19-1660; Rev 1; 1/01 KIT ATION EVALU E L B AVAILA 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter The MAX1760 is a high-efficiency, low-noise, step-up DC-DC converter intended for use in battery-powered wireless applications. It maintains exceptionally low quiescent supply current (100µA) despite its high 1MHz operating frequency. Small external components and a tiny 10-pin µMAX package make this device an excellent choice for small hand-held applications requiring the longest possible battery life. The MAX1760 uses a synchronous-rectified pulsewidth-modulation (PWM) boost topology to generate 2.5V to 5.5V outputs from a wide range of input sources, such as 1 to 3 alkaline or NiCd/NiMH cells or a single lithium-ion (Li+) cell. Maxim's proprietary IdleMode™ circuitry significantly improves efficiency at light load currents while smoothly transitioning to fixedfrequency PWM operation at higher load currents to maintain excellent full-load efficiency. Features ♦ Up to 94% Efficiency ♦ 0.7V to 5.5V Input Range ♦ Up to 800mA Output ♦ Fixed 3.3V Output (or adj from 2.5V to 5.5V) ♦ PWM Synchronous Rectified Topology ♦ Low-Noise, Constant Frequency Operation (1MHz) ♦ 0.1µA Logic-Controlled Shutdown ♦ Synchronizable Switching Frequency ♦ Adjustable Current Limit ♦ Adjustable Soft-Start ♦ 10-Pin µMAX Package Low-noise, forced PWM mode is available for applications requiring constant-frequency operation at all load currents. The MAX1760 may also be synchronized to an external clock to protect sensitive frequency bands in communications equipment. Analog soft-start and adjustable current limit permit optimization of efficiency, external component size, and output voltage ripple. Ordering Information PART Applications Digital Cordless Phones PCS Phones Wireless Handsets Hand-Held Instruments Palmtop Computers Personal Communicators MAX1760EUB TEMP. RANGE PIN-PACKAGE -40°C to +85°C 10 µMAX Two-Way Pagers Typical Operating Circuit Pin Configuration TOP VIEW INPUT = 0.7V TO VOUT ON MAX1760 POUT ISET REF OUT FB GND PGND 10 ON ISET 1 LX CLK/SEL VOUT = 3.3V, 800mA REF 2 GND 3 FB 4 7 PGND OUT 5 6 CLK/SEL MAX1760 9 POUT 8 LX µMAX Idle Mode is a trademark of Maxim Integrated Products. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1760 General Description MAX1760 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS ON, OUT, CLK/SEL to GND .....................................-0.3V to +6V PGND to GND.....................................................................±0.3V LX to PGND ............................................-0.3V to (VPOUT + 0.3V) POUT to OUT ......................................................................±0.3V REF, FB, ISET, POUT to GND...................-0.3V to (VOUT + 0.3V) Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+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 (CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS DC-DC CONVERTER Input Voltage Range (Note 1) 0.7 5.5 V Minimum Startup Voltage (Note 2) ILOAD < 1mA, TA = +25°C 0.9 1.1 V Temperature Coefficient of Startup Voltage ILOAD < 1mA -2.3 mV/°C Frequency in Startup Mode VOUT = 1.5V 125 500 1000 kHz Internal Oscillator Frequency CLK/SEL = OUT 0.8 1 1.2 MHz 80 86 90 % 1.2 MHz Oscillator Maximum Duty Cycle (Note 3) External Clock Frequency Range 0.5 Output Voltage VFB < 0.1V, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A 3.17 3.3 3.38 V FB Regulation Voltage Adjustable output, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A 1.215 1.240 1.270 V FB Input Leakage Current VFB = 1.35V 0.01 100 nA Load Regulation CLK/SEL = OUT, no load