MAX756/MAX V/5V/Adjustable-Output, Step-Up DC-DC Converters. Features

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1 EALUATION KIT AAILABLE AAILABLE MAX75/MAX /5/Adjustable-Output, General Description The MAX75/MAX757 are CMOS step-up DC-DC switching regulators for small, low input voltage or battery-powered systems. The MAX75 accepts a positive input voltage down to.7 and converts it to a higher pinselectable output voltage of 3.3 or 5. The MAX757 is an adjustable version that accepts an input voltage down to.7 and generates a higher adjustable output voltage in the range from 2.7 to 5.5. Typical full-load efficiencies for the MAX75/MAX757 are greater than 7%. The MAX75/MAX757 provide three improvements over previous devices. Physical size is reduced the high switching frequencies (up to.5mhz) made possible by MOSFET power transistors allow for tiny (<5mm diameter) surface-mount magnetics. Efficiency is improved to 7% (% better than with low-voltage regulators fabricated in bipolar technology). Supply current is reduced to µa by CMOS construction and a unique constant-off-time pulse-frequency modulation control scheme. Applications 3.3 to 5 Step-Up Conversion Palmtop Computers Portable Data-Collection Equipment Personal Data Communicators/Computers Medical Instrumentation 2-Cell & 3-Cell Battery-Operated Equipment Glucose Meters Typical Operating Circuit Features Operates Down to.7 Input Supply oltage 7% Efficiency at 2mA µa Quiescent Current 2µA Shutdown Mode with Active Reference and Detector 5kHz Maximum Switching Frequency ±.5% Reference Tolerance Over Temperature Low-Battery Detector (/LBO) -Pin DIP and SO Packages Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX75CPA C to +7 C Plastic DIP MAX75CSA C to +7 C SO MAX75C/D C to +7 C Dice* MAX75EPA -4 C to +5 C Plastic DIP MAX75ESA -4 C to +5 C SO MAX757CPA C to +7 C Plastic DIP MAX757CSA C to +7 C SO MAX757C/D C to +7 C Dice* MAX757EPA -4 C to +5 C Plastic DIP MAX757ESA -4 C to +5 C SO * Dice are tested at T A = +25 C only. Pin Configurations INPUT 2 to TOP IEW 5 5μF 22μH N57 PUT 5 at 2mA or 3.3 at 3mA μf 3/5 LBO MAX75 DIP/SO 7 5.μF 2 3 3/5 MAX75 7 LBO 4 LOW-BATTERY DETECTOR PUT FB LBO MAX757 DIP/SO 7 5 For pricing, delivery, and ordering information, please contact Maxim Direct at , or visit Maxim s website at 9-3; Rev. 2; /95

2 3.3/5/Adjustable-Output, ABSOLUTE MAXIMUM RATINGS Supply oltage ( to )...-.3, +7 Switch oltage ( to )...-.3, +7 Auxiliary Pin oltages (,, LBO,, 3/5, FB to )...-.3, ( +.3) Reference Current (I )...2.5mA Continuous Power Dissipation (T A = +7 C) Plastic DIP (derate 9.9mW/ C above +7 C)...727mW SO (derate 5.mW/ C above +7 C)...47mW Operating Temperature Ranges: MAX75_C... C to +7 C MAX75_E...-4 C to +5 C Junction Temperature...+5 C Storage Temperature Range to + C Lead Temperature (soldering, sec) 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 (Circuits of Figure and Typical Operating Circuit, IN = 2.5, I LOAD = ma, T A = T MIN to T MAX, unless otherwise noted.) Output oltage PARAMETER Minimum Start-Up Supply oltage 2 < IN < 3 I LOAD = ma CONDITIONS MAX75, 3/5 =, ma < I LOAD < 2mA MAX75, 3/5 = 3, ma < I LOAD < 3mA MAX757, = 5, ma < I LOAD < 2mA MIN TYP MAX UNITS Minimum Operating Supply oltage (once started) I LOAD = 2mA.7 Quiescent Supply Current in 3.3 Mode (Note ) I LOAD = ma, 3/5 = 3, =.25, = 3.47, FB =.3 (MAX757 only) µa Battery Quiescent Current Measured at IN in Figure Output set for 3.3 µa Shutdown Quiescent Current (Note ) Reference oltage Reference-oltage Regulation Input Threshold Input Hysteresis LBO Output oltage Low LBO Output Leakage Current, 3/5 Input oltage Low, 3/5 Input oltage High =, =.25, 3/5 = 3, = 3.47, FB =.3 (MAX757 only) No load, C =.µf 3/5 = 3, -2µA < load < 25µA, C =.22µF With falling edge I SINK = 2mA LBO = µa % m µa, 3/5, FB, Input Current =.25, FB =.25, = or 3, 3/5 = or 3 ± na FB oltage Output oltage Range MAX757 MAX757, I LOAD = ma (Note 2) Note : Supply current from the 3.3 output is measured with an ammeter between the 3.3 output and pin. This current correlates directly with actual battery supply current, but is reduced in value according to the step-up ratio and efficiency. Note 2: Minimum value is production tested. Maximum value is guaranteed by design and is not production tested. 2 Maxim Integrated

