Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
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19-1403; 3/01
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
External FETs Required Main Output 350mA Logic Supply Fixed 3.3V Adjustable (2.5V 5.5V) Synchronous Rectification High Efficiency 95%) 300kHz (200kHz 400kHz Synchronizable) Fixed-Frequency Operation Secondary Output +28V -28V Bias Programmable Current Limit 0.7V 5.5V Input Voltage Range 20µA Quiescent Current Shutdown Current Low-Battery Comparator Small 16-Pin QSOP Package
MAX1677
MAX1677 compact, high-efficiency, dual-output boost converter portable devices needing regulated supplies, typically logic liquid crystal displays (LCDs). Operation with inputs 0.7V allows MAX1677 accept 3-cell alkaline, NiCd, NiMH batteries well 1-cell lithium-ion batteries. device requires external FETs maintain regulation while consuming only 20µA, making ideal hand-held pen-input devices operating with low-current "sleep" states. MAX1677's primary regulator supplies 350mA either factory-preset 3.3V adjustable 2.5V 5.5V output. On-chip synchronous rectification provides efficiencies 95%. 300kHz externally clocked) pulse-width-modulation (PWM) operation particularly suitable applications needing noise, such those with wireless features. primary converter also features pin-selectable pulse-frequencymodulation (PFM) operation that consumes only 20µA. shutdown state also minimizes battery drain. MAX1677's secondary step-up converter supplies +28V -28V bias, varactor tuning, other high-voltage, low-current functions. Other MAX1677 features include precision reference, logic control inputs both regulators, uncommitted comparator low-battery detection reset function. MAX1677 supplied Maxim's compact 16-pin QSOP package, which occupies more space than standard SO-8.
Ordering Information
PART MAX1677EEE TEMP RANGE -40°C +85°C PIN-PACKAGE QSOP
Typical Operating Circuit
Applications
PDAs Hand-Held Terminals Portable Phones Portable Instruments
0.7V 5.5V MAINOUT) POUT 3.3V MAIN BOOST OUTPUT
Configuration
VIEW
CLK/SEL LCDON LCDPOL POUT PGND LCDFB
MAX1677
LCDLX LCDFB LCDON CLK/SEL LCDPOL PGND LCDGND
±28V BOOST OUTPUT
MAX1677
LCDGND LCDLX
QSOP Maxim Integrated Products
pricing, delivery, ordering information, please contact Maxim/Dallas Direct! 1-888-629-4642, visit Maxim's website www.maxim-ic.com.
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
ABSOLUTE MAXIMUM RATINGS
OUT, LCDON, POUT, LBI, LBO, .-0.3V CLK/SEL, LCDPOL, REF, LCDFB, .-0.3V (VOUT 0.3V) LCDLX .-0.3V +30V PGND, LCDGND .-0.3V +0.3V POUT OUT.-0.3V +0.3V Continuous Power Dissipation +70°C) 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 +160°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
(VOUT 3.3V, CREF 0.1µF, POUT OUT, +85°C, unless otherwise noted. Typical values +25°C.)
