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19-1403; Rev 0; 11/98
UAL IT MAN TION K SHEET VALUA E TA WS DA FOLLO
Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter
General Description Features
o No External FETs Required o 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 o Secondary Output Up to +28V or -28V for LCD Bias Programmable Current Limit o 0.7V to 5.5V Input Voltage Range o 20A Quiescent Current o 1A Shutdown Current o Low-Battery Comparator o Small 16-Pin QSOP Package
MAX1677
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 20A, 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 20A. A 1A 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.
Ordering Information
PART MAX1677EEE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 QSOP
Typical Operating Circuit
Applications
PDAs Hand-Held Terminals Portable Phones Portable Instruments
VIN = 0.7V to 5.5V (UP TO MAINOUT) LX POUT 3.3V MAIN BOOST OUTPUT
Pin Configuration
TOP VIEW
OUT 1 FB 2 LBI 3 LBO 4 CLK/SEL 5 LCDON 6 LCDPOL 7 REF 8 16 POUT 15 LX 14 PGND OFF PWM PFM ON OFF 11 ON 10 LCDFB 9 GND -VE OUT +VE OUT ON
MAX1677
LCDLX LBI OUT LBO LCDFB LCDON CLK/SEL ON LCDPOL PGND FB LCDGND GND REF
28V LCD BOOST OUTPUT
MAX1677
13 LCDGND 12 LCDLX
QSOP ________________________________________________________________ Maxim Integrated Products 1
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
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 = +70C) 16-Pin QSOP (derate 8.3mW/C above +70C)...........696mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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 = 0.1F, POUT = OUT, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER GENERAL Input Voltage Range Minimum Start-Up Voltage Reference Voltage Reference Load Regulation Reference Line Rejection Supply Current Main DC On, LCD Off Supply Current All On, Main DC-DC in PFM Mode Supply Current All On, Main DC-DC in PWM Mode Supply Current in Shutdown MAIN BOOST DC-DC Output Voltage FB Regulation Voltage FB Input Current Output Voltage Adjustment Range Start-Up to Normal Mode Transition Voltage (Note 4) Line Regulation Load Regulation Frequency in Start-Up Mode LX Leakage Current fSTARTUP ILX(LEAK) VLOCKOUT IOUT = 150mA, VIN = 2V to 3V CLK/SEL = OUT, VIN = 2.4V, I L OAD = 10mA to 200mA VOUT = 1.5V 40 0.2 VOUT VFB(REG) IFB FB = GND, 0 ILX 350mA, CLK/SEL = OUT (Note 3) Adjustable mode, CLK/SEL = OUT (Note 3) VFB = 1.3V 2.5 2.1 0.6 1 300 5 3.20 1.225 3.30 1.25 0.02 3.43 1.275 50 5.5 2.4 V V nA V V % % kHz A ILCDOFF IPFM IPWM VIN VSTARTUP VREF (Note 1) TA = +25C, ILOAD < 1mA IREF = 0 IREF = 0 to 50A (Note 2) VOUT = 2.5V to 5.5V No load, current into OUT No load, current into OUT No load, current into OUT 1.23 0.7 0.9 1.25 2 0.2 20 35 115 0.3 5.5 1.1 1.27 15 5 40 60 300 5 V V V mV mV A A A A SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VOUT = 3.3V, CREF = 0.1F, POUT = OUT, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER LX On-Resistance LX Current Limit P-Channel Synchronous Rectifier Turn-Off Current in PFM Mode Internal Oscillator Oscillator Maximum Duty Cycle