19-1403; Rev 0; 11/98 NUAL KIT MA ATION U EET L H A S V E S DATA W O L L FO Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter The MAX1677 is a compact, high-efficiency, dual-output boost converter for portable devices needing two regulated supplies, typically for logic and liquid crystal displays (LCDs). Operation with inputs as low as 0.7V allows the MAX1677 to accept 1, 2, or 3-cell alkaline, NiCd, or NiMH batteries as well as 1-cell lithium-ion batteries. The device requires no external FETs and can maintain regulation while consuming only 20µA, making it ideal for hand-held pen-input and PDA devices operating with low-current “sleep” states. The MAX1677’s primary regulator supplies up to 350mA at either a factory-preset 3.3V or an adjustable 2.5V to 5.5V output. On-chip synchronous rectification provides efficiencies up to 95%. 300kHz (or externally clocked) pulse-width-modulation (PWM) operation is particularly suitable for applications needing low noise, such as those with wireless features. The primary converter also features pin-selectable pulse-frequencymodulation (PFM) operation that consumes only 20µA. A 1µA shutdown state also minimizes battery drain. The MAX1677’s secondary step-up converter supplies up to +28V or -28V for LCD bias, varactor tuning, or other high-voltage, low-current functions. Other MAX1677 features include precision reference, logic control inputs for both regulators, and an uncommitted comparator for low-battery detection or a reset function. The MAX1677 is supplied in Maxim’s compact 16-pin QSOP package, which occupies no more space than a standard SO-8. Features ♦ No External FETs Required ♦ Main Output Up to 350mA for Logic Supply Fixed 3.3V or Adjustable (2.5V to 5.5V) Synchronous Rectification for High Efficiency (up to 95%) 300kHz (200kHz to 400kHz Synchronizable) Fixed-Frequency PWM Operation ♦ Secondary Output Up to +28V or -28V for LCD Bias Programmable Current Limit ♦ 0.7V to 5.5V Input Voltage Range ♦ 20µA Quiescent Current ♦ 1µA Shutdown Current ♦ Low-Battery Comparator ♦ Small 16-Pin QSOP Package Ordering Information PART MAX1677EEE TEMP. RANGE PIN-PACKAGE -40°C to +85°C 16 QSOP Typical Operating Circuit Applications PDAs Portable Phones Hand-Held Terminals Portable Instruments VIN = 0.7V to 5.5V (UP TO MAINOUT) 3.3V MAIN BOOST OUTPUT POUT LX Pin Configuration MAX1677 TOP VIEW 16 POUT OUT 1 FB 2 15 LX LBI 3 14 PGND LBO 4 MAX1677 13 LCDGND LCDON 6 PWM PFM LBO LCDPOL PGND 9 REF 8 REF CLK/SEL ON +VE OUT -VE OUT ±28V LCD BOOST OUTPUT LCDFB ON 11 ON 10 LCDFB LBI LCDON OFF LCDPOL 7 OUT ON OFF 12 LCDLX CLK/SEL 5 LCDLX FB LCDGND GND GND QSOP ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX1677 General Description MAX1677 Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter ABSOLUTE MAXIMUM RATINGS OUT, LCDON, ON, POUT, LBI, LBO, LX to GND .............................................................-0.3V to +6V CLK/SEL, LCDPOL, REF, LCDFB, FB to GND .............................................-0.3V to (VOUT + 0.3V) LCDLX to GND .......................................................-0.3V to +30V PGND, LCDGND to GND ......................................-0.3V to +0.3V POUT to OUT.........................................................-0.3V to +0.3V Continuous Power Dissipation (TA = +70°C) 16-Pin QSOP (derate 8.3mW/°C above +70°C)...........696mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +160°C Lead Temperature (soldering, 10sec) .............................+300°C Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VOUT = 3.3V, CREF = 0.1µF, POUT = OUT, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 5.5 V V GENERAL Input Voltage Range Minimum Start-Up Voltage Reference Voltage VIN VSTARTUP VREF (Note 1) 0.7 TA = +25°C, ILOAD < 1mA IREF = 0 1.23 0.9 1.1 1.25 1.27 V 2 15 mV Reference Load Regulation IREF = 0 to 50µA (Note 2) Reference Line Rejection VOUT = 2.5V to 5.5V 0.2 5 mV ILCDOFF No load, current into OUT 20 40 µA Supply Current All On, Main DC-DC in PFM Mode IPFM No load, current into OUT 35 60 µA Supply Current All On, Main DC-DC in PWM Mode IPWM No load, current into OUT 115 300 µA 0.3 5 µA V Supply Current Main DC On, LCD Off Supply Current in Shutdown MAIN BOOST DC-DC Output Voltage FB Regulation Voltage FB Input Current VOUT VFB(REG) IFB FB = GND, 0 ≤ ILX ≤ 350mA, CLK/SEL = OUT (Note 3) 3.20 3.30 3.43 Adjustable mode, CLK/SEL = OUT (Note 3) 1.225 1.25 1.275 V 0.02 50 nA 2.5 5.5 V 2.1 2.4 V VFB = 1.3V Output Voltage Adjustment Range Start-Up to Normal Mode Transition Voltage (Note 4) VLOCKOUT Line Regulation IOUT = 150mA, VIN = 2V to 3V 0.6 % Load Regulation CLK/SEL = OUT, VIN = 2.4V, I L OAD = 10mA to 200mA 1 % Frequency in Start-Up Mode fSTARTUP LX Leakage Current ILX(LEAK) 2 VOUT = 1.5V 40 0.2 _______________________________________________________________________________________ 300 kHz 5 µA Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677 ELECTRICAL CHARACTERISTICS (continued) (VOUT = 3.3V, CREF = 0.1µF, POUT = OUT, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER LX On-Resistance LX Current Limit SYMBOL CONDITIONS MIN TYP MAX UNITS RLX(ON)N N-channel 0.22 0.5 RLX(ON)P P-channel 0.4 1.0 ILX(PWM) N-channel PWM mode 550 670 800 ILX(PFM) N-channel PFM mode 250 350 450 40 90 140 mA 240 300 360 kHz 80 85 90 % 400 kHz 1 µA P-Channel Synchronous Rectifier Turn-Off Current in PFM Mode Internal Oscillator f Oscillator Maximum Duty Cycle D CLK/SEL = OUT External Clock Frequency Range 200 Ω mA LOGIC AND CONTROL INPUTS Input Leakage Current ON, LCDON, LCDPOL, CLK/SEL VON(LOW) ON Input Threshold VON(HIGH) LCDON, LCDPOL, CLK/SEL Input Threshold LBI Falling Threshold VIL VIH 1.1V < VOUT < 5.5V VOUT > 2.5V VLBI(TH) 0.2VOUT 0.8VOUT 0.2VOUT 0.8VOUT 599 LBI Hysteresis 614 629 1 LBO Output Low Voltage LBI Input Bias Current LBO Leakage Current V LBO(LO) Sink current = 1mA ILBI(BIAS) I LBO(LEAK) V LBO = 5.5V V V mV % 0.1 V 50 nA 1 µA 28 V LCD BIAS DC-DC LCDLX Voltage LCDLX Switch Current Limit LCDLX Switch Resistance RLCDLX LCDLX Leakage Current LCDPOL = OUT or GND 300 350 450 LCDPOL connected to OUT or GND through 50kΩ 150 225 300 1.0 1.4 Ω 1 µA VOUT = 3.3V VLCDLX = 28V Positive LCD, LCDPOL = OUT LCDFB Set Point Negative LCD, LCDPOL = GND mA 1.225 1.25 1.275 V -15 0 15 mV 50 nA LCDFB Input Bias Current LCD Line Regulation ILOAD = 5mA, VIN = 1.2V to 3.6V, Figure 2 0.1 %/V LCD Load Regulation ILOAD = 0 to 5mA, VIN = 2.4V, Figure 2 0.5 % Maximum LCDLX On-Time Minimum LCDLX Off-Time LCDFB Voltage for Start-Up Mode tON LCD 3.4 4.3 5.2 Operating mode 0.8 1 1.2 Start-up mode (positive or negative) 3.0 4.0 5.0 LCDPOL = OUT 0.75 LCDPOL = GND 0.5 µs µs V _______________________________________________________________________________________ 3 MAX1677 Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter ELECTRICAL CHARACTERISTICS (VOUT = 3.3V, CREF = 0.1µF, POUT = OUT, TA = -40°C to +85°C, unless otherwise noted. ) (Note 5) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS GENERAL General Supply Current Main DC On, LCD Off ILCDOFF No load, current into OUT 40 µA Supply Current All On, Main DC-DC in PFM Mode IPFM No load, current into OUT 60 µA Supply Current All On, Main DC-DC in PWM Mode IPWM No load, current into OUT 300 µA 5 µA Supply Current in Shutdown MAIN Main BOOST DC-DC Output Voltage FB Regulation Voltage Start-Up to Normal Mode Transition Voltage (Note 4) LX Leakage Current LX Current Limit Internal Oscillator VOUT VFB(REG) FB = GND, 0 ≤ ILX ≤ 350mA, CLK/SEL = OUT (Note 3) 3.17 3.4 V Adjustable mode, CLK/SEL = OUT (Note 3) 1.22 1.28 V 2.1 2.4 V 5 µA VLOCKOUT ILX(LEAK) ILX(PWM) N-channel PWM mode 550 900 ILX(PFM) N-channel PFM mode 250 500 CLK/SEL = OUT 240 360 kHz 200 400 kHz f External Clock Frequency Range mA LOGIC AND CONTROL INPUTS ON Input Threshold LCDON, LCDPOL, CLK/SEL Input Threshold LBI Falling Threshold LBO Output Low Voltage VON(LOW) VON(HIGH) 1.1V < VOUT < 5.5V 0.2VOUT 0.8VOUT VIL 0.2VOUT VIH 0.8VOUT VLBI(TH) V LBO(LO) 599 Sink current = 1mA V V 629 mV 0.1 V LCD BIAS DC-DC LCDLX Switch Current Limit LCDFB Set Point LCDPOL = OUT or GND 300 450 LCDPOL connected to OUT or GND through 50kΩ 150 300 Positive LCD, LCDPOL = OUT 1.22 1.28 V Negative LCD, LCDPOL = GND -20 +20 mV mA Note 1: The MAX1677 operates in bootstrap mode (operates from the output voltage). Once started, it will operate down to 0.7V input. If VIN exceeds the set VOUT, VOUT will follow one diode drop below VIN. Note 2: CREF = 0.22µF for applications where IREF > 10µA. Note 3: In low-power mode (CLK/SEL = GND), the output voltage regulates 1% higher than in low-noise mode (CLK/SEL = OUT or synchronized). Note 4: The device is in a start-up mode when VOUT is below this value. Note 5: Specifications to -40°C are guaranteed by design and not production tested. 