PARAMETER GENERAL Input Voltage Range Minimum Startup Voltage Reference Voltage Reference Load Regulation Reference Line Rejection Supply Current Main Supply Current Main DC-DC Mode Supply Current Main DC-DC Mode Supply Current Shutdown MAIN BOOST DC-DC Output Voltage Regulation Voltage Input Current Output Voltage Adjustment Range Startup Normal Mode Transition Voltage (Note Line Regulation Load Regulation Frequency Startup Mode Leakage Current fSTARTUP (LEAK) VLOCKOUT IOUT 150mA, CLK/SEL OUT, 2.4V, ILOAD 10mA 200mA VOUT VOUT (REG) GND, 350mA, CLK/SEL (Note Adjustable mode, CLK/SEL (Note 1.3V 3.20 1.225 3.30 1.25 0.02 3.43 1.275 ILCDOFF IPFM IPWM VSTARTUP VREF (Note +25°C, ILOAD IREF IREF 50µA (Note VOUT 2.5V 5.5V load, current into load, current into load, current into 1.23 1.25 1.27 SYMBOL CONDITIONS UNITS
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VOUT 3.3V, CREF 0.1µF, POUT OUT, +85°C, unless otherwise noted. Typical values +25°C.) PARAMETER On-Resistance Current Limit P-Channel Synchronous Rectifier Turn-Off Current Mode Internal Oscillator Oscillator Maximum Duty Cycle External Clock Frequency Range LOGIC CONTROL INPUTS Input Leakage Current Input Threshold LCDON, LCDPOL, CLK/SEL Input Threshold Falling Threshold Hysteresis Output Voltage Input Bias Current Leakage Current LCDLX Voltage LCDPOL LCDLX Switch Current Limit LCDLX Switch Resistance LCDLX Leakage Current LCDFB Point LCDFB Input Bias Current Line Regulation Load Regulation Maximum LCDLX On-Time Minimum LCDLX Off-Time LCDFB Voltage Startup Mode Operating mode Start-up mode (positive negative) LCDPOL LCDPOL ILOAD 5mA, 1.2V 3.6V, Figure ILOAD 5mA, 2.4V, Figure 0.75 RLCDLX LCDPOL connected through VOUT 3.3V VLCDLX Positive LCD, LCDPOL Negative LCD, LCDPOL 1.225 1.25 LBO(LO) ILBI(BIAS) LBO(LEAK) 5.5V Sink current VON(LOW) VON(HIGH) VLBI(TH) LCDON, LCDPOL, CLK/SEL 1.1V VOUT 5.5V VOUT 2.5V 0.8VOUT 0.2VOUT 0.8VOUT 1.275 0.2VOUT CLK/SEL SYMBOL RLX(ON)N RLX(ON)P ILX(PWM) ILX(PFM) N-channel P-channel N-channel mode N-channel mode CONDITIONS 0.22 UNITS
MAX1677
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
ELECTRICAL CHARACTERISTICS
(VOUT 3.3V, CREF 0.1µF, POUT OUT, -40°C +85°C, unless otherwise noted. (Note PARAMETER GENERAL Supply Current Main Supply Current Main DC-DC Mode Supply Current Main DC-DC Mode Supply Current Shutdown MAIN Output Voltage Regulation Voltage Startup Normal Mode Transition Voltage (Note Leakage Current Current Limit Internal Oscillator External Clock Frequency Range LOGIC Input Threshold LCDON, LCDPOL, CLK/SEL Input Threshold Falling Threshold Output Voltage BIAS DC-DC LCDPOL LCDLX Switch Current Limit LCDPOL connected through Positive LCD, LCDPOL Negative LCD, LCDPOL 1.22 1.28 VON(LOW) VON(HIGH) VLBI(TH) LBO(LO) Sink current 0.8VOUT 1.1V VOUT 5.5V 0.2VOUT 0.8VOUT 0.2VOUT VOUT VFB(REG) VLOCKOUT ILX(LEAK) ILX(PWM) ILX(PFM) N-channel mode N-channel mode CLK/SEL GND, 350mA, CLK/SEL (Note Adjustable mode, CLK/SEL (Note 3.17 1.22 3.46 1.28 ILCDOFF IPFM IPWM load, current into load, current into load, current into SYMBOL CONDITIONS UNITS
LCDFB Point
Note MAX1677 operates bootstrap mode (operates from output voltage). Once started, will operate down 0.7V input. exceeds VOUT, VOUT will follow diode drop below VIN. Note CREF 0.22µF applications where IREF 10µA. Note low-power mode (CLK/SEL GND), output voltage regulates higher than low-noise mode (CLK/SEL synchronized). Note device startup mode when VOUT below this value. Note Specifications -40°C guaranteed design production tested.
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
Typical Operating Characteristics
(Circuits Figures +25°C, unless otherwise noted.)