to full load (0 < ILX < 1.0A) -1.5 Output Voltage Adjust Range 2.5 % 5.5 V 2.15 2.30 V ±0.01 ±50 nA 0.1 5 µA CLK/SEL = GND 100 185 µA CLK/SEL = OUT 2.5 POUT Leakage Current VLX = 0, VOUT = 5.5V 0.1 LX Leakage Current VLX = V ON = VOUT = 5.5V N-channel P-channel 0.25 0.45 Output Voltage Lockout Threshold (Note 4) Rising edge ISET Input Leakage Current VISET = 1.25V Supply Current in Shutdown V ON = 3.6V No-Load Supply Current (Note 5) No-Load Supply Current Forced PWM Mode 2.00 mA DC-DC SWITCHES Switch On-Resistance N-Channel Current Limit P-Channel Turn-Off Current 2 CLK/SEL = GND 10 µA 0.1 10 µA 0.15 0.28 Ω 1.0 1.25 1.6 A 20 60 120 mA _______________________________________________________________________________________ 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter (CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = 0°C to +85°C. Typical values are at TA = +25°C unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS 1.230 1.250 1.270 V 5 15 mV 0.2 5 mV REFERENCES Reference Output Voltage IREF = 0 Reference Load Regulation -1µA < IREF < 50µA Reference Supply Rejection 2.5V < VOUT < 5V LOGIC INPUTS CLK/SEL Input Low Level 2.5V ≤ VOUT ≤ 5.5V CLK/SEL Input High Level 2.5V ≤ VOUT ≤ 5.5V ON Input Low Level (Note 6) ON Input High Level (Note 6) (0.8)VOUT 1.1V ≤ VOUT ≤ 1.8V 0.2 0.4 1.1V ≤ VOUT ≤ 1.8V V V 1.8V ≤ VOUT ≤ 5.5V 1.8V ≤ VOUT ≤ 5.5V Input Leakage Current (0.2)VOUT VOUT - 0.2 V V 1.6 CLK/SEL, ON 0.01 1 µA Minimum CLK/SEL Pulse Width 200 ns Maximum CLK/SEL Rise/Fall Time 100 ns ELECTRICAL CHARACTERISTICS (CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = -40°C to +85°C, unless otherwise noted.) (Note 7) PARAMETER CONDITIONS MIN MAX UNITS DC-DC CONVERTER Output Voltage VFB < 0.1V, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A 3.17 3.38 V FB Regulation Voltage Adjustable output, CLK/SEL = OUT, includes load regulation for 0 < ILX < 0.55A 1.215 1.270 V Internal Oscillator Frequency CLK/SEL = OUT 0.75 1.2 MHz 80 90 % 2.00 2.30 V Oscillator Maximum Duty Cycle (Note 3) Output Voltage Lockout Threshold (Note 4) Rising edge Supply Current in Shutdown V ON = 3.6V No-Load Supply Current (Note 5) CLK/SEL = GND 5 µA 185 µA DC-DC SWITCHES Switch On-Resistance N-channel 0.28 P-channel 0.45 N-Channel Current Limit Ω 1.0 1.6 A 1.230 1.270 V REFERENCE Reference Output Voltage IREF = 0 _______________________________________________________________________________________ 3 MAX1760 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (CLK/SEL = ON = FB = PGND = GND, ISET = REF, OUT = POUT, VOUT = 3.6V, TA = -40°C to +85°C, unless otherwise noted.) (Note 7) PARAMETER CONDITIONS MIN MAX UNITS LOGIC INPUTS 2.5V ≤ VOUT ≤ 5.5V CLK/SEL Input Low Level (0.2)VOUT 2.5V ≤ VOUT ≤ 5.5V CLK/SEL Input High Level ON Input Low Level (Note 6) V 0.2 1.8V ≤ VOUT ≤ 5.5V 0.4 V VOUT + 0.2 1.8V ≤ VOUT ≤ 5.5V V 1.6 CLK/SEL, ON Input Leakage Current V (0.8)VOUT 1.1V ≤ VOUT ≤ 1.8V 1.1V ≤ VOUT ≤ 1.8V ON Input High Level (Note 6) µA 1 Note 1: Operating voltage—since the regulator is bootstrapped to the output, once started the MAX1760 will operate down to 0.7V input. Note 2: Startup is tested with the circuit shown in Figure 6. Note 3: Defines maximum step-up ratio. Note 4: The regulator is in startup mode until this voltage is reached. Do not apply full load current until the output exceeds 2.3V. Note 5: Supply current into the OUT pin. This current correlates directly to the actual battery-supply current, but is reduced in value according to the step-up ratio and efficiency. Note 6: ON has a hysteresis of approximately 0.15 × VOUT. Note 7: Specifications to -40°C are guaranteed by design and not production tested. Typical Operating Characteristics (Circuit of Figure 2, VIN = 2.4V, VOUT = 3.3V, TA = +25°C, unless otherwise noted.) 