ANUAL LUATIO SHEET FOLLO 1.8V Input, Step-Up Controllers µMAX
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ANUAL LUATIO SHEET FOLLO
1.8V Input, Step-Up Controllers µMAX
1.8V Minimum Start-Up Voltage (MAX669) Wide Input Voltage Range (1.8V 28V) Tiny 10-Pin µMAX Package Current-Mode Idle ModeOperation Efficiency over Adjustable 100kHz 500kHz Oscillator SYNC Input 220µA Quiescent Current Logic-Level Shutdown Soft-Start
General Description
MAX668/MAX669 constant-frequency, pulse-widthmodulating (PWM), current-mode DC-DC controllers designed wide range DC-DC conversion applications including step-up, SEPIC, flyback, isolatedoutput configurations. Power levels more controlled with conversion efficiencies over 90%. 1.8V input voltage range supports wide range battery AC-powered inputs. advanced BiCMOS design features operating current (220µA), adjustable operating frequency (100kHz 500kHz), soft-start, SYNC input allowing MAX668/ MAX669 oscillator locked external clock. DC-DC conversion efficiency optimized with 100mV current-sense voltage well with Maxim's proprietary Idle Modecontrol scheme. controller operates mode medium heavy loads lowest noise optimum efficiency, then pulses only needed (with reduced inductor current) reduce operating current maximize efficiency under light loads. logic-level shutdown input also included, reducing supply current 3.5µA. MAX669, optimized input voltages with guaranteed start-up voltage 1.8V, requires bootstrapped operation powered from boosted output). supports output voltages 28V. MAX668 operates with inputs connected either bootstrapped non-bootstrapped powered from input supply other source) configuration. When bootstrapped, restriction output voltage. Both available extremely compact 10-pin µMAX package.
MAX668/MAX669
Applications
Cellular Telephones Telecom Hardware LANs Network Systems Systems
Ordering Information
PART MAX668EUB MAX669EUB TEMP. RANGE -40°C +85°C -40°C +85°C PIN-PACKAGE µMAX µMAX
Idle Mode trademark Maxim Integrated Products.
Typical Operating Circuit
1.8V
Configuration
VIEW
VOUT SYNC/ SHDN FREQ FREQ SYNC/SHDN PGND
MAX668 MAX669
MAX669
PGND
µMAX
Maxim Integrated Products
free samples latest literature: http://www.maxim-ic.com, phone 1-800-998-8800. small orders, phone 1-800-835-8769.
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
ABSOLUTE MAXIMUM RATINGS
.-0.3V +30V PGND GND.±0.3V SYNC/SHDN .-0.3V +30V EXT, GND.-0.3V (VLDO 0.3V) LDO, FREQ, -0.3V Output Current.-1mA +20mA Output Current.-1mA +1mA Short Circuit .Momentary Short Circuit .Continuous Continuous Power Dissipation +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) .444mW Operating Temperature Range .-40°C +85°C Junction Temperature .+150°C Storage Temperature Range .-65°C +150°C Lead Temperature (soldering,10sec) .+300°C
ELECTRICAL CHARACTERISTICS