EFFICIENCY LOAD CURRENT (VOUT 3.3V)
MAX1677-01
EFFICIENCY LOAD CURRENT (VOUT
MAX1677-02
MAXIMUM LOAD CURRENT BATTERY INPUT VOLTAGE (PWM MODE)
MAX1677-03
MODE 2.4V 1.2V EFFICIENCY
LOAD CURENT (mA) VOUT 3.3V VOUT
EFFICIENCY
MODE 3.6V 2.4V 1.2V MODE 3.6V 2.4V 1.2V
MODE 2.4V 1.2V 1000 LOAD CURRENT (mA)
1000
LOAD CURRENT (mA)
INPUT VOLTAGE
EFFICIENCY LOAD CURRENT (LCD VOUT 12V)
MAX1677-04
EFFICIENCY LOAD CURRENT (LCD VOUT 20V)
CIRCUIT FIGURE 3.6V 2.4V 1.2V
MAX1677-05
REFERENCE VOLTAGE REFERENCE CURRENT
MAX1677-06
EFFICIENCY CIRCUIT FIGURE 3.6V 2.4V 1.2V
1.2550
REFERENCE VOLTAGE
EFFICIENCY
1.2525
1.2500
1.2475
1.2450 LOAD CURRENT (mA) REFERENCE CURRENT (µA)
LOAD CURRENT (mA)
LOAD CURRENT STARTUP VOLTAGE
LOAD CURRENT (mA) START-UP VOLTAGE VOUT 3.3V TESTED WITH RESISTIVE LOAD
MAX1677-07
NO-LOAD SUPPLY CURRENT INPUT VOLTAGE (LCD OFF)
MAX1677-08
NO-LOAD SUPPLY CURRENT INPUT VOLTAGE (LCD
SUPPLY CURRENT (mA) VOUT 3.3V MODE VLCD -20V
MAX1677-09
0.20 0.18 0.16 SUPPLY CURRENT (mA) 0.14 0.12 0.10 0.08 0.06 0.04 0.02 VOUT 3.3V MODE
INPUT VOTAGE
INPUT VOLTAGE
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
Typical Operating Characteristics (continued)
(Circuits Figures +25°C, unless otherwise noted.)
MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PWM MODE)
MAX1677-10
MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PFM MODE)
MAX1677-11
100mA/
100mA/
50mV/ 1µs/div 2.4VIN, 3.3VOUT, 200mA IOUT
VRIPPLE
20mV/
VRIPPLE
2µs/div 1.2VIN, 3.3VOUT, 20mA IOUT
MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PFM MODE, 50mA OUTPUT)
MAX1677-12
SWITCHING WAVEFORMS
MAX1677-13
10mV/
100mA/
200mA/ 100mV/
VRIPPLE
50mV/ 10µs/div PFM, 1.2VIN, 3.3VOUT, 50mA IOUT
VRIPPLE 5µs/div LDCLX CURRENT LIMIT 350mA, 2.4VIN, +12VOUT, 10mA LOAD
SWITCHING WAVEFORMS (50k FROM LCDPOL OUT)
MAX1677-14
MAIN BOOST CONVERTER LOAD TRANSIENT
MAX1677-15
10mV/
50mV/
VRIPPLE
200mA/ 100mV/
200mA/ VRIPPLE IOUT
2µs/div LCDLX CURRENT LIMIT 225mA, 2.4VIN, +12VOUT, 10mA LOAD
2ms/div 2.4V, VOUT 3.3V ILOAD 200mA
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
Typical Operating Characteristics (continued)
(Circuits Figures +25°C, unless otherwise noted.)