4 _______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter 100 D 80 EFFICIENCY (%) D B 70 1 10 100 C 500 400 VOUT = 3.3V 300 VOUT = 5V 200 PFM MODE D: VIN = 3.6V E: VIN = 2.4V F: VIN = 1.2V 100 0 0.1 1 10 100 1000 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 LOAD CURRENT (mA) LOAD CURRENT (mA) INPUT VOLTAGE (V) EFFICIENCY vs. LOAD CURRENT (LCD VOUT = 12V) EFFICIENCY vs. LOAD CURRENT (LCD VOUT = 20V) REFERENCE VOLTAGE vs. REFERENCE CURRENT A 70 B 60 REFERENCE VOLTAGE (V) 80 80 A 70 B C 60 50 4.0 MAX1677-06 CIRCUIT OF FIGURE 2 A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V 90 1.2550 MAX1677-05 100 MAX1677-04 CIRCUIT OF FIGURE 2 A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V EFFICIENCY (%) 1.2525 1.2500 1.2475 C 50 0.1 1 10 10 100 0 40 60 80 100 REFERENCE CURRENT (µA) LOAD CURRENT vs. START-UP VOLTAGE NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (LCD OFF) NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (LCD ON) MAX1677-07 0.20 SUPPLY CURRENT (mA) 250 200 150 100 VOUT = 3.3V PFM MODE LCD OFF 0.18 0.16 PWM 300 PFM 1.1 0.14 0.12 0.10 0.08 0.06 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 50 0.02 0.1 0 0 0.5 1.0 1.5 2.0 START-UP VOLTAGE (V) 2.5 3.0 VOUT = 3.3V PFM MODE VLCD = -20V 1.0 0.04 0 20 LOAD CURRENT (mA) VOUT = 3.3V TESTED WITH RESISTIVE LOAD 350 1 LOAD CURRENT (mA) 450 400 1.2450 0.1 100 MAX1677-09 40 SUPPLY CURRENT (mA) EFFICIENCY (%) 40 1000 100 LOAD CURRENT (mA) PWM MODE A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V B 0 60 90 60 20 PWM MODE C = 2.4V D = 1.2V 0.1 E F 600 A MAX1677-08 EFFICIENCY (%) 90 80 700 MAX1677-03 C LOAD CURENT (mA) A MAX1677-02 PFM MODE A = VIN = 2.4V B = VIN = 1.2V MAX1677-01 100 MAXIMUM LOAD CURRENT vs. BATTERY INPUT VOLTAGE (PWM MODE) EFFICIENCY vs. LOAD CURRENT (VOUT = 5V) EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) 0 0 0.5 1.0 1.5 2.0 2.5 INPUT VOTAGE (V) 3.0 3.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 INPUT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX1677 Typical Operating Characteristics (Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.) MAX1677 Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter Typical Operating Characteristics (continued) (Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.) MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PFM MODE) MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PWM MODE) MAX1677-11 MAX1677-10 2V/ div VLX 2V/ div VLX 100mA/ div ILX 100mA/ div ILX 50mV/ div VRIPPLE VRIPPLE 20mV/ div 2µs/div 1µs/div 1.2VIN, 3.3VOUT, 20mA IOUT 2.4VIN, 3.3VOUT, 200mA IOUT MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PFM MODE, 50mA OUTPUT) LCD SWITCHING WAVEFORMS MAX1677-12 MAX1677-13 2V/ div VLX 10mV/ div VLX 100mA/ div ILX 200mA/ div ILX 100mV/ div VRIPPLE 50mV/ div VRIPPLE 10µs/div 5µs/div PFM, 1.2VIN, 3.3VOUT, 50mA IOUT LDCLX CURRENT LIMIT = 350mA, 2.4VIN, +12VOUT, 10mA LOAD LCD SWITCHING WAVEFORMS (50kΩ FROM LCDPOL TO OUT) MAIN BOOST CONVERTER LOAD TRANSIENT MAX1677-14 MAX1677-15 10mV/ div VLX 200mA/ div ILX 50mV/ div VRIPPLE 200mA/ div 100mV/ div 2µs/div LCDLX CURRENT LIMIT = 225mA, 2.4VIN, +12VOUT, 10mA LOAD 6 IOUT VRIPPLE 2ms/div VIN = 2.4V, VOUT = 3.3V ILOAD = 0 to 200mA _______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter LCD LINE TRANSIENT (VLCD = +12V) MAIN BOOST CONVERTER LINE TRANSIENT MAX1677-16 1V/ div MAX1677-17 1V/ div VIN VIN 0V 0V 50mV/ div VOUT 50mV/ div 5ms/div VIN = 2V TO 3V, VOUT = 3.3V, ILOAD = 150mA LCD LINE TRANSIENT (VLCD = -20V) 1V/ div VLCD 5ms/div VIN = 2V TO 3V, VLCD = +12V, IOUT = 5mA MAIN BOOST CONVERTER START-UP DELAY MAX1677-19 MAX1677-18 VIN 0V 50mV/ div 1V/ div ON 1V/ div VOUT VLCD 0V 500µs/div VIN = 2.4V, VOUT = 3.3V, ILOAD = 10mA 5ms/div VIN = 2V TO 3V, VLCD = -20V, IOUT = 5mA LCD START-UP DELAY MAX1677-20 LCDON 2V/ div VLCD 10V/ div 10ms/div VIN = 2.4V, VLCD = -20V, IOUT = 5mA _______________________________________________________________________________________ 7 MAX1677 Typical Operating Characteristics (continued) (Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.) MAX1677 Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter Pin Description PIN NAME 1 OUT 2 FB Dual Mode™ Main Boost Feedback Input. Connect to GND for 3.3V output. Connect a voltage-divider from OUT to FB to adjust the output in the 2.5V to 5.5V range (Figure 5). 3 LBI Low-Battery-Comparator Input. Threshold is 614mV. Set the low-battery trip-point with an external voltage divider (Figure 7). 