(VCC +5V, ROSC 200k, +85°C, unless otherwise noted. Typical values +25°C.) PARAMETER Controller CONTROLLER Input Voltage Range, Input Voltage Range with Tied Threshold Threshold Load Regulation Typically 0.013% CS+; VCS+ range 100mV full load current. Typically 0.012% duty factor EXT; duty factor step-up 100% VIN/VOUT) 1.30V forced 1.30V, SYNC/SHDN GND, (includes dropout) (includes dropout) MAX668 MAX669 1.225 1.250 0.013 1.275 %/mV CONDITIONS UNITS
Threshold Line Regulation Input Current Current Limit Threshold Idle Mode Current-Sense Threshold Input Current Supply Current (Note Shutdown Supply Current (VCC) REFERENCE REGULATORS Reference Regulators
0.012
4.50 2.65 2.40 1.225
5.00
5.50 5.50
Output Voltage
load
Undervoltage Lockout Threshold Output Voltage Load Regulation Undervoltage Lockout Threshold OSCILLATOR Oscillator
Sensed LDO, falling edge, hysteresis MAX668 only load, CREF 0.22µF load 50µA Rising edge, hysteresis ROSC 200k
2.50 1.250
2.60 1.275
Oscillator Frequency
ROSC 100k ROSC 500k
1.8V Input, Step-Up Controllers µMAX
ELECTRICAL CHARACTERISTICS (continued)
(VCC +5V, ROSC 200k, +85°C, unless otherwise noted. Typical values +25°C.) PARAMETER Maximum Duty Cycle Minimum Pulse Width Minimum SYNC Input-Pulse Duty Cycle Minimum SYNC Input Pulse Width SYNC Input Rise/Fall Time SYNC Input Frequency Range SYNC/SHDN Falling Edge Shutdown Delay SYNC/SHDN Input High Voltage SYNC/SHDN Input Voltage SYNC/SHDN Input Current Sink/Source Current On-Resistance 3.0V 1.8V 3.0V (MAX669) 3.0V 1.8V 3.0V (MAX669) SYNC/SHDN SYNC/SHDN forced high 0.45 0.30 tested CONDITIONS ROSC 200k ROSC 100k ROSC 500k UNITS
MAX668/MAX669
ELECTRICAL CHARACTERISTICS
(VCC +5V, ROSC 200k, -40°C +85°C, unless otherwise noted.) (Note PARAMETER Controller CONTROLLER Input Voltage Range, Input Voltage Range with Tied Threshold Input Current Current-Limit Threshold Idle Mode Current-Sense Threshold Input Current Supply Current (Note Shutdown Supply Current (VCC) Reference Regulators REFERENCE REGULATORS load (includes dropout) (includes dropout) 4.50 2.65 2.40 5.50 5.50 2.60 forced 1.30V, SYNC/SHDN GND, 1.30V MAX668 MAX669 1.22 1.28 CONDITIONS UNITS
Output Voltage
Undervoltage Lockout Threshold
Sensed LDO, falling edge, hysteresis MAX669 only
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
ELECTRICAL CHARACTERISTICS (continued)
(VCC +5V, ROSC 200k, -40°C +85°C, unless otherwise noted.) PARAMETER Output Voltage Load Regulation Undervoltage Lockout Threshold OSCILLATOR ROSC 200k Oscillator Frequency ROSC =100k ROSC 500k ROSC 200k Maximum Duty Cycle Minimum SYNC Input-Pulse Duty Cycle Minimum SYNC Input Pulse Width SYNC Input Rise/Fall Time SYNC Input Frequency Range SYNC/SHDN Input High Voltage SYNC/SHDN Input Voltage SYNC/SHDN Input Current On-Resistance 3.0V 1.8V 3.0V (MAX669) 3.0V 1.8V 3.0V (MAX669) SYNC/SHDN SYNC/SHDN high tested 0.45 0.30 ROSC 100k ROSC 500k CONDITIONS load, CREF 0.22µF load 50µA Rising edge, hysteresis 1.22 1.28 UNITS
Note This current consumed when active switching. Does include gate-drive current. Note Limits -40°C guaranteed design.
1.8V Input, Step-Up Controllers µMAX
Typical Operating Characteristics
(Circuits Figures +25°C; unless otherwise noted.)