MAIN BOOST CONVERTER LINE TRANSIENT
MAX1677-16
MAX1677
LINE TRANSIENT (VLCD +12V)
MAX1677-17
50mV/
VOUT
50mV/
VLCD
5ms/div VOUT 3.3V, ILOAD 150mA
5ms/div VLCD +12V, IOUT
LINE TRANSIENT (VLCD -20V)
MAX1677-18
MAIN BOOST CONVERTER STARTUP DELAY
MAX1677-19
50mV/ VLCD 5ms/div VLCD -20V, IOUT 500µs/div 2.4V, VOUT 3.3V, ILOAD 10mA VOUT
STARTUP DELAY
MAX1677-20
LCDON
10V/
VLCD
10ms/div 2.4V, VLCD -20V, IOUT
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
Description
NAME FUNCTION Output Sense Input. device powered from OUT. Bypass with 0.1µF ceramic capacitor. Connect POUT through series resistor. Dual ModeMain Boost Feedback Input. Connect 3.3V output. Connect voltage-divider from adjust output 2.5V 5.5V range (Figure Low-Battery-Comparator Input. Threshold 614mV. low-battery trip-point with external voltage divider (Figure Open-Drain, Low-Battery Output. when below 614mV, otherwise remains high. Sync Clock Select Input. CLK/SEL low: low-power, low-quiescent-current mode. CLK/SEL high: low-noise, high-power mode 300kHz. CLK/SEL driven with external clock 200kHz 400kHz, synchronized high-power mode. Enable Input. Drive high turn boost converter. Main DC-DC must also Polarity Select Input. Sets boost converter polarity peak current output (Table 1.25V Reference Output. Bypass with 0.1µF. Ground Feedback Input. Threshold 1.25V positive with LCDPOL high, negative with LCDPOL low. I.C. Enable Input. Drive high enable MAX1677. Boost Switch Drain Source Internal N-Channel DMOS Boost-Converter Switch Source Internal N-Channel Main Boost-Converter Switch Main Output Boost Internal Switch Drain Boost DC-DC Converter Power Output. Source internal P-channel MOSFET main boost-converter synchronous rectifier.
CLK/SEL
LCDON LCDPOL LCDFB LCDLX LCDGND PGND POUT
Dual Mode trademark Maxim Integrated Products.
Detailed Description
MAX1677 highly-efficient, dual-output power supply battery-powered devices. On-chip complete step-up DC-DC converters power main logic bias (Figure main boost converter (MBC) on-chip P-channel N-channel MOSFETs that provide synchronous-rectified voltage conversion maximum efficiency loads 300mA. Table available output current with typical battery configurations. output voltage factory-preset 3.3V, from 2.5V 5.5V with external resistors (dual-mode operation). Either fixed-frequency low-operating-current operation selected using CLK/SEL input (Table
boost converter (LCD) includes internal Nchannel DMOS switch generate positive negative voltages ±28V. polarity output LCDPOL input (Table Figure shows MAX1677 configured positive output voltage with 3.3V main output. Figure shows MAX1677 configured negative output. LCDPOL also allows current limit LCDLX reduced from 350mA 225mA allow minimum-size inductors low-current applications (typically loads <10mA). Also included MAX1677 precision 1.25V reference that sources 50µA, logic shutdown control (the must operate), low-battery comparator.
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
MAX1677
STARTUP
839k
MAIN DC-DC SUCLK
CLK/SEL POUT PGND
501k
REFERENCE
2.25V ISET/POL SENSE LCDPOL ILCDLX START-UP ILCDLX LCDDRV LCDFB BIAS LCDON LCDLX
LCDGND
Figure Functional Block Diagram
Table Main Boost Converter Available Output Current
NUMBER CELLS Alk/NiCd/NiMH Alk/NiCd/NiMH Alk/NiCd/NiMH Alk/NiCd/NiMH Alk/NiCd/NiMH Li-Ion OUTPUT INPUT CURRENT OUTPUT VOLTAGE (mA) VOLTAGE PWM/PFM 140/150 100/70 350/170 260/125 350/170
Main Boost Converter (MBC)
operates either mode, 300kHz mode, externally synchronized mode selected CLK/SEL input (Table mode offers fixed-frequency operation maximum output power. mode offers lowest operating current. current limit reduced mode increase efficiency minimize output ripple. Mode When CLK/SEL high, MAX1677 operates high-power, low-noise mode, switching 300kHz internal oscillator frequency. MOSFET switch pulse-width modulated control power transferred each switching cycle regulate
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
3.3V MAIN BOOST OUTPUT 0.1µF POUT 100µF 10µH LCDLX LCDON CLK/SEL LCDPOL LCDGND LCDFB MBR0530 BOOST OUTPUT 4.7µF 100µF PWM-MODE CURRENTLIMIT LEVEL FEEDBACK POUT
MAX1677
10µH
PGND
0.1µF
Figure Controller Block Diagram Mode
PGND
Figure Converter Positive Mode
3.3V MAIN BOOST OUTPUT 100µF 100µF 0.1µF 0.1µF LCDGND LCDPOL PGND LCDFB MBR0530 MBR0530