3 3.3/5/Adjustable-Output, Typical Operating Characteristics (Circuit of Figure, T A = +25 C, unless otherwise noted.) MAX75/MAX757 EFFICIENCY (%) EFFICIENCY vs. LOAD CURRENT 3.3 PUT MODE IN =.2 IN = 2. MAX75- EFFICIENCY (%) EFFICIENCY vs. LOAD CURRENT 5 PUT MODE IN = 3.3 IN =.25 IN = 2.5 MAX75-2 MAXIMUM PUT CURRENT (ma) MAXIMUM PUT CURRENT vs. INPUT OLTAGE 3.3 MODE 5 MODE MAX LOAD CURRENT (ma) 4. LOAD CURRENT (ma) INPUT OLTAGE () SWITCHING FREQUENCY (Hz) M k k k SWITCHING FREQUENCY vs. LOAD CURRENT 5 MODE μ μ m m m LOAD CURRENT (A) 3.3 MODE IN = 2.5 MAX75-4 QUIESCENT CURRENT (μa) QUIESCENT CURRENT vs. INPUT OLTAGE CURRENT MEASURED AT IN = INPUT OLTAGE () = 3.3 MAX75-5 SHUTDOWN QUIESCENT CURRENT (μa) SHUTDOWN QUIESCENT CURRENT vs. INPUT OLTAGE CURRENT MEASURED AT IN INPUT OLTAGE () MAX75-. MINIMUM START-UP INPUT OLTAGE vs. LOAD CURRENT MAX75-7 ERENCE OLTAGE LOAD REGULATION MAX75- START-UP INPUT OLTAGE () MODE LOAD REGULATION (m) 4 2 = 3.3. LOAD CURRENT (ma) LOAD CURRENT (μa) Maxim Integrated 3

4 3.3/5/Adjustable-Output, Typical Operating Characteristics (continued) (Circuit of Figure, T A = +25 C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE START-UP DELAY PUT OLTAGE 5m/div 2/div 3 5 PUT CURRENT ma to 2mA 2/div IN = 2.5 HORIZONTAL = 5μs/div 5 Mode IN = 2.5 HORIZONTAL = 5ms/div 5 Mode Pin Description PIN MAX75 MAX757 NAME FUNCTION Shutdown Input disables SMPS when low, but the voltage reference and low-battery comparator remain active. 2 3/5 Selects the main output voltage setting; 5 when low, 3.3 when high. 2 FB LBO 5 5 Feedback Input for adjustable output operation. Connect to an external voltage divider between and..25 Reference oltage Output. Bypass with.22µf to (.µf if there is no external reference load). Maximum load capability is 25µA source, 2µA sink. Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage at drops below Low-Battery Input. When the voltage on drops below +.25, LBO sinks current. Connect to IN if not used. Connect to the regulator output. It provides bootstrapped power to both devices, and also senses the output voltage for the MAX Power Ground. Must be low impedance; solder directly to ground plane. A,.5Ω N-Channel Power MOSFET Drain 4 Maxim Integrated