4 LBO Open-Drain, Low-Battery Output. LBO is low when LBI is below 614mV, otherwise it remains high. 5 CLK/SEL Sync Clock and PWM Select Input. CLK/SEL = low: low-power, low-quiescent-current PFM mode. CLK/SEL = high: low-noise, high-power PWM mode at 300kHz. CLK/SEL = driven with external clock of 200kHz to 400kHz, synchronized PWM high-power mode. 6 LCDON LCD Enable Input. Drive high to turn on LCD boost converter. Main DC-DC must also be on. 7 LCDPOL LCD Polarity Select Input. Sets LCD boost converter polarity and peak current output (Table 2). 8 REF 1.25V Reference Output. Bypass with 0.1µF. 9 GND Ground 10 LCDFB 11 ON 12 LCDLX 13 LCDGND 14 PGND 15 LX 16 POUT FUNCTION Output Sense Input. The device is powered from OUT. Bypass to GND with a 0.1µF ceramic capacitor. Connect OUT to POUT through a 10Ω series resistor. LCD Feedback Input. Threshold is 1.25V for positive with LCDPOL high, and 0 for negative with LCDPOL low. I.C. Enable Input. Drive high to enable the MAX1677. LCD Boost 28V Switch Drain Source of the Internal N-Channel DMOS LCD Boost-Converter Switch Source of the Internal N-Channel Main Boost-Converter Switch Main Output Boost Internal Switch Drain Boost DC-DC Converter Power Output. Source of internal P-channel MOSFET main boost-converter synchronous rectifier. Dual Mode is a trademark of Maxim Integrated Products. _______________ Detailed Description The MAX1677 is a highly efficient dual-output power supply for battery-powered devices. On-chip are two complete step-up DC-DC converters to power main logic and bias an LCD (Figure 1). The main boost converter (MBC) has on-chip P-channel and N-channel MOSFETs that provide synchronous-rectified voltage conversion for maximum efficiency at loads up to 300mA. See Table 1 for available output current with typical battery configurations. The output voltage of the MBC is factory-preset to 3.3V, or can be set from 2.5V to 5.5V with external resistors (dual-mode operation). Either fixed-frequency PWM or low-operating-current PFM operation can be selected for the MBC using the CLK/SEL input (Table 2). 8 The LCD boost converter (LCD) includes an internal Nchannel DMOS switch to generate positive or negative voltages up to ±28V. The polarity of the LCD output is set by LCDPOL input (Table 3). Figure 2 shows the MAX1677 configured for a positive LCD output voltage with a 3.3V main output. Figure 3 shows the MAX1677 configured for a negative LCD output. LCDPOL also allows the current limit of LCDLX to be reduced from 350mA to 225mA to allow minimum-size inductors in low-current LCD applications (typically for LCD loads <10mA). Also included in the MAX1677 are a precision 1.25V reference that sources up to 50µA, logic shutdown control for the MBC and LCD (the MBC must be on for the LCD to operate), and a low-battery comparator. _______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677 OUT MAX1677 839k MAIN DC-DC START-UP OSC OUT ON SUCLK ON QP ON QN 501k REF ON CLK LX EA REF CLK/SEL POUT PGND 90% REF REFERENCE FB LBO 2.25V 50% REF LCD ON ISET/POL SENSE POL LCDPOL LBI IN LCDON ILCDLX START-UP ILCDLX POL ON LCDLX ST LCDDRV LCDFB EA CL LCD BIAS GND LCDGND Figure 1. Functional Block Diagram Table 1. Main Boost Converter Available Output Current NUMBER OF CELLS MBC MBC OUTPUT INPUT OUTPUT CURRENT VOLTAGE VOLTAGE (mA) (V) (V) PWM/PFM 1 Alk/NiCd/NiMH 1.2 3.3 1 Alk/NiCd/NiMH 1.2 5 140/150 100/70 2 Alk/NiCd/NiMH 2.4 3.3 350/170 2 Alk/NiCd/NiMH 2.4 5 260/125 1 Alk/NiCd/NiMH or 1 Li-Ion 3.6 5 350/170 Main Boost Converter (MBC) The MBC operates either in PFM mode, 300kHz PWM mode, or externally synchronized PWM mode as selected by the CLK/SEL input (Table 2). PWM mode offers fixed-frequency operation and maximum output power. PFM mode offers the lowest IC operating current. LX current limit is reduced in PFM mode to increase efficiency and minimize output ripple. PWM Mode When CLK/SEL is high, the MAX1677 operates in its high-power, low-noise PWM mode, switching at the 300kHz internal oscillator frequency. The MOSFET switch pulse-width is modulated to control the power transferred on each switching cycle and regulate the _______________________________________________________________________________________ 9 MAX1677 Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter POUT R3 10Ω P 3.3V MAIN BOOST OUTPUT REF POUT OUT C4 0.1µF FEEDBACK C2 100µF L1 10µH MAX1677 N S VIN LX GND C1 100µF L2 10µH LCDLX LCDON REF CLK/SEL LCDGND LCDPOL PWM-MODE CURRENTLIMIT LEVEL D2 MBR0530 C5 0.1µF OSC C3 4.7µF R1 Figure 4. Controller Block