90 70 60 C 50 40 = AUTO MODE = FPWM MODE A: VIN = 2.4V B: VIN = 1.2V C: VIN = 0.9V 30 20 10 0 0.0001 0.001 0.01 0.1 OUTPUT CURRENT (A) 4 EFFICIENCY (%) B B 80 80 70 C 60 50 40 = AUTO MODE = FPWM MODE A: VIN = 3.6V B: VIN = 2.4V C: VIN = 1.2V 30 20 10 1 A 0 0.0001 MAX1760-03 90 0.9 0.8 OUTPUT CURRENT (A) A MAX1760-02 100 MAX1760 toc01 100 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE EFFICIENCY vs. OUTPUT CURRENT VOUT = 5V EFFICIENCY vs. OUTPUT CURRENT VOUT = 3.3V EFFICIENCY (%) MAX1760 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter VOUT = 3.3V 0.7 0.6 VOUT = 5V 0.5 0.4 0.3 0.2 0.1 0.001 0.01 0.1 OUTPUT CURRENT (A) 1 0.5 1.0 1.5 2.0 2.5 3.0 INPUT VOLTAGE (V) _______________________________________________________________________________________ 3.5 4.0 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE TOTAL SHUTDOWN CURRENT (ILX + IOUT) vs. INPUT VOLTAGE 1.5 1.0 VOUT = 5V 0.5 0.90 3 4 0 5 1 PEAK INDUCTOR CURRENT (A) 2.0 C 1.5 1.0 4 5 -40 -15 10 35 60 85 TEMPERATURE (°C) PEAK INDUCTOR CURRENT vs. VISET HEAVY-LOAD SWITCHING WAVEFORMS MAX1760-08 A 1.0 0.8 B 0.6 0.4 0.2 CIRCUIT OF FIGURE 6 0.0 0.001 0.01 0.0 1 C 0V 0.1 0.3 0.5 0.7 0.9 1.1 VISET (V) LIGHT-LOAD SWITCHING WAVEFORMS LOAD-TRANSIENT RESPONSE 1.3 A = LX PIN, 5V/div B = INDUCTOR CURRENT, 200mA/div C = OUTPUT RIPPLE, 50mV/div AC-COUPLED t = 400ns/div LINE-TRANSIENT RESPONSE MAX1760-11 OUTPUT CURRENT (A) A 0.75 6 INPUT VOLTAGE (V) 1.2 0.5 0.1 3 1.4 MAX1760-07 A = +85°C B = +25°C C = -40°C 2 MAX1760-09 2 MAX1760-10 STARTUP VOLTAGE (V) 0.95 MAX1760-12 1 B 1.00 0.80 0.1 A 1.05 0.85 STARTUP VOLTAGE vs. OUTPUT CURRENT 2.5 MAX1760-06 MAX1760-05 1 INPUT VOLTAGE (V) 3.0 1.15 1.10 VOUT = 3.3V 0.0 0 1.20 FREQUENCY (MHz) SHUTDOWN CURRENT (nA) 2.0 INPUT CURRENT (mA) 10 MAX1760-04 2.5 INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE A A B B B C t = 400ns/div A = LX PIN, 5V/div B = INDUCTOR CURRENT, 200mA/div C = OUTPUT RIPPLE, 50mV/div, AC-COUPLED t = 200ms/div VIN = 1.1V, VOUT = 3.3V, IOUT = 0 AND 0.2A A = IOUT, 100mA/div B = VOUT, 50mV/div, AC-COUPLED 400µs/div VIN = 2.4V TO 1.4V, IOUT = 70mA A = VIN, 1V/div B = VOUT, 5mA/div, AC-COUPLED _______________________________________________________________________________________ 5 MAX1760 Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN = 2.4V, VOUT = 3.3V, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN = 2.4V, VOUT = 3.3V, TA = +25°C, unless otherwise noted.) TURN-ON WAVEFORMS NO SOFT-START COMPONENTS 12 A MAX1760-15 MAX1760-13 16 SOFT-START WAVEFORMS RSS = 500kΩ, CSS = 0.1µF MAX1760-14 NOISE SPECTRUM NOISE (mVRMS) MAX1760 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter A 8 B B 4 C 0 C 0V OV 0.1 1 10 FREQUENCY (MHz) t = 2ms/div 2.00ms/div A = ON, 5V/div B = INPUT CURRENT, 500mA/div C = VOUT, 2V/div A = ON, 5V/div B = INPUT CURRENT, 100mA/div C = VOUT, 2V/div Pin Description PIN 6 NAME FUNCTION 1 ISET N-Channel Current-Limit Control. For maximum current limit, connect to REF. To reduce current, supply a voltage between REF and GND by means of a resistive voltage-divider. If soft-start is desired, connect a capacitor from ISET to GND. When ON = high, or VREF <80% of nominal value, an on-chip 100kΩ switched resistor discharges this pin to GND. 2 REF 1.250V Voltage Reference Bypass. Connect a 0.22µF ceramic bypass capacitor to GND. Up to 50µA of external load current is allowed. 3 GND Ground. Connect to PGND with short trace. 