19-0438; Rev 0; 9/95 NUAL KIT MA ATION EET H S A EVALU T WS DA FOLLO 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller ________________________Applications High-Efficiency DC-DC Converters ____________________________Features 1.8V to 16.5V Input Range 85% Efficiency for 30mA to 1.5A Load Currents Up to 10W Output Power 110A Max Supply Current 5A Max Shutdown Current Preset 5V or Adjustable Output (3V to 16.5V) Current-Limited PFM Control Scheme Up to 300kHz Switching Frequency Evaluation Kit Available ______________Ordering Information PART TEMP. RANGE PIN-PACKAGE Battery-Powered Applications MAX608C/D 0C to +70C Positive LCD-Bias Generators MAX608EPA -40C to +85C 8 Plastic DIP Portable Communicators MAX608ESA -40C to +85C 8 SO Dice* * Contact factory for dice specifications. __________Typical Operating Circuit INPUT 1.8V TO VOUT ON/OFF __________________Pin Configuration TOP VIEW OUTPUT 5V MAX608 SHDN REF EXT CS FB AGND GND OUT N EXT 1 8 CS OUT 2 7 GND FB 3 6 AGND SHDN 4 5 REF MAX608 DIP/SO ________________________________________________________________ Maxim Integrated Products Call toll free 1-800-998-8800 for free samples or literature. 1 MAX608 _______________General Description The MAX608 low-voltage step-up controller operates from a 1.8V to 16.5V input voltage range. Pulse-frequency-modulation (PFM) control provides high efficiency at heavy loads, while using only 85A (typical) when operating with no load. In addition, a logic-controlled shutdown mode reduces supply current to 2A typical. The output voltage is factory-set at 5V or can be adjusted from 3V to 16.5V with an external resistor divider. The MAX608 is ideal for two- and three-cell batterypowered systems. An operating frequency of up to 300kHz allows use with small surface-mount components. The MAX608 operates in "bootstrapped" mode only (with the chip supply, OUT, connected to the DC-DC output). For a 12V output without external resistors, or for nonbootstrapped applications (chip supply connected to input voltage), refer to the pin-compatible MAX1771. The MAX608 is available in 8-pin DIP and SO packages. MAX608 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller ABSOLUTE MAXIMUM RATINGS Supply Voltage OUT to GND.............................................................-0.3V, 17V EXT, CS, REF, SHDN, FB to GND ...............-0.3V, (VOUT + 0.3V) GND to AGND.............................................................0.1V, -0.1V Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) ............727mW SO (derate 5.88mW/C above +70C) .........................471mW 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 = 5V, ILOAD = 0mA, TA = -40C to +85C where indicated. TA = -25C to +85C for all other limits. Typical values are at TA = +25C.) PARAMETER SYMBOL MIN TYP MAX TA = -25C to +85C 1.8 16.5 TA = -40C to +85C (Note 1) 1.9 16.5 Minimum Start-Up Voltage No load Supply Current Output Voltage (Note 3) UNITS V 1.6 1.8 V 85 110 A VOUT = 16.5V, SHDN 0.4V TA = -25C to +85C VOUT = 10V, SHDN 1.6V TA = -25C to +85C VIN = 2.0V to 5.0V, over full load range, circuit of Figure 2a TA = -25C to +85C 4.825 5.0 5.175 TA = -40C to +85C (Note 1) 4.800 5.0 5.200 TA = -40C to +85C (Note 1) 120 2 TA = -40C to +85C (Note 1) 5 10 A V Output Voltage Line Regulation (Note 4) VIN = 2.7V to 4.0V, VOUT = 5V, ILOAD = 500mA, circuit of Figure 2a 7 mV/V Output Voltage Load Regulation (Note 4) VIN = 2V, VOUT = 5V, ILOAD = 0mA to 500mA, circuit of Figure 2a 60 mV/A Maximum Switch On-Time tON(max) 12 16 20 s Minimum Switch Off-Time 1.8 2.3 2.8 s tOFF(min) VIN = 4V, VOUT = 5V, ILOAD = 500mA, circuit of Figure 2a Efficiency Reference Voltage 2 CONDITIONS Input Voltage Range (Note 2) VREF IREF = 0A 87 TA = -25C