External Clock Frequency Range LOGIC AND CONTROL INPUTS Input Leakage Current ON Input Threshold LCDON, LCDPOL, CLK/SEL Input Threshold LBI Falling Threshold LBI Hysteresis LBO Output Low Voltage LBI Input Bias Current LBO Leakage Current LCD BIAS DC-DC LCDLX Voltage LCDPOL = OUT or GND LCDLX Switch Current Limit LCDLX Switch Resistance LCDLX Leakage Current LCDFB Set Point LCDFB Input Bias Current LCD Line Regulation LCD Load Regulation Maximum LCDLX On-Time Minimum LCDLX Off-Time LCDFB Voltage for Start-Up Mode tON LCD Operating mode Start-up mode (positive or negative) LCDPOL = OUT LCDPOL = GND ILOAD = 5mA, VIN = 1.2V to 3.6V, Figure 2 ILOAD = 0 to 5mA, VIN = 2.4V, Figure 2 3.4 0.8 3.0 0.1 0.5 4.3 1 4.0 0.75 0.5 5.2 1.2 5.0 RLCDLX LCDPOL connected to OUT or GND through 50k VOUT = 3.3V VLCDLX = 28V Positive LCD, LCDPOL = OUT Negative LCD, LCDPOL = GND 1.225 -15 1.25 0 300 150 350 225 1.0 28 450 300 1.4 1 1.275 15 50 mA A V mV nA %/V % s s V V V LBO(LO) ILBI(BIAS) I LBO(LEAK) V LBO = 5.5V Sink current = 1mA VON(LOW) VON(HIGH) VIL VIH VLBI(TH) ON, LCDON, LCDPOL, CLK/SEL 1.1V < VOUT < 5.5V VOUT > 2.5V 0.8VOUT 0.2VOUT 0.8VOUT 599 614 1 0.1 50 1 629 1 0.2VOUT A V V mV % V nA A f D CLK/SEL = OUT SYMBOL RLX(ON)N RLX(ON)P ILX(PWM) ILX(PFM) N-channel P-channel N-channel PWM mode N-channel PFM mode 550 250 40 240 80 200 CONDITIONS MIN TYP 0.22 0.4 670 350 90 300 85 MAX 0.5 1.0 800 450 140 360 90 400 UNITS mA mA kHz % kHz
MAX1677
3
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
ELECTRICAL CHARACTERISTICS
(VOUT = 3.3V, CREF = 0.1F, POUT = OUT, TA = -40C to +85C, unless otherwise noted. ) (Note 5) PARAMETER GENERAL General Supply Current Main DC On, LCD Off Supply Current All On, Main DC-DC in PFM Mode Supply Current All On, Main DC-DC in PWM Mode Supply Current in Shutdown MAIN BOOST DC-DC Main Output Voltage FB Regulation Voltage Start-Up to Normal Mode Transition Voltage (Note 4) LX Leakage Current LX Current Limit Internal Oscillator External Clock Frequency Range LOGIC AND CONTROL INPUTS ON Input Threshold LCDON, LCDPOL, CLK/SEL Input Threshold LBI Falling Threshold LBO Output Low Voltage LCD BIAS DC-DC LCDPOL = OUT or GND LCDLX Switch Current Limit LCDPOL connected to OUT or GND through 50k Positive LCD, LCDPOL = OUT Negative LCD, LCDPOL = GND 300 150 1.22 -20 450 300 1.28 +20 mA V mV VON(LOW) VON(HIGH) VIL VIH VLBI(TH) V LBO(LO) Sink current = 1mA 0.8VOUT 599 629 0.1 1.1V < VOUT < 5.5V 0.2VOUT 0.8VOUT 0.2VOUT V V mV V VOUT VFB(REG) VLOCKOUT ILX(LEAK) ILX(PWM) ILX(PFM) f N-channel PWM mode N-channel PFM mode CLK/SEL = OUT 550 250 240 200 FB = GND, 0 ILX 350mA, CLK/SEL = OUT (Note 3) Adjustable mode, CLK/SEL = OUT (Note 3) 3.17 1.22 2.1 3.4 1.28 2.4 5 900 500 360 400 V V V A mA kHz kHz ILCDOFF IPFM IPWM No load, current into OUT No load, current into OUT No load, current into OUT 40 60 300 5 A A A A SYMBOL CONDITIONS MIN MAX UNITS
LCDFB Set Point
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 = 0.22F for applications where IREF > 10A. 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 -40C are guaranteed by design and not production tested.
