19-1307; Rev 0a; 11/97 KIT ATION EVALU E L B AVAILA Step-Up/Down DC-DC Converter in QSOP Package ____________________________Features The MAX1672 integrates a step-up DC-DC converter with a linear regulator to provide step-up/down voltage conversion. This device provides a constant output voltage for inputs that vary above and below the output voltage. It has a 1.8V to 11V input range and a preset 3.3V or 5V output. The output can also be set from 1.25V to 5.5V using two resistors. Typical efficiency is 85%. The MAX1672’s step-up/linear-regulator configuration permits the use of a single, physically smaller inductor than can be used with competing SEPIC and flyback configurations. Switch current is also selectable, permitting the use of smaller inductors in low-current applications. The linear regulator also acts as a filter to reduce output ripple voltage. ♦ Step-Up/Down Voltage Conversion ♦ 1.8V to 11V Input Range ♦ 3.3V/5V or Adjustable Output Voltage Range The MAX1672 has a low 85µA quiescent supply current, which is further reduced to 0.1µA in logic-controlled shutdown. The output voltage is disconnected from the input in shutdown. The MAX1672 also has a PGI/PGO low-battery detector. The MAX1672 comes in a 16-pin QSOP package (same size as a standard 8-pin SO). For a larger device that delivers more output current, refer to the MAX710/MAX711. The preassembled MAX1672 evaluation kit is available to speed designs. ________________________Applications Single-Cell, Lithium-Powered Portable Devices 2-Cell to 4-Cell AA Alkaline Hand-Held Equipment 3.3V and Other Low-Voltage Systems Battery-Powered Devices with AC Input Adapters ♦ Output Current: 300mA at 5V (VIN ≥ 2.5V) 150mA at 5V (VIN ≥ 1.8V) ♦ Smaller Inductor than SEPIC and Flybacks ♦ Load Disconnects from Input in Shutdown ♦ Supply Current from Battery: 85µA (No-Load) 0.1µA (Shutdown) ♦ PGI/PGO Low-Battery Comparator ♦ 16-Pin QSOP Package (same footprint as 8-pin SO) ♦ No External FETs Required ♦ Thermal and Short-Circuit Protection ______________Ordering Information PART TEMP. RANGE MAX1672C/D 0°C to +70°C MAX1672EEE -40°C to +85°C PIN-PACKAGE Dice* 16 QSOP *Dice are tested at TA = +25°C. Digital Cameras __________Typical Operating Circuit INPUT 1.8V TO 11V __________________Pin Configuration TOP VIEW IN LX PS PGI 16 LX LX 1 15 PGND PGND 2 MAX1672 ON OFF ONA ON 3.3V OFF ONB 5V 3/5 0.5A 0.8A ILIM ONB 3 OUT PG0 FB REF PGND GND 3.3V/5V OUTPUT LOW-BATTERY DETECTOR OUTPUT ONA 4 14 GND MAX1672 13 REF 3/5 5 12 IN PGI 6 11 PS PGO 7 10 FB ILIM 8 9 OUT 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 408-737-7600 ext. 3468. MAX1672 ________________General Description MAX1672 Step-Up/Down DC-DC Converter in QSOP Package ABSOLUTE MAXIMUM RATINGS IN, PS, LX, OUT, PGO to GND ......................... -0.3V to +11.5V ILIM, ONA, ONB, FB, 3/5, REF, PGI to GND......................................-0.3V to (VPS + 0.3V) PGND to GND .......................................................-0.3V to +0.3V OUT Short Circuit to GND ..........................................Continuous Output Current ..................................................................350mA Continuous Power Dissipation (TA = +70°C) 16-Pin QSOP (derate above +70°C by 8.3mW/°C).......667mW 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 (VPS = 6V, CREF = 0.1µF, COUT = 4.7µF, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1) PARAMETER CONDITIONS MIN Input Voltage Startup Voltage Output Voltage TYP 1.8 MAX UNITS 11.0 V 0.9 FB = GND, IOUT = 0mA to 150mA 3/5 = GND 3/5 = PS TA = 0°C to +85°C 4.8 TA = -40°C to +85°C 4.75 TA = 0°C to +85°C 3.17 TA = -40°C to +85°C 3.13 Output Voltage Adjustment Range V 5.2 5.00 5.25 3.43 3.30 1.25 V 3.47 5.5 V Output Load Regulation VIN = 2V, 3/5 = GND, FB = GND, IOUT = 10mA to 150mA 0.003 %/mA Output Line Regulation VIN = 3V to 5V, 3/5 = GND, IOUT = 100mA 0.15 %/V Quiescent Current ONA = PS or ONB = GND, current measured into PS pin, IOUT = 0mA Shutdown Quiescent Current ONA = GND, ONB = PS, current measured into PS pin Reference Voltage IREF = 0mA 85 0.1 TA = 0°C to +85°C 1.21 1.25 1.29 TA = -40°C to +85°C 1.20 Hysteresis = 15mV typical 70 FB Input Current VFB = 1.3V 1 IN Input Current VIN = GND to 11V nA µA 6 1.3 VPS = 2.7V, ILX = 50mA 0.9 2.0 ILIM = PS 0.1 1 TA = 0°C to +85°C 0.35 0.5 0.65 TA = -40°C to +85°C 0.3 0.5 0.7 TA = 0°C to +85°C 0.6 0.8 1.0 TA = -40°C to +85°C 0.5 0.8 1.1 VPS = 5.5V, IOUT = 50mA 1.2 2.4 VPS = 2.7V, IOUT = 50mA 2.3 4.6 Output PFET Leakage Current VOUT = 0V, ONA = GND, ONB = PS VPS = 5.5V 0.35 V mV 3 VLX = 11V, ONA = GND, ONB = PS V 50 0.6 ILIM = GND 2 1.30 VPS = 5.5V, ILX = 50mA LX Current Limit Output PFET Current Limit µA 1.29 FB Dual-Mode Trip Threshold Output PFET Resistance 1 1.25 OUT = FB LX Leakage Current µA 1.21 FB Voltage LX On-Resistance 125 Ω µA A Ω 0.1 1 µA 0.7 1.4 A _______________________________________________________________________________________ Step-Up/Down DC-DC Converter in QSOP Package (VPS = 6V, CREF = 0.1µF, COUT = 4.7µF, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1) PARAMETER CONDITIONS MIN TYP MAX UNITS Thermal Shutdown Threshold 150 °C Thermal Shutdown Hysteresis 20 °C PGI/PGO COMPARATOR PGI Input Bias Current VPGI = 1.3V 1 Hysteresis 50 nA 30 PGI Threshold Voltage TA = 0°C to +85°C 1.21 TA = -40°C to +85°C 1.19 mV 1.29 1.25 V 1.31 PGO Output Leakage VPGO = 11V 0.1 1 µA PGO Output Low Voltage IPGO = 2mA, VPGI = 1.2V 0.1 0.4 V 0.4 V 100 nA LOGIC AND CONTROL INPUTS Input Low Voltage ONA, ONB, 3/5; ILIM Input High Voltage ONA, ONB, 3/5; ILIM Input Bias Current ONA, ONB, 3/5, ILIM 1.6 V 1 Note 1: Specifications to -40°C are guaranteed by design. __________________________________________Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) 70 VIN = 1.8V 90 VIN = 2.7V EFFICIENCY (%) VIN = 2.7V 80 80 70 60 50 1 10 100 OUTPUT CURRENT (mA) 70 VOUT = 3.3V 60 VIN = 0.9V 50 50 0.1 VOUT = 5V 80 VIN = 1.8V VIN = 0.9V 60 VIN = 3.3V 90 EFFICIENCY (%) EFFICIENCY (%) VIN = 3.6V 100 MAX1672-02 VIN = 5V 90 100 MAX1672-01 100 EFFICIENCY vs. INPUT VOLTAGE (IOUT = 10mA) EFFICIENCY vs. OUTPUT CURRENT (VOUT = 3.3V) MAX1672-03 EFFICIENCY vs. OUTPUT CURRENT (VOUT = 5V) 1000 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 0 1 2 3 4 5 6 7 8 9 10 INPUT VOLTAGE (V) _______________________________________________________________________________________ 3 MAX1672 ELECTRICAL CHARACTERISTICS (continued) _____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) ILIM = PS (0.8A) ILIM = GND (0.5A) 100 300 ILIM = PS (0.8A) 200 ILIM = GND (0.5A) 100 400 MAXIMUM RECOMMENDED OUTPUT CURRENT VOUT = 5V 300 200 VOUT = 3.3V 100 VOUT = 5V TA = +25°C TA = +85°C 0 0 0 1 2 3 4 VOUT = 3.3V 0 0 1 2 3 4 4 5 6 7 8 9 MAX1672-06 MAXIMUM RECOMMENDED OUTPUT CURRENT MAXIMUM RECOMMENDED INPUT VOLTAGE 300 400 MAX1672-05 MAXIMUM RECOMMENDED OUTPUT CURRENT MAXIMUM OUTPUT CURRENT (mA) MAX1672-04 MAXIMUM OUTPUT CURRENT (mA) 400 200 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (POWER DISSIPATION LIMIT) MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (VOUT = 3.3V) MAXIMUM OUTPUT CURRENT (mA) MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (VOUT = 5V) 10 11 12 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE SHUTDOWN CURRENT vs. INPUT VOLTAGE LINEAR REGULATOR POWER-SUPPLY REJECTION RATIO vs. FREQUENCY 100 VOUT = 3.3V MAX1672-09 60 50 PSRR (dB) SUPPLY CURRENT (µA) VOUT = 5V 70 MAX1672-08 1 MAX1672-07 1000 SUPPLY CURRENT (µA) MAX1672 Step-Up/Down DC-DC Converter in QSOP Package 0.1 40 30 20 10 10 0.01 0 1 2 3 4 5 6 7 8 9 10 11 0 INPUT VOLTAGE (V) 1 2 3 4 5 6 7 8 0.01 9 10 11 LOAD-TRANSIENT RESPONSE LINE-TRANSIENT RESPONSE 1 10 100 MAX1672-12 A A 1000 OUTPUT RIPPLE (MEDIUM LOAD) MAX1672-11 MAX1672-10 0.1 FREQUENCY (kHz) INPUT VOLTAGE (V) A B B B 2ms/div A: VOUT = 5V (100mV/div, AC COUPLED) B: VIN = 2V TO 4V (IOUT = 100mA) 4 2ms/div A: VOUT = 5V (50mV/div, AC COUPLED) B: IOUT = 10mA TO 100mA (VIN = 2V) 10µs/div A: VOUT = 5V (20mV/div, AC COUPLED) B: IL1 (500mA /div) (VIN = 2.7V, IOUT = 