to +85C 1.4625 TA = -40C to +85C (Note 1) 1.4475 1.5 % 1.5375 1.5525 V REF Load Regulation 0A IREF 100A -4 10 mV REF Line Regulation 3V VOUT 16.5V 40 100 V/V TA = -25C to +85C 1.4625 TA = -40C to +85C (Note 1) 1.4475 1.5 1.5375 FB Trip Point Voltage (Note 5) VFB FB Input Current IFB SHDN Input High Voltage VIH VOUT = 1.8V to 16.5V SHDN Input Low Voltage VIL VOUT = 1.8V to 16.5V 0.4 SHDN Input Current IIN VOUT = 16.5V, SHDN = 0V or 16.5V 1 1.5525 -4 TA = -25C to +85C TA = -40C to +85C (Note 1) 20 40 1.6 _______________________________________________________________________________________ V nA V V A 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller (VOUT = 5V, ILOAD = 0mA, TA = -40C to +85C where indicated. TA = -25C to +85C for all other limits. Typical values are at TA = +25C.) PARAMETER SYMBOL Current-Limit Trip Level VCS CS Input Current ICS CONDITIONS VOUT = 3V to 16.5V MIN TYP MAX TA = -25C to +85C 85 100 115 TA = -40C to +85C (Note 1) 80 UNITS mV 120 0.01 EXT Rise Time VOUT = 5V, 1nF from EXT to GND 50 EXT Fall Time VOUT = 5V, 1nF from EXT to GND 50 EXT On-Resistance EXT = high or low 15 A 1 ns 30 Note 1: Limits over this temperature range are guaranteed by design. Note 2: The MAX608 must be operated in bootstrapped mode with OUT connected to the DC-DC circuit output. The minimum output voltage set point is +3V. Note 3: Output voltage guaranteed using preset voltages. See Figures 4a-4d for output current capability versus input voltage. Note 4: Output voltage line and load regulation depend on external circuit components. Note 5: Operation in the external-feedback mode is guaranteed to be accurate to the VFB trip level, and does not include resistor tolerance. __________________________________________Typical Operating Characteristics (TA = +25C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT (VOUT = 12V) VIN = 9.0V VIN = 3.0V 100 VIN = 2.0V VIN = 2.0V VIN = 3.0V 70 80 60 1 1000 10 1 10 400 400 300 200 100 1000 LOAD CURRENT (mA) SUPPLY CURRENT vs. INPUT VOLTAGE VOUT = 12V CIRCUIT OF FIGURE 2b EXTERNAL FET THRESHOLD LIMITS FULL-LOAD START-UP BELOW 3.6V 200 300 200 MAX608-06 500 LOAD CURRENT (mA) VOUT = 5V CIRCUIT OF FIGURE 2a EXTERNAL FET THRESHOLD LIMITS FULL-LOAD START-UP BELOW 3.7V MAX608-04 700 500 1000 LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE 600 100 LOAD CURRENT (mA) SUPPLY CURRENT (A) 100 LOAD CURRENT (mA) MAX608-05 10 VIN = 2.0V 70 60 1 VIN = 3.0V 90 80 60 LOAD CURRENT (mA) VIN = 5.0V EFFICIENCY (%) 80 70 VIN = 6.0V 90 VIN = 3.5V EFFICIENCY (%) EFFICIENCY (%) 90 100 MAX608-03 VIN = 4.0V MAX608-01 100 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) MAX608-02 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V) 150 100 50 100 100 0 0 0 1.8 2.2 2.6 3.0 3.4 3.8 MINIMUM START-UP VOLTAGE (V) 4.0 1.8 2.2 2.6 3.0 3.4 3.8 MINIMUM START-UP VOLTAGE (V) 4.0 0 1 2 3 4 5 INPUT VOLTAGE (V) _______________________________________________________________________________________ 3 MAX608 ELECTRICAL CHARACTERISTICS (continued) ____________________________Typical Operating Characteristics (continued) (TA = +25C, unless otherwise noted.) REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE CEXT = 2200pF CEXT = 1000pF CEXT = 470pF CEXT = 100pF 100 50 MAX608-08 10A 1.502 150 100 50A 6 4 1.494 12 10 8 1.492 -60 -40 -20 0 20 40 60 80 100 120 140 MAXIMUM SWITCH ON-TIME vs. TEMPERATURE SHUTDOWN CURRENT (A) 3.0 2.5 2.0 V+ = 15V 1.5 V+ = 8V V+ = 4V 2.20 0 90 120 150 2.25 1.0 0.5 60 2.30 MAX608-11 3.5 15.5 30 MINIMUM SWITCH OFF-TIME vs. TEMPERATURE SHUTDOWN CURRENT vs. TEMPERATURE 16.0 0 20 40 60 80 100 120 140 TEMPERATURE (C) 4.0 MAX608-10 16.5 0 -60 -40 -20 TEMPERATURE (C) SUPPLY VOLTAGE (V) -60 -30 1.498 tOFF(min) (s) 2 1.500 