4 FB 5 OUT 6 CLK/SEL 7 PGND 8 LX 9 POUT 10 ON DC-DC Converter Feedback Input. To set fixed output voltage of +3.3V, connect FB to ground. For adjustable output of 2.5V to 5.5V, connect to a resistive divider from OUT to GND. FB set point = 1.24V. IC Power, Supplied from the Output. Bypass to GND with a 0.68µF ceramic capacitor, and connect to POUT with a series 4.7Ω resistor (Figure 2). Clock Input for the DC-DC Converter. Also serves to program operating mode of switcher as follows: CLK/SEL = LO: Normal operation—operates at a fixed frequency, automatically switching to low-power mode if load is minimized. CLK/SEL = HI: Forced PWM mode—operates in low-noise, constant-frequency mode at all loads. CLK/SEL = Clocked: Forced PWM mode with the internal oscillator synchronized to CLK in 500kHz to 1200kHz range. Source of N-Channel Power MOSFET Switch Inductor Connection Power Output. P-channel synchronous-rectifier source. Shutdown Control Input. When ON = high, the IC is in shutdown. Connect ON to GND for normal operation. _______________________________________________________________________________________ 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760 UNDERVOLTAGE LOCKOUT OUT IC POWER CONTROLLER POUT 2.15V STARTUP OSCILLATOR EN ON ON REF 1.25V LX RDY REFERENCE REF GND GND PCH D Q EN EN OSC OSC NCH OSCILLATOR 1MHz CLK/SEL MODE MODE PGND CLK/SEL FB FB ISET ISET MAX1760 Figure 1. Functional Diagram Detailed Description The MAX1760 is a highly efficient, low-noise power supply for portable RF and hand-held instruments. It combines a boost switching regulator, N-channel power MOSFET, P-channel synchronous rectifier, precision reference, and shutdown control (Figure 1). The DC-DC converter boosts a 1-cell to 3-cell battery voltage input to a fixed 3.3V or adjustable voltage between 2.5V and 5.5V. An external Schottky diode is required for output voltages greater than 4V. The MAX1760 guarantees startup with an input voltage as low as 1.1V and remains operational down to an input of just 0.7V. It is optimized for use in cellular phones and other applications requiring low noise and low quiescent current for maximum battery life. It features fixed-frequency operation at medium and heavy loads, but at light loads, switches only as needed for optimum efficiency. This device is also capable of constant-frequency (1MHz), low-noise PWM operation at all load currents, or frequency-synchronized PWM operation when connected to an external clock. Table 1 lists some typical outputs. Shutdown reduces quiescent current to just 1µA. Figure 2 shows the standard application circuit for the MAX1760. 3.3µH VIN = 2.4V 33µF ON LX CLK/SEL MAX1760 ISET REF 0.22µF FB VOUT = 3.3V, 800mA POUT 4.7Ω 100µF OUT GND PGND 0.68µF Figure 2. Standard Application Circuit Step-Up Converter During DC-DC converter operation, the internal N-channel MOSFET switch turns on for the first part of each cycle, allowing current to ramp up in the inductor and store energy in a magnetic field. During the second part of each cycle, the MOSFET turns off and inductor current flows through the synchronous rectifier to the _______________________________________________________________________________________ 7 MAX1760 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter Table 1. Typical Available Output Current NUMBER OF NiCd/NiMH CELLS INPUT VOLTAGE (V) OUTPUT VOLTAGE (V) OUTPUT CURRENT (mA) 1 1.2 3.3 350 2.4 3.3 800 2.4 5.0 500 3.6 5.0 750 2 3 Table 2. Selecting the Operating Mode CLK/SEL MODE FEATURES 0 Normal Operation 1 Forced PWM Low noise, fixed frequency at all