EFFICIENCY LOAD CURRENT (VOUT
EFFICIENCY EFFICIENCY 1000 10,000 LOAD CURRENT (mA) BOOTSTRAPPED FIGURE 200k 2.7V 3.6V 3.3V
MAX668 toc01
MAX668/MAX669
MAX668 EFFICIENCY LOAD CURRENT (VOUT 12V)
MAX668 toc02
MAX668 EFFICIENCY LOAD CURRENT (VOUT 24V)
EFFICIENCY
MAX668 toc03
NON-BOOTSTRAPPED FIGURE 200k 1000 10,000
NON-BOOTSTRAPPED FIGURE 200k 1000 LOAD CURRENT (mA) 10,000
LOAD CURRENT (mA)
MAX669 MINIMUM START-UP VOLTAGE LOAD CURRENT
MAX668 toc04
SUPPLY CURRENT SUPPLY VOLTAGE
MAX668 toc05
NO-LOAD SUPPLY CURRENT SUPPLY VOLTAGE
3500 3000 2500 2000 1500 1000 VOUT BOOTSTRAPPED FIGURE 200k
MAX668 toc06
MINIMUM START-UP VOLTAGE VOUT VOUT
1200 1000 SUPPLY CURRENT (µA) MAX668 MAX669 CURRENT INTO ROSC 500k
4000 NO-LOAD SUPPLY CURRENT (µA)
BOOTSTRAPPED FIGURE 1000 LOAD CURRENT (mA)
SUPPLY VOLTAGE
SUPPLY VOLTAGE
SHUTDOWN CURRENT SUPPLY VOLTAGE
MAX669
MAX668 toc07
SUPPLY CURRENT TEMPERATURE
MAX668 toc08
DROPOUT VOLTAGE CURRENT
MAX668 toc09
SHUTDOWN CURRENT (µA) CURRENT INTO
SUPPLY CURRENT (µA) ROSC 200k ROSC 500k ROSC 100k
4.5V
MAX668
DROPOUT VOLTAGE (mV)
CURRENT (mA)
SUPPLY VOLTAGE
TEMPERATURE (°C)
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
Typical Operating Characteristics (continued)
(Circuits Figures +25°C; unless otherwise noted.)
REFERENCE VOLTAGE TEMPERATURE
MAX668 toc10
SWITCHING FREQUENCY ROSC
SWITCHING FREQUENCY (kHz)
MAX668 toc11
1.250 1.249 REFERENCE VOLTAGE 1.248 1.247 1.246 1.245 1.244 1.243 1.242 1.241 1.240
TEMPERATURE (°C)
ROSC
SWITCHING FREQUENCY TEMPERATURE
100k SWITCHING FREQUENCY (kHz) 165k 499k TEMPERATURE (°C)
MAX668 toc12
RISE/FALL TIME CAPACITANCE
MAX668 toc13
RISE/FALL TIME (ns) 3.3V 1000 CAPACITANCE (pF) 3.3V
10,000
1.8V Input, Step-Up Controllers µMAX
Typical Operating Characteristics (continued)
(Circuits Figures +25°C; unless otherwise noted.)
EXITING SHUTDOWN
MAX668 toc14
MAX668/MAX669
ENTERING SHUTDOWN
MAX668 toc15
OUTPUT VOLTAGE 5V/div INDUCTOR CURRENT 2A/div SHUTDOWN VOLTAGE 5V/div 500µs/div MAX668, VOUT 12V, LOAD 1.0A, ROSC 100k, VOLTAGE, NON-BOOTSTRAPPED SHUTDOWN VOLTAGE 5V/div
OUTPUT VOLTAGE 5V/div 200µs/div MAX668, VOUT 12V, LOAD 1.0A, VOLTAGE, NON-BOOTSTRAPPED
HEAVY-LOAD SWITCHING WAVEFORM
MAX668 toc16
LIGHT-LOAD SWITCHING WAVEFORM
VOUT 100mV/div AC-COUPLED
MAX668 toc17
VOUT 200mV/div AC-COUPLED
DRAIN 5V/div
DRAIN 5V/div
1A/div 1µs/div MAX668, VOUT 12V, ILOAD 1.0A, VOLTAGE, NON-BOOTSTRAPPED
1A/div 1µs/div MAX668, VOUT 12V, ILOAD 0.1A, VOLTAGE, NON-BOOTSTRAPPED
LOAD-TRANSIENT RESPONSE
MAX668 toc18
LINE-TRANSIENT RESPONSE
MAX668 toc19
OUTPUT VOLTAGE AC-COUPLED 100mV/div
OUTPUT VOLTAGE 100mV/div AC-COUPLED
LOAD CURRENT 1A/div 1ms/div MAX668, VOUT 12V, ILOAD 0.1A 1.0A, VOLTAGE, NON-BOOTSTRAPPED
INPUT VOLTAGE 5V/div 20ms/div MAX668, VOUT 12V, LOAD 1.0A, HIGH VOLTAGE, NON-BOOTSTRAPPED
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
Description