80mA) _______________________________________________________________________________________ Step-Up/Down DC-DC Converter in QSOP Package (TA = +25°C, unless otherwise noted.) OUTPUT RIPPLE (HEAVY LOAD) TURN-OFF DELAY START-UP DELAY MAX1672-13 MAX1672-15 MAX1672-14 A A A B B B 10µs/div A: VOUT = 5V (20mV/div, AC COUPLED) B: IL1 (500mA /div) (VIN = 2.7V, IOUT = 250mA) 50µs/div A: VOUT = (2V/div) B: ONB (2V/div) (VIN = 2.7V, RLOAD = 50Ω) 200µs/div A: VOUT = (2V/div) B: ONB (2V/div) (VIN = 2.7V, RLOAD = 50Ω) ______________________________________________________________Pin Description PIN NAME FUNCTION 1 LX 2 PGND 3 ONB On Control Input. When ONB = low or ONA = high, the IC is on. Connect ONB to GND for normal operation (Table 1). 4 ONA On Control Input. When ONA = low and ONB = high, the IC is off. Connect ONA to PS for normal operation (Table 1). 5 3/5 Output Voltage Selection Input. Connect to PS for 3.3V output and to GND for 5V output. With VFB > 80mV, the state of the 3/5 pin is ignored. (Table 2). 6 PGI Low-Battery Detector Input (1.25V threshold) 7 PGO Low-Battery Detector Output (open drain). PGO pulls low when VPGI is greater than 1.25V. 8 ILIM Inductor-Current-Limit Selection Input. Connect to PS for 0.8A current limit and to GND for 0.5A current limit. 9 OUT Regulator Output. Drain of internal PFET linear regulator. Bypass with a 4.7µF capacitor to GND. 10 FB Feedback Input. For 3.3V or 5V output, connect to GND. For adjustable output, connect to feedback resistordivider network. With VFB > 70mV, the state of the 3/5 pin is ignored. 11 PS Bootstrapped Power Supply. Output of step-up switch-mode regulator and source of internal PFET linear regulator. The IC is powered from this pin. 12 IN Input Voltage Sense Input. Connect to input supply. 13 REF Reference Voltage Output. Bypass with a 0.1µF capacitor to GND. 14 GND Analog Ground 15 PGND Power Ground 16 LX Inductor Connection to the Drain of the Internal N-Channel Power MOSFET Power Ground Inductor Connection to the Drain of the Internal N-Channel Power MOSFET _______________________________________________________________________________________ 5 MAX1672 _____________________________Typical Operating Characteristics (continued) MAX1672 Step-Up/Down DC-DC Converter in QSOP Package LX REF ∆TON FIXED TOFF GENERATOR ERROR AMP 2 DRV N FB OFF ILIM 100mV IN CURRENT-LIMIT COMPARATOR INPUT MONITOR PGND OUT MAX1672 PS REF ONA REFERENCE GENERATOR ONB REF + OFFSET PS REF ERROR AMP 1 P FB2 OUT FB FB1 FB 70mV 3/5 PGO REF N N PGI Figure 1. Functional Diagram 6 _______________________________________________________________________________________ GND Step-Up/Down DC-DC Converter in QSOP Package The MAX1672 integrates a step-up, switch-mode DCDC converter with a linear regulator to provide stepup/down voltage conversion. The step-up converter contains an N-channel power MOSFET switch, while the linear regulator contains a P-channel MOSFET pass element (Figure 1). The step-up converter and the linear regulator share the same precision voltage reference. The MAX1672’s input range is from +1.8V to +11V, and the regulated output is internally preset to +3.3V or +5V, or can be adjusted with two external resistors. Boost efficiency typically exceeds 80% over a 2mA to 200mA load range. The device is bootstrapped with chip power derived from the stepped-up voltage output at PS. The MAX1672 typically starts up with a 0.9V input. The MAX1672’s step-up/linear-regulator configuration permits the use of a physically smaller inductor than competing SEPIC and flyback configurations because the 1/2LI2 requirements of a step-up converter are half those of SEPIC and flyback converters. Also, high-frequency switching and selectable peak inductor current limit allow for low inductor value (10µH) and low current saturation rating, respectively, further reducing the inductor’s physical dimensions. The MAX1672 maximizes efficiency in both step-up and step-down operation. In step-up mode, when VIN < VOUT, only the step-up