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
Typical Operating Characteristics
(Circuits of Figures 2 and 3, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX1677-01
EFFICIENCY vs. LOAD CURRENT (VOUT = 5V)
MAX1677-02
MAXIMUM LOAD CURRENT vs. BATTERY INPUT VOLTAGE (PWM MODE)
MAX1677-03
100 PFM MODE A = VIN = 2.4V B = VIN = 1.2V 90 EFFICIENCY (%) A C
100
D A
700 600 LOAD CURENT (mA) 500 400 300 200 100 0 VOUT = 3.3V VOUT = 5V
80 EFFICIENCY (%) E F
60
80
B
D
B 40 C
PWM MODE A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V PFM MODE D: VIN = 3.6V E: VIN = 2.4V F: VIN = 1.2V
70 PWM MODE C = 2.4V D = 1.2V 60 0.1 1 10 100 1000 LOAD CURRENT (mA)
20
0 0.1 1 10
100
1000
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
EFFICIENCY vs. LOAD CURRENT (LCD VOUT = 12V)
MAX1677-04
EFFICIENCY vs. LOAD CURRENT (LCD VOUT = 20V)
MAX1677-05
REFERENCE VOLTAGE vs. REFERENCE CURRENT
MAX1677-06
100 90 EFFICIENCY (%) 80 A 70 60 50 40 0.1 1 10 B C CIRCUIT OF FIGURE 2 A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V
100 CIRCUIT OF FIGURE 2 A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V
1.2550
90 EFFICIENCY (%)
REFERENCE VOLTAGE (V) 100
1.2525
80 A B 60 C 50
1.2500
70
1.2475
1.2450 0.1 1 10 0 20 40 60 80 100 LOAD CURRENT (mA) REFERENCE CURRENT (A)
100
LOAD CURRENT (mA)
LOAD CURRENT vs. START-UP VOLTAGE
MAX1677-07
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (LCD OFF)
MAX1677-08
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (LCD ON)
1.0 0.9 SUPPLY CURRENT (mA) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 VOUT = 3.3V PFM MODE VLCD = -20V
MAX1677-09
450 400 350 LOAD CURRENT (mA) 300 250 200 150 100 50 0 0 0.5 1.0 1.5 2.0 2.5 PFM VOUT = 3.3V TESTED WITH RESISTIVE LOAD
0.20 0.18 0.16 SUPPLY CURRENT (mA) 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0 VOUT = 3.3V PFM MODE LCD OFF
1.1
PWM
3.0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
START-UP VOLTAGE (V)
INPUT VOTAGE (V)
INPUT VOLTAGE (V)
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
Typical Operating Characteristics (continued)
(Circuits of Figures 2 and 3, TA = +25C, unless otherwise noted.)
MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PWM MODE)
MAX1677-10
MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PFM MODE)
MAX1677-11
2V/ div 100mA/ div
VLX
2V/ div
VLX
ILX
100mA/ div
ILX
50mV/ div 1s/div 2.4VIN, 3.3VOUT, 200mA IOUT
VRIPPLE
20mV/ div
VRIPPLE
2s/div 1.2VIN, 3.3VOUT, 20mA IOUT
MAIN BOOST CONVERTER SWITCHING WAVEFORMS (PFM MODE, 50mA OUTPUT)
MAX1677-12
LCD SWITCHING WAVEFORMS
MAX1677-13
2V/ div
VLX
10mV/ div
VLX
100mA/ div
ILX
200mA/ div 100mV/ div
ILX
VRIPPLE
50mV/ div 10s/div PFM, 1.2VIN, 3.3VOUT, 50mA IOUT
VRIPPLE
5s/div LDCLX CURRENT LIMIT = 350mA, 2.4VIN, +12VOUT, 10mA LOAD
LCD SWITCHING WAVEFORMS (50k FROM LCDPOL TO OUT)
MAX1677-14
MAIN BOOST CONVERTER LOAD TRANSIENT
MAX1677-15
10mV/ div
VLX
50mV/ div
VRIPPLE
200mA/ div 100mV/ div
ILX 200mA/ div VRIPPLE IOUT
2s/div LCDLX CURRENT LIMIT = 225mA, 2.4VIN, +12VOUT, 10mA LOAD
2ms/div VIN = 2.4V, VOUT = 3.3V ILOAD = 0 to 200mA
6
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter
Typical Operating Characteristics (continued)
(Circuits of Figures 2 and 3, TA = +25C, unless otherwise noted.)