output voltage. mode, supply 350mA. Switching harmonics generated fixedfrequency operation consistent easily filtered. During operation, rising edge internal clock sets flip-flop, which turns N-channel MOSFET (Figure switch turns when voltage-error, slope-compensation, currentfeedback signals trips multi-input comparator resets flip-flop; switch remains rest cycle. Changes output voltage error signal shift inductor current level modulate MOSFET pulse width. Clock-Synchronized MAX1677 operates clock-synchronized current-mode when clock signal (200kHz 400kHz) applied CLK/SEL. This allows switching harmonics positioned avoid sensitive frequency bands, such those near frequencies wireless applications. Power Mode Pulling CLK/SEL places MAX1677 low-power standby mode. During standby mode, operation regulates output voltage transferring fixed amount energy during each cycle, then modulating switching frequency control power delivered output. device switches only needed service load, resulting highest possible efficiency light loads operating current only 20µA. supply 170mA when mode (Table
0.1µF
POUT
MAX1677
10µH 10µH
LCDLX LCDON CLK/SEL
-LCD BOOST OUTPUT 4.7µF
Figure Converter Negative Mode
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
During operation, error comparator detects when output voltage regulation sets flip-flop, turning N-channel MOSFET switch (Figure When inductor current ramps mode current limit (350mA), current-sense comparator resets flip-flop. flip-flop turns N-channel switch turns P-channel synchronous rectifier. energy stored inductor transferred output through P-channel switch. second flip-flop, previously reset switch's "on" signal, inhibits next cycle until inductor current depleted output regulation. This forces operation with discontinuous inductor current mode. Startup Oscillator employs low-voltage startup oscillator ensure 1.1V (0.9V typical) startup voltage. startup, output voltage less than 2.25V, P-channel switch stays N-channel pulses duty cycle. When output voltage exceeds 2.25V, normal control circuitry takes over. Once regulation, operate with inputs down 0.7V since internal power taken from OUT. cannot supply full output current until reaches 2.5V. Synchronous Rectifier MAX1677 features internal P-channel synchronous rectifier. Synchronous rectification typically improves efficiency more over similar nonsynchronous step-up designs. mode, synchronous rectifier turns during second half each cycle. mode, internal comparator turns synchronous rectifier when voltage exceeds output, then turns when inductor current drops below 90mA (typ). on-chip synchronous rectifier allows external Schottky diode omitted designs that operate from inputs exceeding 1.4V. circuits operating below 1.4V (1-cell inputs, example), connecting Schottky diode parallel with internal synchronous rectifier (from POUT) provides lowest startup voltage.
MAX1677
Boost Converter (LCD)
converter configured positive negative output setting LCDPOL using appropriate circuit (Figures Table combination peak current limiting pair one-shot timers control switching. During oncycle internal N-channel DMOS switch turns inductor current ramps until either switch peak current limit reached 5.2µs maximum ontime expires (typically input voltages). After on-cycle terminates, switch turns output capacitor charges. switch remains until error comparator initiates another cycle. LCDLX current limit LCDPOL, outlined Table lower, 225mA peak current setting allows tiny low-current "chip" inductors used when powering smaller (less than square inches) liquid crystal panels. following equation determine which LCDLX current-limit setting required. ILCD (0.7 IPK(LCD) VIN(MIN)) VLCD(MAX)) where ILCD output current, VIN(MIN) minimum expected input voltage, VLCD(MAX) maximum required output voltage, PK(LCD) 350mA 225mA LCDPOL. term correction factor conservatively account typical switch, inductor, diode losses. boost enabled when both LCDON high, output voltage within value. soft-start startup mode with increased
Table Selecting Operating Mode
CLK/SEL Clock (200Hz 400kHz) MODE Low-Power Synchronized FEATURES Lowest Supply Current High Output Current, Fixed-Frequency Ripple High Output Current, Synchronized Ripple Frequency
LOGIC HIGH POUT
VREF CURRENT LIMIT LEVEL
PGND
Figure Controller Block Diagram Mode
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
Table Setting Output Polarity Peak Inductor Current
OUTPUT POLARITY Positive Negative Positive Negative LCDPOL CONNECTED through through LCDLX PEAK INDUCTOR CURRENT (mA)
high, series combination source current into summing node current flows into IC).