loads. External Clock 500kHz to 1.2MHz Synchronized PWM Low noise, fixed frequency at all loads. High efficiency at all loads. Fixed frequency at all but light loads. output filter capacitor and the load. As the energy stored in the inductor is depleted, the current ramps down and the synchronous rectifier turns off. At light loads, the device operates at fixed frequency or only as needed to maintain regulation, depending on the CLK/SEL setting (Table 2). Normal Operation Pulling CLK/SEL low selects the MAX1760’s normal operating mode. In this mode, the device operates in PWM when driving medium-to-heavy loads, and automatically switches to PFM if the load requires less power. PFM operation allows higher efficiency than PWM under light load conditions. Forced PWM Operation When CLK/SEL is high, the MAX1760 operates in a lownoise PWM-only mode. During forced PWM operation, the MAX1760 switches at a constant frequency (1MHz), and modulates the MOSFET switch pulse width to control the power transferred per cycle to regulate the output voltage. Switching harmonics generated by fixed-frequency operation are consistent and easily filtered. See the Noise Spectrum plot in the Typical Operating Characteristics. Synchronous Rectifier The MAX1760 features an internal, P-channel synchronous rectifier to enhance efficiency. Synchronous rectification provides 5% improved efficiency over similar nonsynchronous boost regulators. In PWM mode, the synchronous rectifier is turned on during the second half of each switching cycle. In low-power mode, an internal comparator turns on the synchronous rectifier when the voltage at LX exceeds the boost regulator output, and turns it off when the inductor current drops below 60mA. When setting output voltages greater than 4V, an external 0.5A Schottky diode must be connected in parallel with the on-chip synchronous rectifier. Low-Voltage Startup Oscillator The MAX1760 uses a CMOS, low-voltage startup oscillator for a 1.1V guaranteed minimum startup input voltage. At startup, the low-voltage oscillator switches the N-channel MOSFET until the output voltage reaches 2.15V. Above this level, the normal boost-converter feedback and control circuitry take over. Once the device is in regulation, it can operate down to 0.7V input since internal power for the IC is bootstrapped from the output through OUT. Do not apply full load until the output exceeds 2.3V. Synchronized PWM Operation The MAX1760 can be synchronized in PWM mode to an external frequency of 500kHz to 1.2MHz by applying an external clock signal to CLK/SEL. This allows interference to be minimized in wireless applications. The synchronous rectifier is active during synchronized PWM operation. 8 Shutdown The MAX1760 has a shutdown mode that reduces quiescent current to 0.1µA. During shutdown (ON = high), the reference and all feedback and control circuitry are off. During shutdown, the output voltage is one diode drop below the input voltage. _______________________________________________________________________________________ 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter MAX1760 3.3µH VIN = 2.4V 33µF CLK/SEL LX ON MAX1760 ISET REF 0.22µF PGND GND POUT VOUT 100µF 4.7Ω OUT FB 0.68µF R1 R1 = R2 R2 ( ) VOUT -1 VFB VFB = 1.24V Figure 3. Connecting Resistors for External Feedback REF REF 0.22µF RSS ILIM = 1.25A MAX1760 ILIM = 1.25A 0.22µF RSS1 tSS = RSSCSS MAX1760 ( RSS2 RSS1 + RSS2 ) tSS = (RSS1 II RSS2) CSS ISET ISET RSS2 CSS RSS ≥ 470k Figure 4. Soft-Start with Maximum Switch Limit Current Reference The MAX1760 has an internal 1.250V ±1% reference. Connect