NAME FUNCTION On-Chip Regulator Output. This regulator powers internal circuitry including gate driver. Bypass with greater ceramic capacitor. Oscillator Frequency Input. resistor from FREQ sets oscillator from 100kHz (ROSC 500k) 500kHz (ROSC 100k). fOSC 1010 ROSC. ROSC still required external clock used SYNC/SHDN. (See SYNC/SHDN FREQ Inputs section.) Analog Ground 1.25V Reference Output. source 50µA. Bypass with 0.22µF ceramic capacitor. Feedback Input. threshold 1.25V. Positive Current-Sense Input. Connect current-sense resistor, RCS, between PGND. Power Ground Gate Driver Negative Current-Sense Input External MOSFET Gate-Driver Output. swings from PGND. Input Supply On-Chip Regulator. accepts inputs 28V. Bypass with 0.1µF ceramic capacitor. Shutdown control Synchronization Input. There three operating modes: SYNC/SHDN low: DC-DC off. SYNC/SHDN high: DC-DC with oscillator frequency FREQ ROSC. SYNC/SHDN clocked: DC-DC with operating frequency SYNC clock input. DC-DC conversion cycles initiate rising edge input clock.
FREQ PGND
SYNC/ SHDN
Detailed Description
MAX668/MAX669 current-mode controllers operate wide range DC-DC conversion applications, including boost, SEPIC, flyback, isolated output configurations. Optimum conversion efficiency maintained over wide range loads employing both operation Maxim's proprietary Idle Mode control minimize operating current light loads. Other features include shutdown, adjustable internal operating frequency synchronization external clock, soft start, adjustable current limit, wide (1.8V 28V) input range.
28V. Bootstrapping required because MAX669 does have undervoltage lockout, instead drives with open-loop, duty-cycle start-up oscillator when below 2.5V. switches closed-loop operation only when exceeds 2.5V. non-bootstrapped connection used with MAX669 (the input voltage) remains below 2.7V, output voltage will soar above regulation point. Table recommends appropriate device each biasing option.
Table MAX668/MAX669 Comparison
FEATURE Input Range Bootstrapped nonbootstrapped. connected input, output, other voltage source such logic supply. stops switching below 2.5V. MAX668 MAX669 1.8V Must bootstrapped (VCC must connected boosted output voltage, VOUT). When above 2.5V
MAX668 MAX669 Differences
Differences between MAX668 MAX669 relate their bootstrapped non-bootstrapped circuits (Table MAX668 operates with inputs connected either bootstrapped non-bootstrapped powered from input supply other source) configuration. When bootstrapped, MAX668 restriction output voltage. When bootstrapped, output cannot exceed 28V. MAX669 optimized input voltages (down 1.8V) requires bootstrapped operation powered from VOUT) with output voltages greater than
Operation
Undervoltage Lockout Soft-Start
1.8V Input, Step-Up Controllers µMAX
Controller
heart MAX668/MAX669 current-mode controller BiCMOS multi-input comparator that simultaneously processes output-error signal, current-sense signal, slope-compensation ramp (Figure main comparator direct summing, lacking traditional error amplifier associated phase shift. direct summing configuration approaches ideal cycle-by-cycle control over output voltage since there conventional error feedback path. mode, controller uses fixed-frequency, current-mode operation where duty ratio input/output voltage ratio (duty ratio (VOUT VIN) boost configuration). current-mode feedback loop regulates peak inductor current function output error signal. light loads controller enters Idle Mode. During Idle Mode, switching pulses provided only needed service load, operating current minimized provide best light-load efficiency. minimum-current comparator threshold 15mV, full-load value (IMAX) 100mV. When controller synchronized external clock, Idle Mode occurs only very light loads.