regulator is active, while the linear regulator behaves as a 1.2Ω (at 5V output) PFET switch. This provides optimum efficiency (typically 85%). In low-dropout, step-down operation, when VIN is slightly greater than VOUT, both the step-up regulator and linear regulator are active. The step-up regulator is automatically enabled to maintain headroom across the linear regulator (typically 1V above the 5V output). In this case, boost ripple is rejected by the linear regulator, and OUT remains in regulation with no dropout. In normal step-down operation, when VIN is significantly greater than VOUT, only the linear regulator is active. The mode of operation is automatically controlled onchip through the IN pin, which compares V IN and VOUT. Transitions between step-up, low-dropout stepdown, and normal step-down operation are stable, but can be seen as small variations in the output DC level and output ripple. Step-Up Switch-Mode Converter A pulse-frequency-modulation (PFM) control scheme, with a constant 1µs off-time and variable on-time, controls the N-channel MOSFET switch. A pulse is initiated whenever OUT falls out of regulation. The N-channel switch then turns off when the inductor current reaches the peak current limit or after the 4µs maximum on-time, whichever occurs first. This control architecture provides high-efficiency, discontinuous inductor current under light loads as well as continuous inductor current under heavy loads. The switching frequency and output ripple are a function of load current and input voltage. Linear Regulator The low-dropout linear regulator consists of a reference, an error amplifier, and a P-channel MOSFET. The reference is connected to the error amplifier input. The error amplifier compares this reference with the selected feedback voltage and amplifies the difference. The difference is conditioned and applied to the P-channel pass transistor’s gate. ILIM The current-limit-select input, ILIM, selects between the two peak inductor current limits: 0.8A (ILIM = PS) and 0.5A (ILIM = GND). If the application requires low output current (see Typical Operating Characteristics), select 0.5A. The lower peak current limit allows for a smaller, lower-cost inductor, and reduced output ripple. On/Off Control The MAX1672 is turned on or off by logic inputs ONA and ONB (Table 1). When ONA = 1 or ONB = 0, the device is on. When ONA = 0 and ONB = 1, the device shuts down (see the Applications Information section). For normal (on) operation, connect ONA to PS and ONB to GND. Shutdown mode turns off the MAX1672 completely, disconnecting the input from the output and actively pulling OUT to GND. Table 1. On/Off Logic Control ONA ONB MAX1672 0 0 On 0 1 Off 1 0 On 1 1 On _______________________________________________________________________________________ 7 MAX1672 _______________Detailed Description MAX1672 Step-Up/Down DC-DC Converter in QSOP Package __________________Design Procedure Output Voltage Selection For fixed output voltages of 3.3V or 5V, connect 3/5 to PS or GND and connect FB to GND (Table 2). Alternatively, adjust the output voltage from 1.25V to 5.5V by connecting two resistors, R1 and R2 (Figure 2), which form a voltage divider between OUT and FB. Choose resistor values as follows: R1 = R2[(VOUT / VREF) -1] where VREF = 1.25V. Since the input bias current at FB has a maximum value of 50nA, R1 and R2 can be large with no significant accuracy loss. Choose R2 in the 100kΩ to 270kΩ range and calculate R1 using the above formula. For 1% error, the current through R1 should be at least 100 times FB’s bias current. Whenever the voltage at FB exceeds 70mV above GND, the state of the 3/5 pin is ignored. Connect 3/5 to GND when adjusting VOUT with a resistor divider. Never leave 3/5 unconnected. L1 10µH INPUT 1.8V TO 11V C1 100µF R3 IN PGI LX PS C2 100µF R4 MAX1672 ON OFF ONA ON OFF ONB 3/5 0.5A 0.8A C3 0.1µF 1.25V TO 5.5V OUTPUT OUT R1 ILIM PG0 REF FB PGND GND R5 1M C4 4.7µF LOW-BATTERYDETECTOR OUTPUT R2 Low-Battery