Diagram in PWM Mode output voltage. In PWM mode, the MBC can supply up to 350mA. Switching harmonics generated by the fixedfrequency operation are consistent and easily filtered. During PWM operation, the rising edge of the internal clock sets a flip-flop, which turns on the N-channel MOSFET (Figure 4). The switch turns off when the sum of the voltage-error, slope-compensation, and currentfeedback signals trips the multi-input comparator and resets the flip-flop; the switch remains off for the rest of the cycle. Changes in the output voltage error signal shift the inductor current level and modulate the MOSFET pulse width. LCDFB PGND R2 Figure 2. LCD Converter in Positive Mode R3 10Ω C4 0.1µF 3.3V MAIN BOOST OUTPUT POUT OUT GND C2 100µF L1 10µH MAX1677 VIN C1 100µF LX L2 10µH LCDLX LCDON ON C6 0.1µF C5 0.1µF D3 MBR0530 FB LCDGND R2 R1 LCDPOL PGND LCDFB Figure 3. LCD Converter in Negative Mode 10 D2 MBR0530 REF CLK/SEL PGND LCD BOOST OUTPUT FB ON LX R Q -LCD BOOST OUTPUT C3 4.7µF Clock-Synchronized PWM The MAX1677 operates as a clock-synchronized current-mode PWM when a clock signal (200kHz to 400kHz) is applied to CLK/SEL. This allows switching harmonics to be positioned to avoid sensitive frequency bands, such as those near IF frequencies in wireless applications. Low Power PFM Mode Pulling CLK/SEL low places the MAX1677 in low-power standby mode. During standby mode, PFM operation regulates the output voltage by transferring a fixed amount of energy during each cycle, and then modulating the switching frequency to control the power delivered to the output. The device switches only as needed to service the load, resulting in the highest possible efficiency at light loads and an operating current of only 20µA. The MBC can supply up to 170mA when in PFM mode (Table 1). During PFM operation, the error comparator detects when the output voltage is out of regulation and sets a ______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter Start-Up Oscillator The MBC employs a low-voltage start-up oscillator to ensure a 1.1V (0.9V typical) start-up voltage. On startup, if the output voltage is less than 2.25V, the P-channel switch stays off and the N-channel pulses at a 25% duty cycle. When the output voltage exceeds 2.25V, the normal PWM or PFM control circuitry takes over. Once the MBC is in regulation, it can operate with inputs down to 0.7V since the internal power for the IC is taken from OUT. The MBC cannot supply full output current until OUT reaches 2.5V. Table 2. Selecting MBC Operating Mode CLK/SEL 0 MBC MODE Low-Power PFM FEATURES Lowest Supply Current 1 PWM High Output Current, Fixed-Frequency Ripple Ext Clock (200Hz to 400kHz) Synchronized PWM High Output Current, Synchronized Ripple Frequency Q D LOGIC HIGH POUT Q Synchronous Rectifier The MAX1677 MBC features an internal 1Ω P-channel synchronous rectifier. Synchronous rectification typically improves efficiency by 5% or more over similar nonsynchronous step-up designs. In PWM mode, the synchronous rectifier turns on during the second half of each cycle. In PFM mode, an internal comparator turns on the synchronous rectifier when the voltage at LX exceeds the MBC output, and then turns it off when the inductor current drops below 90mA (typ). The on-chip synchronous rectifier allows the external Schottky diode to be omitted in designs that operate from inputs exceeding 1.4V. In circuits operating below 1.4V (1-cell inputs, for example), connecting a Schottky diode in parallel with the internal synchronous rectifier (from LX to POUT) provides the lowest start-up voltage. LCD Boost Converter (LCD) The LCD converter can be configured for a positive or negative output by setting the LCDPOL pin and using the appropriate circuit (Figures 2 and 3, and Table 3). A combination of peak current limiting and a pair of one-shot timers control LCD switching. During the oncycle the internal N-channel DMOS switch turns on, and inductor current ramps up until either the switch peak current limit is reached or the 5.2µs maximum ontime expires (typically at low input voltages). After the on-cycle terminates, the switch turns off and the output capacitor charges. The switch remains off until the error comparator initiates another cycle. The LCDLX current limit is set by LCDPOL, as outlined in Table 3. The lower, 225mA peak current setting allows tiny low-current “chip” inductors to be used when powering smaller (less than 15 square