Bootstrapped/Non-Bootstrapped Operation
Low-Dropout Regulator (LDO) Several biasing options, including bootstrapped non-bootstrapped operation, made possible on-chip, low-dropout regulator. regulator input VCC, while output LDO. MAX668/MAX669 functions, including EXT, internally powered from LDO. -to-LDO dropout voltage typically 200mV (300mV 12mA), that when less than 5.2V, typically 200mV. When dropout, MAX668/MAX669 still operate with long exceeds 2.7V), with reduced amplitude drive EXT. maximum input voltage 28V. supply 12mA power supply gate charge through external FET, supply small external loads. When driving particularly large FETs high switching rates, little current available external loads. example, when switched 500kHz, large with 20nC gate charge requires 20nC 500kHz, 10mA. allow variety biasing connections optimize efficiency, circuit quiescent current, fullload start-up behavior different input output voltage ranges. Connections shown Figures characteristics each outlined Table
MAX668/MAX669
MAX669 ONLY 1.25V ANTISAT 552k UVLO 276k 276k CURRENT SENSE SLOPE COMPENSATION 100mV IMAX 1.25V MAIN COMPARATOR PGND LOW-VOLTAGE START-UP OSCILLATOR (MAX669 ONLY)
MAX668 MAX669
SYNC/SHDN FREQ
BIAS
15mV
IMIN
Figure MAX668/MAX669 Functional Diagram
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
1.8V 68µF 4.7µH
0.1µF
VOUT 0.5A MBRS340T3 IRF7401 0.02 68µF 68µF 0.1µF 218k
MAX669
SYNC/ SHDN 0.22µF FREQ
PGND
220pF
24.9k
100k
Figure MAX669 High-Voltage Bootstrapped Configuration
1.8V
68µF
4.7µH
0.02
VOUT MBRS340T3 FDS6680 IRF7401 68µF 68µF 0.1µF
MAX669
SYNC/ SHDN 0.22µF FREQ
PGND
220pF
24.9k
100k
Figure MAX669 Low-Voltage Bootstrapped Configuration
Bootstrapped Operation With bootstrapped operation, powered from circuit output This improves efficiency when input voltage low, since drives with higher gate voltage than would available from low-voltage input. Higher gate voltage reduces on-resistance, increasing efficiency. Other (undesirable) characteristics bootstrapped operation increased operating power (since higher operating voltage) reduced ability start with high load current input voltages. input volt10
range extends below 2.7V, then bootstrapped operation with MAX669 only option. With connected VOUT, Figure voltage swing when 5.2V more, 0.2V when less than 5.2V. output voltage does exceed 5.5V, on-chip regulator disabled connecting (Figure This eliminates forward drop supplies maximum gate drive external FET.
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
68µF 4.7µH VOUT MBRS340T3 FDS6680 0.02 PGND 220pF 24.9k 68µF 68µF 0.1µF 218k
0.1µF
MAX668
SYNC/ SHDN 0.22µF 100k FREQ
Figure MAX668 High-Voltage Non-Bootstrapped Configuration
2.7V 5.5V
68µF PGND
4.7µH 0.02 VOUT MBRS340T3 FDS6680 68µF 68µF 0.1µF 218k
MAX668
SYNC/ SHDN 0.22µF 100k FREQ
220pF
24.9k
Figure MAX668 Low-Voltage Non-Bootstrapped Configuration
Non-Bootstrapped Operation With non-bootstrapped operation, powered from input voltage (VIN) another source, such logic supply. Non-bootstrapped operation (Figure recommended (but required) input voltages above since amplitude (limited LDO) this voltage range higher than would with bootstrapped operation. Note that non-bootstrapped operation required output voltage exceeds 28V, since this level high safely con-
nect VCC. Also note that only MAX668 used with non-bootstrapped operation. input voltage does exceed 5.5V, on-chip regulator disabled connecting (Figure This eliminates regulator forward drop supplies maximum gate drive external lowest on-resistance. Disabling regulator also reduces non-bootstrapped minimum input voltage from 2.7V.