a 0.22µF ceramic bypass capacitor from REF to GND within 0.2in (5mm) of the REF pin. REF can source up to 50µA of external load current. Design Procedure Setting the Output Voltages For a fixed 3.3V output, connect FB to GND. To set other output voltages between 2.5V and 5.5V, connect a resistor voltage-divider to FB from OUT to GND (Figure 3). The input bias current into FB is <20nA, allowing large-value divider resistors without sacrificing accuracy. Connect the resistor voltage-divider as close to the IC as possible, within 0.2in (5mm) of FB. Choose R2 of 270kΩ or less, then calculate R1 using: V R1 = R2 OUT − 1 VFB CSS RSS1 ≥ 470k Figure 5. Soft-Start with Reduced Switch Current Limit where VFB, the boost-regulator feedback set point, is 1.24V. Setting the Switch Current Limit and Soft-Start The ISET pin adjusts the inductor current limit and implements soft-start. With ISET connected to REF, the inductor current limits at 1.25A. With ISET connected to a resistive divider set from REF to GND, the current limit is reduced according to: V ILIM = 1.25A ISET 1.25V Implement soft-start by placing a resistor from ISET to REF and a capacitor from ISET to GND. In shutdown, ISET is discharged to GND through an on-chip 100kΩ resistor. At power-up, ISET is 0V and the LX current is zero. As the capacitor voltage rises, the current increases and the output voltage rises. The soft-start _______________________________________________________________________________________ 9 MAX1760 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter Table 3. Component Selection Guide PRODUCTION METHOD INDUCTORS CAPACITORS DIODES Coilcraft DS3316P AVX TPS series EIC SB series Coilcraft LPT3305 Kemet T510 series Motorola MBR0520L Sumida CDRH5D18 Sanyo POSCAP series — Sumida CD43 — — Surface Mount Table 4. Component Suppliers SUPPLIER PHONE AVX USA: 843-448-9411 Coilcraft USA: 847-639-6400 EIC USA: 916-941-0712 Kemet USA: 810-287-2536 Motorola USA: 408-629-4789 Japan: 81-45-474-7030 Sumida USA: 847-956-0666 Japan: 011-81-3-3667-3302 Note: Please indicate that you are using the MAX1760 when contacting these component suppliers. External Diode For output voltages greater than 4V, an external Schottky diode must be connected from LX to POUT, in parallel with the on-chip synchronous rectifier (Figure 6). The diode should be rated for 0.5A. Representative devices are Motorola MBR0520L, Nihon EP05Q03L, or generic 1N5817. This external diode is also recommended for applications that must start with input voltages at or below 1.8V. The Schottky diode carries current during startup and after the synchronous rectifier turns off; thus, its current rating only needs to be 500mA. Connect the diode as close to the IC as possible. Do not use ordinary rectifier diodes; their slow switching speeds and long reverse-recovery times render them unacceptable. For circuits that do not require startup with inputs below 1.8V, and have an output of 4V or less, no external diode is needed. time constant is: Input and Output Filter Capacitors t SS = RSS CSS where RSS ≥ 470k. Placing a capacitor across the lower resistor of the current-limiting resistive divider provides both current-limit and soft-start features simultaneously (Figures 4 and 5). Choose input and output filter capacitors that will service the input and output peak currents with acceptable voltage ripple. Choose input capacitors with working voltage ratings over the maximum input volt- Inductor Selection The MAX1760’s high switching frequency allows the use of a small 3.3µH surface-mount inductor. The chosen inductor