Detection The MAX1672 contains a comparator for low-battery detection. If the voltage at PGI falls below VREF (typically 1.25V), the open-drain comparator output (PGO) goes high. Hysteresis is typically 30mV. Set the lowbattery detector’s threshold with resistors R3 and R4 (Figure 2) using the following equation: R3 = R4[(VPGT / VREF) -1] where VPGT is the desired threshold of the low-battery detector and VREF = 1.25V. Since the input bias current at PGI has a maximum value of 50nA, R3 and R4 can be large to minimize input loading with no significant accuracy loss. Choose R4 in the 100kΩ to 270kΩ range and calculate R3 using the above formula. For 1% error, the current through R3 should be at least 100 times PGI’s bias current. The PGO output is open-drain and should be pulled high with external resistor R5 for normal operation. If the low-battery comparator is not used, connect PGI and PGO to GND. Table 2. Output Voltage Control 8 5 3/5 FB 0 GND +5 1 GND +3.3 X >70mV +1.25 to +5.5 VOUT (V) Figure 2. Adjustable Output Voltage Configuration Inductor Selection A 10µH inductor performs well in most MAX1672 applications. Smaller inductor values typically offer a smaller physical size for a given series resistance, but may increase switching losses. Larger inductor values exhibit higher output current capability and larger physical dimensions for a given series resistance. For optimum performance, choose an inductor value from Table 3 or by using the following equation: (VOUT + VDIODE ) t ILIM < OFF < L (VIN(min) + VSWITCH ) 2t ILIM ON(max) where ILIM is the peak switch-current limit, which is 0.8A for ILIM = PS and 0.5A for ILIM = GND. The inductor’s incremental saturation current rating should also be greater than the peak switch-current limit. However, it is generally acceptable to bias most inductors into saturation by as much as 20% with slightly reduced efficiency. The inductor’s DC resistance significantly affects efficiency. See Tables 4 and 5 for a list of suggested inductors and suppliers. _______________________________________________________________________________________ Step-Up/Down DC-DC Converter in QSOP Package 5 3/5 ILIM INDUCTOR VALUE (µH) 0 (5V) 0 (0.5A) 10 to 22 0 (5V) 1 (0.8A) 10 1 (3.3V) 0 (0.5A) 10 1 (3.3V) 1 (0.8A) 4.7 to 10 MAX1672 Table 3. Suggested Inductor Values Table 4. Suggested Components INDUCTORS L1 10µH Sumida CD43-100 (1.04A, 0.182Ω) CD54-100 (1.44A, 0.100Ω) CDRH73-100 (1.68A, 0.072Ω) Coilcraft DT1608C-103 (0.7A, 0.095Ω) CAPACITORS Capacitor Selection The equivalent series resistance (ESR) of both bypass and filter capacitors affects efficiency and output ripple. Output voltage ripple is the product of peak inductor current and filter capacitor ESR. Use low-ESR capacitors for best performance, or connect two or more filter capacitors in parallel. A 100µF, 16V, input bypass capacitor (C1) with low ESR reduces peak battery currents and reflected noise due to inductor current ripple. Smaller ceramic capacitors may also be used for light loads or in applications that can tolerate higher input ripple. A 100µF, 16V, surface-mount (SMT) tantalum PS filter capacitor (C2) with 0.1Ω ESR typically exhibits 20mV output ripple (at OUT) when stepping up from 2V to 5V at 100mA load. Smaller capacitors (down to 10µF with higher ESR) are acceptable for light loads or in applications that can tolerate higher output ripple. Only 4.7µF is needed at OUT (C4) to maintain linear regulator stability. During boost operation, this capacitor reduces output voltage spikes from the step-up converter by forming an R-C lowpass filter along with the P-channel MOSFET on-resistance. Output ripple can be further reduced by increasing C4. See Tables 4 and 5 for a list of suggested capacitors and suppliers. Diode Selection The MAX1672’s high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817 or MBRS130T3, are recommended. Make sure the diode’s current rating exceeds the maximum load current. See Tables 4 and 5 for a list of suggested diodes