (µF) COUT (µF) (pF) (1%) (1%) RCOMP RFREQ (µH) 0.047 40.2 0.02 CMSH5-40 FDS6685 0.22 1200 95.3 0.02 CMSH5-40 FDS6375 0.068 1800 0.05 CMR1U-02 IRFR5410 1800 1800 0.05 CMR1U-02 IRFR5410
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
Component Suppliers
SUPPLIER Central Semiconductor Coilcraft Dale Fairchild International Rectifier Kemet Semiconductor Panasonic Sanyo Siliconix Sprague Sumida Vitramon COMPONENT Capacitors Diodes Inductors Resistors MOSFETs MOSFETs Resistors Capacitors MOSFETs, Diodes Capacitors, Resistors Capacitors MOSFETs Capacitors Inductors Resistors PHONE 803-946-0690 516-435-1110 847-639-6400 402-564-3131 408-721-2181 310-322-3331 512-992-7900 864-963-6300 602-303-5454 201-348-7522 619-661-6835 408-988-8000 603-224-1961 847-956-0666 203-268-6261 WEBSITE www.avxcorp.com www.centralsemi.com www.coilcraft.com www.fairchildsemi.com www.irf.com www.irctt.com www.kemet.com www.onsemi.com www.panasonic.com www.secc.co.jp www.siliconix.com www.remtechcorp.com
Note: Please indicate that using MAX1846/MAX1847 when contacting these component suppliers.
Configurations
VIEW
FREQ COMP FREQ SYNC
Chip Information
TRANSISTOR COUNT: 2441 PROCESS TECHNOLOGY: BiCMOS
MAX1846
PGND COMP N.C. SHDN
MAX1847
PGND N.C.
10-PIN µMAX
16-PIN QSOP
High-Efficiency, Current-Mode, Inverting Controller
Package Information
10LUMAX.EPS
MAX1846/MAX1847
High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX1847
Package Information (continued)
QSOP.EPS
Maxim cannot assume responsibility circuitry other than circuitry entirely embodied Maxim product. circuit patent licenses implied. Maxim reserves right change circuitry specifications without notice time.
_Maxim Integrated Products, Gabriel Drive, Sunnyvale, 94086 408-737-7600 2001 Maxim Integrated Products Printed registered trademark Maxim Integrated Products.

High-Efficiency, Current-Mode, Inverting Controller MAX1846/MAX18

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