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
Table Bootstrapped Non-Bootstrapped Configurations
CONFIGURATION FIGURE WITH: INPUT VOLTAGE RANGE* OUTPUT VOLTAGE RANGE COMMENTS
High-Voltage, Bootstrapped
Figure
MAX669
Connect VOUT. Provides maximum external gate drive low-voltage (Input <3V) highvoltage (output >5.5V) boost circuits. VOUT cannot exceed 28V. Connect VOUT LDO. Provides maximum possible external gate drive low-voltage designs, limits VOUT 5.5V less. Connect VCC. Provides widest input output range, external gate drive reduced below Connect LDO. gate-drive amplitude logic-supply (input 5.5V) high-voltage (output >5.5V) boost circuits. operating power less than Figure since current does pass through regulator. Connect separate supply (VBIAS) that powers only gate-drive amplitude VBIAS. Input power source (VIN) output voltage range (VOUT) restricted, except that VOUT must exceed VIN.
Low-Voltage, Bootstrapped
Figure
MAX669
High-Voltage, Non-Bootstrapped
Figure
MAX668
Low-Voltage, Non-Bootstrapped
Figure
MAX668
Extra supply, Non-Bootstrapped
None
MAX668
Restricted
standard step-up DC-DC circuits Figures regulation cannot maintained exceeds VOUT. SEPIC transformer-based circuits have this limitation.
addition configurations shown Table following guidelines help when selecting configuration: ever below 2.7V, must bootstrapped VOUT MAX669 must used. VOUT never exceeds 5.5V, shorted VOUT eliminate dropout voltage regulator. greater than 3.0V, powered from VIN, rather than from VOUT (non-bootstrapped). This save quiescent power consumption, especially when large. never exceeds 5.5V, shorted eliminate dropout voltage regulator.
4.5V range (i.e., 1-cell Li-Ion 3-cell NiMH battery range), bootstrapping from VOUT, although required, increase overall efficiency increasing gate drive (and reducing resistance) expense quiescent power consumption. always exceeds 4.5V, should tied since bootstrapping from does increase gate drive from does increase quiescent power dissipation.
1.8V Input, Step-Up Controllers µMAX
SYNC/SHDN FREQ Inputs
SYNC/SHDN provides both external-clock synchronization desired) shutdown control. When SYNC/SHDN low, functions shut down. logic high SYNC/SHDN selects operation frequency ROSC, connected from FREQ GND. relationship between fOSC ROSC ROSC 1010 fOSC 500kHz operating frequency, example, with ROSC 100k. Rising clock edges SYNC/SHDN interpreted synchronization inputs. sync signal lost while SYNC/SHDN high, internal oscillator takes over last cycle frequency returned rate ROSC. sync lost with SYNC/SHDN low, waits 70µs before shutting down. This maintains output regulation even with intermittent sync signals. When external sync signal used, Idle Mode switchover 15mV current-sense threshold disabled that Idle Mode only occurs very light loads. Also, ROSC should frequency below SYNC clock rate: ROSC(SYNC) 1010 (0.85 fSYNC) Noise considerations dictate setting synchronizing) fOSC above below certain frequency band frequencies, particularly applications. 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; however, MAX668/ MAX669's Idle Mode feature substantially increases light-load efficiency. Higher frequencies exhibit poorer overall efficiency more transition losses FET; however, this shortcoming often nullified trading some inductor capacitor size benefits lower-resistance components. oscillator frequency resistor, ROSC, connected from FREQ GND. ROSC must connected whether part externally synchronized ROSC each case: ROSC 1010 fOSC when using external clock. ROSC(SYNC) 1010 (0.85 fSYNC) when using external clock, fSYNC.
MAX668/MAX669
Soft-Start
MAX668/MAX669 feature "digital" soft start which preset requires external capacitor. Upon start-up, peak inductor increments from value RCS, full current-limit value, five steps over 1024 cycles fOSC fSYNC. example, with 200kHz, complete soft-start sequence takes 5ms. Typical Operating Characteristics photo soft-start operation. Softstart implemented: when power first applied when exiting shutdown with power already applied, when exiting undervoltage lockout. MAX669's soft-start sequence does start until reaches 2.5V.
Setting Output Voltage
output voltage external resistors Figures First select value range. then given [(VOUT VREF) where VREF 1.25V.