1.496 100A 50 0 0 1.504 200 MAX608-12 150 REFERENCE vs. TEMPERATURE 1.506 REFERENCE (V) MAX608-07 200 250 REFERENCE OUTPUT RESISTANCE () EXT RISE/FALL TIME (ns) 250 MAX608-09 EXT RISE/FALL TIME vs. SUPPLY VOLTAGE tON(max) (s) MAX608 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller -60 -40 -20 TEMPERATURE (C) 0 20 40 60 80 100 120 140 -60 -30 0 30 VOUT VOUT A B 90 MEDIUM-LOAD SWITCHING WAVEFORMS (VOUT = 5V) HEAVY-LOAD SWITCHING WAVEFORMS (VOUT = 5V) A 60 TEMPERATURE (C) TEMPERATURE (C) 0V 0V ILIM ILIM B 0A 0A C C 2s/div VIN = 3V, IOUT = 930mA, VOUT = 5V A = EXT VOLTAGE, 5V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED 4 20s/div VIN = 3V, IOUT = 490mA, VOUT = 5V A = EXT VOLTAGE, 5V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED _______________________________________________________________________________________ 120 150 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MEDIUM-LOAD SWITCHING WAVEFORMS (VOUT = 12V) HEAVY-LOAD SWITCHING WAVEFORMS (VOUT = 12V) VOUT VOUT A A 0V 0V ILIM ILIM B B 0A 0A C C 10s/div 2s/div VIN = 4V, IOUT = 300mA, VOUT = 12V A = EXT VOLTAGE, 10V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED VIN = 4V, IOUT = 490mA, VOUT = 12V A = EXT VOLTAGE, 10V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED LINE-TRANSIENT RESPONSE (VOUT = 5V) LOAD-TRANSIENT RESPONSE (VOUT = 5V) A 500mA 4.0V A 0A 2.7V B B 2ms/div 5ms/div IOUT = 500mA, VOUT = 5V A = VIN, 2.7V TO 4.0V, 1V/div B = VOUT RIPPLE, 100mV/div, AC-COUPLED VIN = 2V, VOUT = 5V A = LOAD CURRENT, 0mA TO 500mA, 500mA/div B = VOUT RIPPLE, 50mV/div, AC-COUPLED EXITING SHUTDOWN A 0V 5V B 0V 200s/div IOUT = 500mA, VIN = 3.5V A = SHDN, 2V/div B = VOUT, 2V/div _______________________________________________________________________________________ 5 MAX608 ____________________________Typical Operating Characteristics (continued) (TA = +25C, unless otherwise noted.) MAX608 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller ______________________________________________________________Pin Description PIN NAME FUNCTION 1 EXT Gate Drive for External N-Channel Power Transistor 2 OUT Power-Supply and Voltage-Sense Input. Always connect OUT to circuit output. 3 FB 4 SHDN 5 REF 6 AGND 7 GND 8 CS Feedback Input for Adjustable-Output Operation. Connect to ground for fixed-output operation. Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section. Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, VOUT is a diode drop below the input voltage (due to the DC path from the input voltage to the output). Connect to ground for normal operation. 1.5V Reference Output that can source 100A for external loads. Bypass to GND with 0.1F. The reference is disabled in shutdown. Analog Ground High-Current Ground Return for the Output Driver Positive Input to the Current-Sense Amplifier. Connect the current-sense resistor between CS and AGND. REF FB DUAL-MODE COMPARATOR SHDN MAX608 BIAS CIRCUITRY 50mV 1.5V REFERENCE ERROR COMPARATOR LOW-VOLTAGE START-UP COMPARATOR MIN OFF-TIME ONE-SHOT Q TRIG N OUT 2.3s F/F S Q R MAX ON-TIME ONE-SHOT LOW-VOLTAGE OSCILLATOR 2.5V EXT TRIG Q 16s CURRENT-SENSE AMPLIFIER 0.1V CS Figure 1. Functional Diagram 6 _______________________________________________________________________________________ 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller C2 0.1F 5 REF C3 0.1F 4 SHDN C2 0.1F 2 2 OUT OUT MAX608 3 FB EXT 1 5 REF C1 150F L1 22H D1 1N5817 C3 0.1F VOUT = 5V @ 0.5A 4 SHDN MAX608 6 N AGND L1 22H EXT CS 1 CS 8 FB RSENSE 50m GND 7 R2 = (R1) D1 1N5817 VOUT = 12V @ 0.3A C4 200F 8 RSENSE 50m 3 GND 7 C4 200F C1 150F N MMFT3055EL MMFT3055EL 6 AGND MAX608 VIN = 2V VIN = 2V R2 402k R1 58k ( VVOUT -1) REF VREF = 1.5V Figure 2a. 5V Preset Output