5 3.3/5/Adjustable-Output, Detailed Description Operating Principle The MAX75/MAX757 combine a switch-mode regulator with an N-channel MOSFET, precision voltage reference, and power-fail detector in a single monolithic device. The MOSFET is a sense-fet type for best efficiency, and has a very low gate threshold voltage to ensure start-up under low-battery voltage conditions (. typ). Pulse-Frequency Modulation Control Scheme A unique minimum off time, current-limited, pulse-frequency modulation (PFM) control scheme is a key feature of the MAX75/MAX757. This PFM scheme combines the advantages of pulse-width modulation (PWM) (high output power and efficiency) with those of a traditional PFM pulse-skipper (ultra-low quiescent currents). There is no oscillator; at heavy loads, switching is accomplished through a constant peak-current limit in the switch, which allows the inductor current to self-oscillate between this peak limit and some lesser value. At light loads, switching frequency is governed by a pair of one-shots, which set a minimum off-time (µs) and a maximum on-time (4µs). The switching frequency depends on the load and the input voltage, and can range as high as 5kHz. The peak switch current of the internal MOSFET power switch is fixed at A ±.2A. The switch's on resistance is typically.5ω, resulting in a switch voltage drop ( SW ) of about 5m under high output loads. The value of SW decreases with light current loads. Conventional PWM converters generate constant-frequency switching noise, whereas this architecture produces variable-frequency switching noise. However, the noise does not exceed the switch current limit times the filter-capacitor equivalent series resistance (ESR), unlike conventional pulse-skippers. oltage Reference The precision voltage reference is suitable for driving external loads such as an analog-to-digital converter. It has guaranteed 25µA source-current and 2µA sink-current capability. The reference is kept alive even in shutdown mode. If the reference drives an external load, bypass it with.22µf to. If the reference is unloaded, bypass it with at least.µf. Control-Logic Inputs The control inputs (3/5, ) are high-impedance MOS gates protected against ESD damage by normally reverse-biased clamp diodes. If these inputs are driven from signal sources that exceed the main supply voltage, the diode current should be limited by a series resistor (MΩ suggested). The logic input threshold level is the same (approximately ) in both 3.3 and 5 modes. Do not leave the control inputs floating. Design Procedure Output oltage Selection The MAX75 output voltage can be selected to 3.3 or 5 under logic control, or it can be left in one mode or the other by tying 3/5 to or. Efficiency varies depending upon the battery and the load, and is typically better than % over a 2mA to 2mA load range. The device is internally bootstrapped, with power derived from the output voltage (via ). When the output is set at 5 instead of 3.3, the higher internal supply voltage results in lower switch-transistor on resistance and slightly greater output power. Bootstrapping allows the battery voltage to sag to less than once the system is started. Therefore, the battery voltage range is from + D to less than (where D is the forward drop of the Schottky rectifier). If the battery voltage exceeds the programmed output voltage, the output will follow the battery voltage. In many systems this is acceptable; however, the output voltage must not be forced above 7. The output voltage of the MAX757 is set by two resistors, R and R2 (Figure ), which form a voltage divider between the output and the FB pin. The output voltage is set by the equation: = ( ) [(R2 + R) / R2] where =.25. To simplify resistor selection: R = (R2) [( / ) - ] Since the input bias current at FB has a maximum value of na, large values (kω to 2kΩ) can be used for R and R2 with no significant loss of accuracy. For % error, the current through R should be at least times FB s bias current. Low-Battery Detection The MAX75/MAX757 contain on-chip circuitry for lowbattery detection. If the voltage at falls below the regulator s internal reference voltage (.25), LBO (an opendrain output) sinks current to. The low-battery monitor's threshold is set by two resistors, R3 and R4 (Figure ), which forms a voltage divider between the input voltage and the pin. The threshold voltage is set by R3 and R4 using the following equation: R3 = [( IN / ) - ] (R4) Maxim Integrated 5