Determining Inductance Value
most MAX668/MAX669 boost designs, inductor value (LIDEAL) derived from following equation, which picks optimum value stability based MAX668/MAX669's internally slope compensation: LIDEAL VOUT IOUT fOSC) MAX668/MAX669 allow significant latitude inductor selection LIDEAL convenient value. This happen LIDEAL standard inductance (such 10µH, 22µH, etc.), LIDEAL large obtained with suitable resistance saturation-current rating desired size. Inductance values smaller than LIDEAL used with adverse stability effects; however, peak-to-peak inductor current (ILPP) will rise reduced. This effect raising required ILPK given output power also requiring larger output capacitance maintain
Design Procedure
MAX668/MAX669 operate number DCDC converter configurations including step-up, SEPIC (single-ended primary inductance converter), flyback. following design discussions limited step-up, although SEPIC flyback examples shown Application Circuits section.
Setting Operating Frequency
MAX668/MAX669 operate from 100kHz 500kHz. Choice operating frequency will depend number factors:
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
given output ripple. inductance value larger than LIDEAL also used, output-filter capacitance must increased same proportion that LIDEAL. Capacitor Selection section more information determining output filter values. MAX668/MAX669's high switching frequencies, inductors with ferrite core equivalent recommended. Powdered iron cores recommended their high losses frequencies over 50kHz. NFETs that specify on-resistance with gatesource voltage (VGS) 2.7V less. When selecting NFET, parameters include: Total gate charge (Qg) Reverse transfer capacitance charge (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 RDS(ON), which predicts losses. includes capacitances associated with charging gate. addition, this parameter helps predict current needed drive gate selected operating frequency. continuous current gate IGATE fOSC example, MMFT3055L typical 5V); therefore, IGATE current 500kHz 3.5mA. manufacturer's typical value above equation, since maximum value supplied) usually conservative estimating IGATE.
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 (VIN where forward voltage drop across Schottky rectifier diode (D1), drop across external FET, when (VIN (VOUT fOSC (VOUT where inductor 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 (IOUT VOUT VIN)2 where inductor series resistance. Once peak inductor current selected, currentsense resistor (RCS) determined ILPP 85mV ILPEAK high peak inductor currents (>1A), Kelvin sensing connections should used connect PGND RCS. PGND should tied together ground side RCS.
Diode Selection
MAX668/MAX669'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 adequate using diode manufacturer's data, approximate with following formula: IDIODE IOUT LPEAK Also, diode reverse breakdown voltage must exceed VOUT. high output voltages (50V above), Schottky diodes practical because this voltage requirement. these cases, high-speed silicon rectifier with adequate reverse voltage.
Capacitor Selection
Output Filter Capacitor minimum output filter capacitance that ensures stability (7.5V IDEAL COUT(MIN) (2RCS VIN(MIN) fOSC
where VIN(MIN) minimum expected input voltage. Typically COUT(MIN), though sufficient stability, will
Power MOSFET Selection
MAX668/MAX669 drive wide variety N-channel power MOSFETs (NFETs). Since limits output gate drive more than logic-level NFET required. Best performance, especially input voltages (below 5V), achieved with low-thresh14
1.8V Input, Step-Up Controllers µMAX
adequate output voltage ripple. Since output ripple boost DC-DC designs dominated capacitor equivalent series resistance (ESR), capacitance value times larger than COUT(MIN) typically needed. Low-ESR types must used. Output ripple VRIPPLE(ESR) ILPEAK ESRCOUT bootstrapped configurations with MAX668 MAX669, there circumstances where full load current only applied after circuit started output near value. input voltage drops, this limitation becomes more severe. This characteristic bootstrapped designs occurs when MOSFET gate fully driven until output voltage rises. This problematic because heavily loaded output cannot rise until MOSFET on-resistance. such situations, low-threshold FETs (VTH VIN(MIN)) most effective solution. Typical Operating Characteristics section shows plots startup voltage versus load current typical bootstrapped design.