MAIN BOOST CONVERTER LINE TRANSIENT
MAX1677-16
MAX1677
LCD LINE TRANSIENT (VLCD = +12V)
MAX1677-17
1V/ div
VIN
1V/ div
VIN
0V
0V
50mV/ div
VOUT
50mV/ div
VLCD
5ms/div VIN = 2V TO 3V, VOUT = 3.3V, ILOAD = 150mA
5ms/div VIN = 2V TO 3V, VLCD = +12V, IOUT = 5mA
LCD LINE TRANSIENT (VLCD = -20V)
MAIN BOOST CONVERTER START-UP DELAY
MAX1677-18 MAX1677-19
1V/ div
VIN 1V/ div ON
0V
1V/ div VLCD 0V 5ms/div VIN = 2V TO 3V, VLCD = -20V, IOUT = 5mA 500s/div VIN = 2.4V, VOUT = 3.3V, ILOAD = 10mA
VOUT
50mV/ div
LCD START-UP DELAY
MAX1677-20
2V/ div
LCDON
10V/ div
VLCD
10ms/div VIN = 2.4V, VLCD = -20V, IOUT = 5mA
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
Pin Description
PIN 1 2 3 4 NAME OUT FB LBI LBO FUNCTION Output Sense Input. The device is powered from OUT. Bypass to GND with a 0.1F ceramic capacitor. Connect OUT to POUT through a 10 series resistor. Dual ModeTM 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). Low-Battery-Comparator Input. Threshold is 614mV. Set the low-battery trip-point with an external voltage divider (Figure 7). Open-Drain, Low-Battery Output. LBO is low when LBI is below 614mV, otherwise it remains high. 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. LCD Enable Input. Drive high to turn on LCD boost converter. Main DC-DC must also be on. LCD Polarity Select Input. Sets LCD boost converter polarity and peak current output (Table 2). 1.25V Reference Output. Bypass with 0.1F. Ground 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.
5
CLK/SEL
6 7 8 9 10 11 12 13 14 15 16
LCDON LCDPOL REF GND LCDFB ON LCDLX LCDGND PGND LX POUT
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).
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 50A, logic shutdown control for the MBC and LCD (the MBC must be on for the LCD to operate), and a low-battery comparator.
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
OUT
MAX1677
START-UP OSC OUT ON
839k
MAIN DC-DC SUCLK EA CLK QP
CLK/SEL POUT
LX ON QN PGND 90% REF FB
REF ON ON REF
501k
REFERENCE
2.25V LCD ON ISET/POL SENSE POL LCDPOL IN ILCDLX START-UP ILCDLX ST LCDDRV LCDFB EA CL LCD BIAS GND POL ON
LBO 50% REF
LBI LCDON LCDLX
LCDGND
Figure 1. Functional Block Diagram
Table 1. Main Boost Converter Available Output Current
NUMBER OF CELLS 1 Alk/NiCd/NiMH 1 Alk/NiCd/NiMH 2 Alk/NiCd/NiMH 2 Alk/NiCd/NiMH 1 Alk/NiCd/NiMH or 1 Li-Ion MBC MBC OUTPUT INPUT OUTPUT CURRENT VOLTAGE VOLTAGE (mA) (V) (V) PWM/PFM 1.2 1.2 2.4 2.4 3.6 3.3 5 3.3 5 5 140/150 100/70 350/170 260/125 350/170
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
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
R3 10 3.3V MAIN BOOST OUTPUT OUT C4 0.1F GND POUT C2 100F VIN L2 10H LCDLX LCDON CLK/SEL ON LCDPOL FB LCDGND LCDFB R2 R1 D2 MBR0530 LCD BOOST OUTPUT C3 4.7F C1 100F PWM-MODE CURRENTLIMIT LEVEL OSC FEEDBACK REF RQ N S LX POUT P
MAX1677
LX
L1 10H
PGND
REF
C5 0.1F
Figure 4. Controller Block Diagram in PWM Mode
PGND
Figure 2. LCD Converter in Positive Mode
R3 10 3.3V MAIN BOOST OUTPUT C2 100F VIN C1 100F C6 0.1F C5 0.1F FB LCDGND LCDPOL PGND LCDFB R2 R1 D3 MBR0530 D2 MBR0530
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.