6 3.3/5/Adjustable-Output, R3 R4 C3.μF 5 3 C 5μF MAX757 7 FB 2 LBO 4 Figure. Standard Application Circuit IN L 22μH D N57 R R2 C2 μf The inductor s DC resistance significantly affects efficiency. For highest efficiency, limit L s DC resistance to.3ω or less. See Table for a list of suggested inductor suppliers. Table. Component Suppliers PRODUCTION METHOD Surface-Mount Miniature Through-Hole INDUCTORS Sumida CD54-22 (22µH) CoilCraft DT Coiltronics CTX2- Sumida RCH54-22 CAPACITORS AX TPS series Sprague 595D series Sanyo OS-CON OS-CON series low-esr organic semiconductor where IN is the desired threshold of the low-battery detector, R3 and R4 are the input divider resistors at, and is the internal.25 reference. Since the current is less than na, large resistor values (typically kω to 2kΩ) can be used for R3 and R4 to minimize loading of the input supply. When the voltage at is below the internal threshold, LBO sinks current to. A pull-up resistor of kω or more connected from LBO to can be used when driving CMOS circuits. Any pull-up resistor connected to LBO should not be returned to a voltage source greater than. When is above the threshold, the LBO output is off. The low-battery comparator and reference voltage remain active when the MAX75/MAX757 is in shutdown mode. If the low-battery comparator is not used, connect to IN and leave LBO open. Inductor Selection The inductors should have a saturation (incremental) current rating equal to or greater than the peak switchcurrent limit, which is.2a worst-case. However, it s generally acceptable to bias the inductor into saturation by 2%, although this will reduce the efficiency. The 22µH inductor shown in the typical applications circuit is sufficient for most MAX75/MAX757 application circuits. Higher input voltages increase the energy transferred with each cycle, due to the reduced input/output differential. Minimize excess ripple due to increased energy transfer by reducing the inductor value (µh suggested). Low-Cost Through-Hole CoilCraft PCH Nichicon PL series low-esr electrolyic United Chemi-Con F series AX USA: (27) 22-5, FAX (27) () CoilCraft USA: (7) 39-4, FAX (7) Coiltronics USA: (47) 24-77, FAX (47) Collmer Semiconductor USA: (24) Motorola USA: (2) , FAX (2) Nichicon USA: (7) 43-75, FAX (7) Japan: , FAX (+-) Nihon USA: (5) , FAX (5) Japan: , FAX (+-) Sanyo OS-CON USA: (9) -35 Japan: , FAX (+-72-) 7-74 Sprague USA: (3) 224-9, FAX (3) Sumida USA: (7) 95- Japan: , FAX (+-3-) United Chemi-Con USA: (7) 9-2, FAX (7) 4-3 Capacitor Selection A µf, surface-mount (SMT) tantalum capacitor typically provides 5m output ripple when stepping up from 2 to 5 at 2mA. Smaller capacitors, down to µf, are acceptable for light loads or in applications that can tolerate higher output ripple. Maxim Integrated

7 3.3/5/Adjustable-Output, The ESR of both bypass and filter capacitors affects efficiency. Best performance is obtained by using specialized low-esr capacitors, or connecting two or more filter capacitors in parallel. The smallest low-esr SMT tantalum capacitors currently available are Sprague 595D series, which are about half the size of competing products. Sanyo OS-CON organic semiconductor through-hole capacitors also exhibit very low ESR, and are especially useful for operation at cold temperatures. Table lists suggested capacitor suppliers. Rectifier Diode For optimum performance, a switching Schottky diode, such as the N57, is recommended. N57 equivalent diodes are also available in surface-mount packages from Collmer Semiconductor in Dallas, TX, phone (24) The part numbers are SE4 or SE24. For low output power applications, a pn junction switching diode, such as the N44, will also work well, although efficiency will suffer due to the greater forward voltage drop of the pn junction diode. IN MINIMUM OFF-TIME ONE-SHOT Q ONE-SHOT F/F TRIG S Q N 3/5 R MAXIMUM ON-TIME ONE-SHOT TRIG ONE-SHOT Q MAX75 LBO N ERENCE Figure 2. MAX75 Block Diagram Maxim Integrated 7

8 3.3/5/Adjustable-Output, PC Layout and Grounding The MAX75/MAX757 high peak currents and high-frequency operation make PC layout important for minimizing ground bounce and noise. The distance between the MAX75/MAX757 s pin and the ground leads of C and C2 in Figure must be kept to less than.2" (5mm). All connections to the FB and pins should also be kept as short as possible. To obtain maximum output power and efficiency and minimum output ripple voltage, use a ground plane and solder the MAX75/MAX757 (pin 7) directly to the ground plane. Chip Topography 3/5 (MAX75) FB (MAX757).22" (3.mm) LBO." (2.3mm) TRANSISTOR COUNT: 75 SUBSTRATE CONNECTED TO Package Information E H DIM A A B C D E e H h L α MIN INCHES MAX BSC MILLIMETERS MIN MAX BSC A e D B A A.27mm.4in. C h x 45 L α -PIN PLASTIC SMALL-LINE PACKAGE Maxim Integrated

9 3.3/5/Adjustable-Output, 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. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. Maxim Integrated Rio Robles, San Jose, CA 9534 USA Maxim Integrated The Maxim logo and Maxim Integrated are trademarks of Maxim Integrated Products, Inc.