MAX668/MAX669
Input Capacitor input capacitor (CIN) boost designs reduces current peaks drawn from input supply reduces noise injection. value largely determined source impedance input supply. High source impedance requires high input capacitance, particularly input voltage falls. Since step-up DCDC converters "constant-power" loads their input supply, input current rises input voltage falls. Consequently, low-input-voltage designs, increasing and/or lowering many five percentage points conversion efficiency. good starting point same capacitance value COUT. Bypass Capacitors addition COUT, three ceramic bypass capacitors also required with MAX668/MAX669. Bypass with 0.22µF more. Bypass with more. bypass with 0.1µF more. bypass capacitors should located close their respective pins possible. Compensation Capacitor Output ripple voltage COUT affects loop stability introducing left half-plane zero. small capacitor connected from forms pole with feedback resistance that cancels zero. optimum compensation value
ESRCOUT COUT where feedback resistors (Figures calculated value results non-standard capacitance value, values from 0.5CFB 1.5CFB will also provide sufficient compensation.
Layout Considerations
high current levels fast switching waveforms that radiate noise, proper board layout essential. Protect sensitive analog grounds using star ground configuration. Minimize ground noise connecting GND, PGND, input bypass-capacitor ground lead, output-filter ground lead single point (star ground configuration). Also, minimize trace lengths reduce stray capacitance, trace resistance, radiated noise. trace between external gain-setting resistors must extremely short, must trace between PGND.
Application Circuits
Low-Voltage Boost Circuit Figure shows MAX669 operating low-voltage boost application. MAX669 configured bootstrapped mode improve input voltage performance. IRF7401 N-channel MOSFET selected this application because very 0.7V gate threshold voltage (VGS). This circuit provides output greater than output current operates with input voltages 1.8V. Efficiency typically range. +12V Boost Application Figure shows MAX668 operating boost application. This circuit provides output currents greater than typical efficiency 92%. MAX668 operated non-bootstrapped mode minimize input supply current. This achieves maximum light-load efficiency. input voltages below used, should operated bootstrapped mode achieve best low-voltage performance. 4-Cell SEPIC Power Supply Figure shows MAX668 SEPIC (single-ended primary inductance converter) configuration. This configuration useful when input voltage either
Applications Information
Starting Under Load
non-bootstrapped configurations (Figures MAX668 start with combination output load input voltage which operate when already started. other words, there special limitations start-up non-bootstrapped circuits.
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
larger smaller than output voltage, such when converting four NiMH, NiCd, Alkaline cells output. SEPIC configuration often good choice combined step-up/step-down applications. N-channel MOSFET (Q1) must selected withstand drain-to-source voltage (VDS) greater than input output voltages. coupling capacitor (C2) must low-ESR type achieve maximum efficiency. must also able handle high ripple currents; ordinary tantalum capacitors should used high-current designs. circuit Figure provides greater than output current when operating with input voltage from 25V. Efficiency will typically between 85%, depending upon input voltage output current.
Isolated Power Supply circuit Figure provides isolated output 400mA from input power supply. Transformer provides electrical isolation forward path converter, while TLV431 shunt regulator MOC211 opto-isolator provide isolated feedback error voltage converter. output voltage resistors such that mid-point divider 1.24V (threshold TLV431). Output voltage adjusted from 1.24V selecting proper ratio output voltages greater than substitute TL431 TLV431, 2.5V voltage midpoint voltagedivider.
22µF
4.9µH CTX5-4 VOUT 10µF FDS6680 68µF
FREQ
SHDN
MAX668
100k 0.22µF
0.02
PGND
MBR5340T3, SCHOTTKY DIODE WSL-2512-R020F, 0.02 TPSZ686M020R0150, 68µF, 150m
520pF
Figure MAX668 SEPIC Configuration
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
MBR0540L 220µF SHDN IRF7603 220µF RETURN 47µH 400mA
MBR0540L
MAX668
PGND FREQ
0.22µF 100k MOC211 301k
0.1µF TLV431 100k
0.068µF
COILTRONICS CTX03-14232
Figure Isolated 400mA Power Supply
Chip Information
TRANSISTOR COUNT: 1861
1.8V Input, Step-Up Controllers µMAX MAX668/MAX669
Package Information
10LUMAXB.EPS
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