inches) liquid crystal panels. Use the following equation to determine which LCDLX current-limit setting is required. ILCD = (0.7 · IPK(LCD) · VIN(MIN)) / (2 · VLCD(MAX)) R P LX VFB S Q N VREF CURRENT LIMIT LEVEL R PGND where ILCD is the output current, VIN(MIN) is the minimum expected input voltage, VLCD(MAX) is the maximum required LCD output voltage, and I PK(LCD) is 350mA or 225mA as set by LCDPOL. The 0.7 term is a correction factor to conservatively account for typical switch, inductor, and diode losses. The LCD boost is enabled when both ON and LCDON are high, and the MBC output voltage is within 90% of its set value. A soft-start start-up mode with increased off time reduces transient input current when the LCD is activated. Figure 5. Controller Block Diagram in PFM Mode ______________________________________________________________________________________ 11 MAX1677 flip-flop, turning on the N-channel MOSFET switch (Figure 5). When the inductor current ramps to the PFM mode current limit (350mA), the current-sense comparator resets a flip-flop. The flip-flop turns off the N-channel switch and turns on the P-channel synchronous rectifier. The energy stored in the inductor is transferred to the output through the P-channel switch. A second flip-flop, previously reset by the switch’s “on” signal, inhibits the next cycle until the inductor current is depleted and the output is out of regulation. This forces operation with discontinuous inductor current in PFM mode. MAX1677 Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter Table 3. Setting LCD Output Polarity and Peak Inductor Current LCD OUTPUT POLARITY LCDPOL CONNECTED TO: LCDLX PEAK INDUCTOR CURRENT (mA) Design Procedure The MBC feedback pin (FB) features Dual Mode operation. With FB grounded, the MBC output is preset to 3.3V. It can also be adjusted from 2.5V to 5.5V with external resistors, R3 and R4, as shown in Figure 8. To set the output voltage externally, select resistor R4 in the 10kΩ to 200kΩ range. Calculate R3 using: R3 = R4 [(VOUT / 1.25V) – 1] Positive OUT 350 Negative GND 350 Positive OUT through 50kΩ 225 Setting the LCD Output Voltage Negative GND through 50kΩ 225 For either positive or negative LCD output voltages, set the voltage with two external resistors, R1 and R2, as shown in Figures 2 and 3. Since the input current at FB has a maximum of 50nA, large resistors can be used without significant accuracy loss. Begin by selecting R2 in the 10kΩ to 200kΩ range and calculate R1 using one of the following two equations (for positive or negative output). Shutdown: ON and LCDON A logic-low level at ON shuts down all MAX1677 circuits including the LCD converter, reference, and LBI comparator. A logic-high level at LCDON activates the LCD boost converter. The LCD boost converter can only be activated when ON is high. When ON is low, the MAX1677 draws 1µA. Low-Battery Comparator The MAX1677 has an on-chip comparator for low-battery detection. If the voltage at LBI falls below 614mV, LBO (an open-drain output) sinks current to GND. The low-battery trip level is set by two resistors (Figure 6). Since the LBI input current is less than 50nA, large resistor values (R6 ≤ 130kΩ) can be used to minimize input loading. Calculate R5 as follows: R5 = R6 [(VTRIP / 0.614V) – 1] Connect a pull-up resistor (R8) to LBO when driving CMOS logic. LBO is an open-drain output and can be pulled as high as 6V regardless of the voltage at OUT. When LBI is above 0.614V, LBO is high impedance. If the LBI comparator is not used, ground LBI. Since the low-battery comparator is noninverting, hysteresis can be added by connecting a resistor (R7) from LBI to LBO as shown in Figure 7. When LBO is high, the series combination of R8 and R7 source current into the summing node at LBI (no current flows into the IC). VIN (VTRIP: VH, VL) POUT R5 MAX1677 R8 100k LBI LBO R6 R7 R5 + R5 VH = 0.614V 1+ R7 R6 [ ] VL = 0.614V 1+ R7 - (VPOUT - 0.614V) (R7 + R8) R8 0.614V (R5 + R6) [ WHERE VH IS THE RISING VTRIP LEVEL AND VL IS THE FALLING VTRIP LEVEL. Figure 7. Adding External Hysteresis to the LBI Comparator MBC OUTPUT POUT VIN (VTRIP) LOGIC POWER POUT R5 LBI FB R8 MAX1677 R3 MAX1677 R4 LBO LOW-BATTERY OUTPUT R6 GND Figure 6. Setting the Low-Battery Trip Threshold 12 Figure 8. Setting the MBC Output Voltage Externally ______________________________________________________________________________________ ] Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter For positive LCD output: R1 = R2 · VLCD / 1.25V To minimize ripple in the LCD output and prevent subharmonic noise caused by switching pulse grouping, it may be necessary in some PC board layouts to connect a small capacitor in parallel with R1. For R1 values in 500kΩ to 2MΩ range, 22pF is usually adequate. Many LCD bias applications require an adjustable output voltage. In Figure 9, an external control voltage (generated by a potentiometer, DAC, filtered PWM control signal, or other source) is coupled to LCDFB through the resistor RADJ. The output voltage of this circuit, for both positive and negative outputs, is given by: VOUT = VINIT + (R1 / RADJ)(VLCDFB – VADJ) where VINIT is the initial output obtained without the added adjust voltage, as calculated in one of the preceding two equations. VLCDFB is 1.25V for the positive configuration, and 0 for the negative configuration. R ADJ sets the output adjustment span, which is 1.25V · R1 / RADJ for either polarity output. Note that raising VADJ lowers VOUT in positive output designs, while in negative output designs, raising VADJ increases the magnitude of the negative output. Higher LCD Output Voltages If the application requires LCD output voltages greater than +28V, use the connection in Figure 10. This circuit adds one capacitor-diode charge pump stage to increase the output voltage without increasing the voltage stress on the LCDLX pin. The maximum output voltage of the circuit is +55V and output current is slightly less than half that available from the standard circuit in Figure 2. In Figure 10, diodes D1, D2, and D3 should be at least 30V-rated Schottky diodes such as 1N5818 or MBR0530L or equivalent. Capacitors C1 and C2 should also be rated for 30V, while C3 must be rated for the maximum set output voltage. Applications Information Inductor Selection The MAX1677’s high switching frequency allows the use of small surface-mount inductors. The 10µH values shown in Figures 2 and 3 are recommended for most applications, although values between 4.7µH and 47µH are suitable. Smaller inductance values typically offer a smaller physical size for a given series resistance, allowing the smallest overall circuit dimensions. Larger inductance values exhibit higher output current capability, but larger physical dimensions. Use inductors with a ferrite core or equivalent; powder iron cores are not recommended for use with the MAX1677’s high switching frequencies. The inductor’s incremental saturation rating ideally should exceed the OUT LCDPOL LCDLX 1 7 L2 10µH C1 1µF 30V D1 VADJ R2 MAX1677 GND (REF) Figure 9. Adjusting LCD Output Voltage C3 2.2µF LCDFB C2 2.2µF 30V R1 2M R1 FB D2 12 VLCD RADJ D3 +40V/5mA (SET TO NO MORE THAN 55V) VIN MAX1677 10 R2 65k D1, D2, D3 = 30V RATED SCHOTTKY DIODES: MBR0530L OR EQUIVALENT. Figure 10. Higher LCD Output Voltage ______________________________________________________________________________________ 13 MAX1677 For a positive LCD output, connect LCDPOL to OUT as shown in Figure 2. This sets the threshold at LCDFB to 1.25V. Select R2 and the desired output voltage (VLCD), and calculate R1: For positive LCD output: R1 = R2 [(VLCD / 1.25V) – 1] Figure 3 shows the standard circuit for generating a negative LCD supply. This connection limits V LCD to values between -VIN and -28V. If a smaller negative output voltage is required, D2’s cathode can be connected to VIN rather than ground. This alternate connection permits output voltages from 0 to –28 – VIN. For a negative LCD output voltage, connect LCDPOL to GND. The feedback threshold voltage of LCDFB is set to 0. Select R2 and the desired output voltage (VLCD), and calculate R1: MAX1677 Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter selected current limit, however it is generally acceptable to bias most inductors into saturation by as much as 20% (although this may reduce efficiency). For best efficiency, select inductors with resistance no greater than the internal N-channel FET resistance in each boost converter (220mΩ for the MBC, and 1Ω for the LCD). The inductor is effectively in series with the input at all times, so inductor wire losses can be roughly approximated by IIN2 · RL. See Table 4 for a list of inductor suppliers. The LCD boost converter (LCD) features selectable inductor/switch current limit of 350mA or 225mA. The higher current setting provides the greatest output current, while the lower setting allows