should generally have a saturation current rating exceeding the N-channel switch current limit; however, it is acceptable to bias the inductor current into saturation by as much as 20% if a slight reduction in efficiency is acceptable. Lower current-rated inductors may be used if ISET is employed to reduce the peak inductor current (see Setting the Switch Current Limit and Soft-Start). For high efficiency, choose an inductor with a high-frequency ferrite core material to reduce core losses. To minimize radiated noise, use a toroid or shielded inductor. See Table 3 for suggested components and Table 4 for a list of component suppliers. Connect the inductor from the battery to the LX pin as close to the IC as possible. 10 ___________________________________________________ 3.3µH VIN = 0.7V TO VOUT 33µF CLK/SEL MRB0520L MAX1760 100µF POUT ISET REF 0.22µF VOUT LX ON PGND GND 4.7Ω OUT FB 0.68µF Figure 6. Connection with External Schottky Diode for Output Voltages Greater than 4V, or to Assist Low-Voltage Startup 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter The output filter capacitor reduces output ripple voltage and provides the load with transient peak currents when necessary. For the output, a 100µF, low-equivalent-series-resistance (ESR) capacitor is recommended for most applications. Sanyo POSCAP, Panasonic SP/CB, and Kemet T510 are good low-ESR capacitors. Low-ESR tantalum capacitors offer a good tradeoff between price and performance. Do not exceed the ripple current ratings of tantalum capacitors. Avoid aluminum electrolytic capacitors; their high ESR typically results in higher output ripple voltage. Other External Components bypass OUT to GND with a 0.68µF ceramic capacitor, and connect OUT to POUT with a 4.7Ω resistor. Each of these components should be placed as close to its respective IC pins as possible, within 0.2in (5mm). Table 4 lists suggested suppliers. Layout Considerations High switching frequencies and large peak currents make PC board layout a critical part of design. Poor design will cause excessive EMI and ground bounce, both of which can cause instability or regulation errors by corrupting the voltage and current feedback signals. Power components—such as the inductor, converter IC, filter capacitors, and output diode—should be placed as close together as possible, and their traces should be kept short, direct, and wide. Keep the voltage feedback network very close to the IC, within 0.2in (5mm) of the FB pin. Keep noisy traces, such as those from the LX pin, away from the voltage feedback network and guarded from them using grounded copper. Refer to the MAX1760 EV kit for a full PC board example. Chip Information TRANSISTOR COUNT: 1361 Two ceramic bypass capacitors are required for proper operation. Bypass REF to GND with 0.22µF. Also, ______________________________________________________________________________________ 11 MAX1760 age, and output capacitors with working voltage ratings higher than the output. The input filter capacitor reduces peak currents drawn from the input source and also reduces input switching noise. The input voltage source impedance determines the required value of the input capacitor. When operating directly from one or two NiMH cells placed close to the MAX1760, use a single 33µF low-ESR input filter capacitor. With higher impedance batteries, such as alkaline and Li+, a higher value input capacitor may improve efficiency. 0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter 10LUMAX.EPS MAX1760 Package Information Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.