OUT C4 0.1F GND
POUT
MAX1677
LX
L1 10H L2 10H
LCDLX LCDON CLK/SEL ON
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 20A. 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
REF -LCD BOOST OUTPUT C3 4.7F
Figure 3. LCD Converter in Negative Mode
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter
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.
Synchronous Rectifier The MAX1677 MBC features an internal 1 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.
MAX1677
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.
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 5.2s 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)) 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.
Table 2. Selecting MBC Operating Mode
CLK/SEL 0 1 Ext Clock (200Hz to 400kHz) MBC MODE Low-Power PFM PWM Synchronized PWM FEATURES Lowest Supply Current High Output Current, Fixed-Frequency Ripple High Output Current, Synchronized Ripple Frequency
Q Q R
D
LOGIC HIGH POUT
P
LX VFB VREF CURRENT LIMIT LEVEL R S Q N
PGND
Figure 5. Controller Block Diagram in PFM Mode
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
Table 3. Setting LCD Output Polarity and Peak Inductor Current
LCD OUTPUT POLARITY Positive Negative Positive Negative LCDPOL CONNECTED TO: OUT GND OUT through 50k GND through 50k LCDLX PEAK INDUCTOR CURRENT (mA) 350 350 225 225
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 10k to 200k range. Calculate R3 using: R3 = R4 [(VOUT / 1.25V) - 1]
Setting the LCD Output Voltage
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 10k to 200k range and calculate R1 using one of the following two equations (for positive or negative output).
VIN (VTRIP: VH, VL) POUT R5 LBI LBO R6
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 1A.
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 130k) 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) POUT R5 LBI R6 R8 LBO LOW-BATTERY OUTPUT LOGIC POWER
MAX1677
R8 100k
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
POUT R3
MBC OUTPUT
MAX1677
FB R4 GND
MAX1677
Figure 6. Setting the Low-Battery Trip Threshold
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Figure 8. Setting the MBC Output Voltage Externally
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter
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: 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 500k to 2M 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.
VLCD R1 RADJ FB R2 GND (REF) VADJ
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.
MAX1677
Applications Information
Inductor Selection
The MAX1677's high switching frequency allows the use of small surface-mount inductors. The 10H values shown in Figures 2 and 3 are recommended for most applications, although values between 4.7H and 47H 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
MAX1677
OUT LCDPOL 1 7
VIN D3 L2 10H C1 1F 30V D1
+40V/5mA (SET TO NO MORE THAN 55V) C3 2.2F
D2
LCDLX
12 C2 2.2F 30V
LCDFB
10
R1 2M
MAX1677
R2 65k D1, D2, D3 = 30V RATED SCHOTTKY DIODES: MBR0530L OR EQUIVALENT.
Figure 9. Adjusting LCD Output Voltage
Figure 10. Higher LCD Output Voltage
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
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 (220m for the MBC, and 1 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. 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.
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
Table 4. Component Suppliers
SUPPLIER INDUCTORS INDUCTORS Coilcraft: DO and DT series Murata: LQH4 and LQH3C series Sumida: CD, CDR, and RCH series TDK: NLC Series CAPACITORS CAPACITORS AVX: TPS series Matsuo: 267 series Sanyo: OS-CON and GX series Sprague: 595D series DIODES Motorola: MBR0520 Nihon: EC11 FS1 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 FAX
Input Bypass Capacitors
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. 100F 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 100F, 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 4.7F 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 2.2F. Ceramic capacitors will exhibit lower ripple than equivalent value (or even higher value) tantalums due to lower ESR.
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter
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.
Chip Information
TRANSISTOR COUNT: 1221
MAX1677
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Compact, High-Efficiency, Dual-Output Step-Up and LCD Bias DC-DC Converter MAX1677
Package Information
QSOP.EPS
16
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