the smallest inductor size. External Diodes The MAX1677’s on-chip synchronous rectifier allows the normally required external Schottky diode to be omitted from the MBC in designs whose input exceeds Input Bypass Capacitors Table 4. Component Suppliers SUPPLIER PHONE FAX Coilcraft: DO and DT series 847-639-6400 847-639-1469 Murata: LQH4 and LQH3C series 814-237-1431 814-238-0490 Sumida: CD, CDR, and RCH series 847-956-0666 847-956-0702 TDK: NLC Series 847-390-4373 847-390-4428 INDUCTORS INDUCTORS CAPACITORS CAPACITORS AVX: TPS series 803-946-0690 803-626-3123 Matsuo: 267 series 714-969-2591 714-960-6492 Sanyo: OS-CON and GX series 619-661-6835 619-661-1055 Sprague: 595D series 603-224-1961 603-224-1430 Motorola: MBR0520 602-303-5454 602-994-6430 Nihon: EC11 FS1 series 805-867-2555 805-867-2698 DIODES 14 1.4V. In circuits that need to operate below 1.4V (1-cell inputs for example), connecting a Schottky diode in parallel with the internal synchronous rectifier (from LX to POUT) provides the lowest start-up voltage. Suitable devices are the 1N5817 or MBR0520L, however the diode current rating need not match the peak switch current, since most of the current is handled by the onchip synchronous rectifier. Since the LCD boost converter (LCD) does not have synchronous rectification, an external diode is always needed. High switching speed demands a high-speed rectifier. For best efficiency, Schottky diodes such as the 1N5818 and MBR0530L are recommended. Be sure that the diode current rating exceeds the peak current set by LCDPOL, and that the diode voltage rating exceeds the LCD output voltage. In particularly cost-sensitive applications, and if the LCD’s 225mA peak current is set, a high-speed silicon signal diode (such as an 1N4148) may be used instead of a Schottky diode, but with reduced efficiency. A low-ESR input capacitor connected in parallel with the battery will reduce peak currents and input-reflected noise. Battery bypassing is especially helpful at low input voltages and with high-impedance batteries (such as alkaline types). Benefits include improved efficiency and lower useful end-of-life voltage for the battery. 100µF is typically recommended for 2-cell applications. Small ceramic capacitors may also be used for light loads or in applications that can tolerate higher input ripple. Only one input bypass capacitor is typically needed for both the MBC and LCD. Output Filter Capacitors For most applications, a 100µF, 10V, low-ESR output filter capacitor is recommended for the MBC output. A surface-mount tantalum capacitor typically exhibits 30mV ripple when the MBC is stepping up from 1.2V to 3.3V at 100mA. OS-CON and ceramic capacitors offer lowest ESR, while low-ESR tantalums offer a good balance between cost and performance. The LCD output typically exhibits less than 1% peak-topeak ripple with 4.7µF of filter capacitance. This can be either a ceramic or tantalum type, but be sure that the capacitor voltage rating exceeds the LCD output voltage. If the LCD’s 225mA peak switch current setting is used, the designer can choose lower output ripple or reduce the output filter to 2.2µF. Ceramic capacitors will exhibit lower ripple than equivalent value (or even higher value) tantalums due to lower ESR. ______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter Chip Information TRANSISTOR COUNT: 1221 ______________________________________________________________________________________ 15 MAX1677 Layout Considerations The MAX1677’s high-frequency operation makes PC board layout important for minimizing ground bounce and noise. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting PGND, the input bypass capacitor ground terminal, and the output filter capacitor ground terminal to a single point (star ground configuration). Also, minimize lead lengths to reduce stray capacitance and trace resistance. Where an external resistor-divider is used to set output voltage, the trace from FB or LCDFB to the feedback resistors should be extremely short to minimize coupling from LX and LCDLX. To maximize efficiency and minimize output ripple, use a ground plane and connect the MAX1677 GND and PGND pins directly to the ground plane. Consult the MAX1677 evaluation kit for a full PC board example. Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter QSOP.EPS MAX1677 Package Information 16 ______________________________________________________________________________________