Design Procedure
feedback (FB) features Dual Mode operation. With grounded, output preset 3.3V. also adjusted from 2.5V 5.5V with external resistors, shown Figure output voltage externally, select resistor 200k range. Calculate using: [(VOUT 1.25V)
time reduces transient input current when activated.
Setting Output Voltage
either positive negative output voltages, voltage with external resistors, shown Figures Since input current maximum 50nA, large resistors used without significant accuracy loss. Begin selecting
(VTRIP: POUT
Shutdown: LCDON
logic-low level shuts down MAX1677 circuits including converter, reference, comparator. logic-high level LCDON activates boost converter. boost converter only activated when high. When low, MAX1677 draws 1µA.
Low-Battery Comparator
MAX1677 on-chip comparator low-battery detection. voltage falls below 614mV, open-drain output) sinks current GND. low-battery trip level resistors (Figure Since input current less than 50nA, large resistor values 130k) used minimize input loading. Calculate follows: [(VTRIP 0.614V) Connect pullup resistor (R8) when driving CMOS logic. open-drain output pulled high regardless voltage OUT. When above 0.614V, high impedance. comparator used, ground LBI. Since low-battery comparator noninverting, hysteresis added connecting resistor (R7) from shown Figure When
(VTRIP) POUT LOW-BATTERY OUTPUT LOGIC POWER
MAX1677
100k
0.614V
0.614V 0.614V VPOUT 0.614V
WHERE RISING VTRIP LEVEL FALLING VTRIP LEVEL.
Figure Adding External Hysteresis Comparator
POUT
OUTPUT
MAX1677
MAX1677
Figure Setting Low-Battery Trip Threshold
Figure Setting Output Voltage Externally
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
200k range calculate using following equations (for positive negative output). positive output, connect LCDPOL shown Figure This sets threshold LCDFB 1.25V. Select desired output voltage (VLCD), calculate positive output: [(VLCD 1.25V) Figure shows standard circuit generating negative supply. This connection limits VLCD values between -VIN -28V. smaller negative output voltage required, D2's cathode connected rather than ground. This alternate connection permits output voltages from VIN. negative output voltage, connect LCDPOL GND. feedback threshold voltage LCDFB Select desired output voltage (VLCD), calculate negative output: VLCD 1.25V minimize ripple output prevent subharmonic noise caused switching pulse grouping, necessary some board layouts connect small capacitor parallel with values 500k range, 22pF usually adequate. Many bias applications require adjustable output voltage. Figure external control voltage (generated potentiometer, DAC, filtered control signal, other source) coupled LCDFB through resistor RADJ. output voltage this circuit, both positive negative outputs, given VOUT VINIT RADJ)(VLCDFB VADJ) where VINIT initial output obtained without added adjust voltage, calculated preceding equations. VLCDFB 1.25V positive configuration, negative configuration. sets output adjustment span, which 1.25V RADJ either polarity output. Note that
VLCD RADJ (REF) VADJ
raising VADJ lowers VOUT positive output designs, while negative output designs, raising VADJ increases magnitude negative output.
MAX1677
Higher Output Voltages
application requires output voltages greater than +28V, connection Figure This circuit adds capacitor-diode charge pump stage increase output voltage without increasing voltage stress LCDLX pin. maximum output voltage circuit +55V output current slightly less than half that available from standard circuit Figure Figure diodes should least 30V-rated Schottky diodes such 1N5818 MBR0530L equivalent. Capacitors should also rated 30V, while must rated maximum output voltage.
Applications Information
Inductor Selection
MAX1677's high switching frequency allows small surface-mount inductors. 10µH values shown Figures recommended most applications, although values between 4.7µH 47µH suitable. Smaller inductance values typically offer smaller physical size given series resistance, allowing smallest overall circuit dimensions. Larger inductance values exhibit higher output current capability, larger physical dimensions.
MAX1677
LCDPOL
10µH
+40V/5mA (SET MORE THAN 55V) 2.2µF
LCDLX
2.2µF
LCDFB
MAX1677
RATED SCHOTTKY DIODES: MBR0530L EQUIVALENT.