and suppliers. Tantalum AVX TPSE Series Sprague 593D or 595D Series DIODES Motorola MBRS130LT3 (1.0A, 30V) MBR0520LT3 (0.5A, 20V) Schottky International Rectifier 10BQ40 (1.0A, 40V) 1N5817 Equivalent Table 5. Component Suppliers SUPPLIER PHONE FAX AVX (803) 946-0690 (803) 626-3123 Coilcraft (847) 639-6400 (847) 639-1469 International Rectifier (310) 322-3331 (310) 322-3332 Motorola (602) 303-5454 (602) 994-6430 Sanyo (619) 661-6835 (619) 661-1055 Sprague (603) 224-1961 (603) 224-1430 Sumida (847) 956-0666 (847) 956-0702 _______________________________________________________________________________________ 9 MAX1672 Step-Up/Down DC-DC Converter in QSOP Package __________Applications Information Using a Single, Pushbutton On/Off Switch A single pushbutton switch can be used to turn the MAX1672 on and off. As shown in Figure 3, ONA is pulled low and ONB is pulled high when the part is off. When the momentary switch is pressed, ONB is pulled low and the regulator turns on. The switch should be on long enough for the µC to exit reset. The controller issues a logic high to ONA, which guarantees the part will stay on regardless of the switch state. To turn off the regulator, press the switch again. The controller reads the switch status and pulls ONA low. When the switch is released, ONB goes high, turning off the MAX1672. 1M VDD ONB OUT I/O MAX1672 µC I/O ONA Thermal Overload Protection Thermal overload protection limits total power dissipation in the MAX1672. When the junction temperature exceeds TJ = +150°C, the pass transistor turns off, allowing the MAX1672 to cool. The pass transistor turns on again after the IC’s junction temperature cools by 20°C, resulting in a pulsed output during thermal overload conditions. Thermal overload protection is designed to protect the MAX1672 if fault conditions occur. It is not intended to be used as an operating mode. Prolonged operation in thermal shutdown mode may reduce the IC’s reliability. For continual operation, do not exceed the absolute maximum junction temperature rating TJ = +150°C. Power Dissipation and Operating Region The MAX1672’s maximum power dissipation in stepdown mode depends on the thermal resistance of the case and circuit board, the temperature difference between the die junction and ambient air, and the air flow rate. The power dissipated in the device is P = IOUT (VIN - VOUT) during step-down operation. The maximum power dissipation is as follows: PMAX = (TJ - TA)/(θJB + θBA) where (TJ - TA) is the temperature difference between the MAX1672 die junction and the surrounding air, θJB (or θJC) is the thermal resistance of the package, and θBA is the thermal resistance throughout the printed circuit board, copper traces, and other materials to the surrounding air. The MAX1672’s thermal resistance is 120°C/W. See the Typical Operating Characteristics for Maximum Output Current vs. Input Voltage. 10 1M Figure 3. Momentary Pushbutton On/Off Control Layout Considerations Proper PC board layout is essential to minimize noise due to high inductor current levels and fast switching waveforms. To maximize output power and efficiency and minimize output ripple voltage and ground noise, use the following guidelines when designing your board: • Use a ground plane. • Keep the IC’s GND pin and the ground leads of C1 and C2 (Figure 2) less than 0.2in. (5mm) apart. • Make all connections to the FB and LX pins as short as possible. • Solder the IC’s GND pin directly to the ground plane. Refer to the MAX1672 EV kit for a suggested PC board layout. ______________________________________________________________________________________ Step-Up/Down DC-DC Converter in QSOP Package QSOP.EPS ______________________________________________________________________________________ 11 MAX1672 ________________________________________________________Package Information MAX1672 Step-Up/Down DC-DC Converter in QSOP Package NOTES 12 ______________________________________________________________________________________