Figure Adjusting Output Voltage
Figure Higher Output Voltage
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
inductors with ferrite core equivalent; powder iron cores recommended with MAX1677's high switching frequencies. inductor's incremental saturation rating ideally should exceed selected current limit, however generally acceptable bias most inductors into saturation much (although this reduce efficiency). best efficiency, select inductors with resistance greater than internal N-channel resistance each boost converter (220m MBC, LCD). inductor effectively series with input times, inductor wire losses roughly approximated IIN2 Table list inductor suppliers. boost converter (LCD) features selectable inductor/switch current limit 350mA 225mA. higher current setting provides greatest output current, while lower setting allows smallest inductor size.
MAX1677
External Diodes
MAX1677's on-chip synchronous rectifier allows normally required external Schottky diode omitted from designs whose input exceeds 1.4V. circuits that need operate below 1.4V (1-cell inputs example), connecting Schottky diode parallel with internal synchronous rectifier (from POUT) provides lowest start-up voltage. Suitable devices 1N5817 MBR0520L, however diode current rating need match peak switch current, since most current handled onchip synchronous rectifier. Since boost converter (LCD) does have synchronous rectification, external diode always needed. High switching speed demands high-speed rectifier. best efficiency, Schottky diodes such 1N5818 MBR0530L recommended. sure that diode current rating exceeds peak current LCDPOL, that diode voltage rating exceeds output voltage. particularly cost-sensitive applications, LCD's 225mA peak current set, high-speed silicon signal diode (such 1N4148) used instead Schottky diode, with reduced efficiency.
Table Component Suppliers
SUPPLIER INDUCTORS INDUCTORS Coilcraft: series Murata: LQH4 LQH3C series Sumida: CDR, series TDK: Series CAPACITORS CAPACITORS AVX: series Matsuo: series Sanyo: OS-CON series Sprague: 595D series DIODES Motorola: MBR0520 Nihon: EC11 series 602-303-5454 805-867-2555 602-994-6430 805-867-2698 803-946-0690 714-969-2591 619-661-6835 603-224-1961 803-626-3123 714-960-6492 619-661-1055 603-224-1430 847-639-6400 814-237-1431 847-956-0666 847-390-4373 847-639-1469 814-238-0490 847-956-0702 847-390-4428 PHONE
Input Bypass Capacitors
low-ESR input capacitor connected parallel with battery will reduce peak currents input-reflected noise. Battery bypassing especially helpful input voltages with high-impedance batteries (such alkaline types). Benefits include improved efficiency lower useful end-of-life voltage battery. 100µF typically recommended 2-cell applications. Small ceramic capacitors also used light loads applications that tolerate higher input ripple. Only input bypass capacitor typically needed both LCD.
Output Filter Capacitors
most applications, 100µF, 10V, low-ESR output filter capacitor recommended output. surface-mount tantalum capacitor typically exhibits 30mV ripple when stepping from 1.2V 3.3V 100mA. OS-CON ceramic capacitors offer lowest ESR, while low-ESR tantalums offer good balance between cost performance. output typically exhibits less than peak-topeak ripple with 4.7µF filter capacitance. This either ceramic tantalum type, sure that capacitor voltage rating exceeds output voltage. LCD's 225mA peak switch current setting used, designer choose lower output ripple
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter
reduce output filter 2.2µF. Ceramic capacitors will exhibit lower ripple than equivalent value even higher value) tantalums lower ESR.
Chip Information
TRANSISTOR COUNT: 1221
MAX1677
Layout Considerations
MAX1677's high-frequency operation makes board layout important minimizing ground bounce noise. Protect sensitive analog grounds using star ground configuration. Minimize ground noise connecting PGND, input bypass-capacitor ground terminal, output filter-capacitor ground terminal single point (star ground configuration). Also, minimize lead lengths reduce stray capacitance trace resistance. Where external resistor-divider used output voltage, trace from LCDFB feedback resistors should extremely short minimize coupling from LCDLX. maximize efficiency minimize output ripple, ground plane connect MAX1677 PGND pins directly ground plane. Consult MAX1677 evaluation full board example.
Compact, High-Efficiency, Dual-Output Step-Up Bias DC-DC Converter MAX1677
Package Information
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.
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