Figure 2b. 12V Output _______________Detailed Description The MAX608 is a BiCMOS, step-up, switch-mode power-supply controller that provides a preset 5V output, in addition to adjustable-output operation. Its unique control scheme combines the advantages of pulse-frequency modulation (low supply current) and pulse-width modulation (high efficiency with heavy loads), providing high efficiency over a wide output current range, as well as increased output current capability over previous PFM devices. In addition, the external sense resistor and power transistor allow the user to tailor the output current capability for each application. Figure 1 shows the MAX608 functional diagram. The device has a shutdown mode that reduces the supply current to 5A max. Figure 2 shows the standard application circuits. The IC is powered from the output, and the input voltage range is 1.8V to VOUT (this configuration is commonly known as bootstrap operation). The voltage applied to the gate of the external power transistor is switched from VOUT to ground. The MAX608's output voltage can be set to 5V by connecting FB to ground; it can also be adjusted from 3V to 16.5V using external resistors. Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2b, 2c) to achieve an overall output voltage accuracy of 5%. VIN = 2V C2 0.1F 2 5 REF C3 0.1F 4 SHDN MAX608 6 L1 22H OUT AGND EXT CS FB GND 7 R2 = (R1) ( VVOUT -1) 1 C1 150F D1 1N5817 N SI6426 VOUT = 3.3V @ 0.6A C4 200F 8 3 RSENSE 50m R1 50k R2 60k C5 47pF REF VREF = 1.5V Figure 2c. 3.3V Output PFM Control Scheme The MAX608 uses a proprietary current-limited PFM control scheme to provide high efficiency over a wide range of load currents. This control scheme combines the ultralow supply current of PFM converters (or pulse skippers) with the high full-load efficiency of PWM converters. _______________________________________________________________________________________ 7 MAX608 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller Unlike traditional PFM converters, the MAX608 uses a sense resistor to control the peak inductor current. The device also operates with high switching frequencies (up to 300kHz), allowing the use of miniature external components. As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses the output is out of regulation. However, unlike traditional PFM converters, the MAX608 switch uses the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16s) and minimum offtime (2.3s); there is no oscillator. Once off, the minimum off-time one-shot holds the switch off for 2.3s. After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. The control circuitry allows the IC to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16s later), or 2) the switch current reaches the peak current limit set by the current-sense resistor. The MAX608 switching frequency is variable (depending on load current and input voltage), causing variable switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the filter capacitor equivalent series resistance (ESR). For example, when generating a 5V output at 500mA from a 2V input, only 75mV of output ripple occurs, using the circuit of Figure 2a. Low-Voltage Start-Up Oscillator The MAX608 features a low input voltage start-up oscillator that guarantees start-up with no load for input voltages down to 1.8V. At these low voltages, the output voltage is not large enough for proper error-comparator operation and internal biasing. The start-up oscillator has a fixed 50% duty cycle and the MAX608 disregards the error-comparator output when the output voltage is less than 2.5V. Above 2.5V, the error-comparator and normal one-shot timing circuitry are used. R2 VOUT FB MAX608 R1 C5* R1 = 10k TO 500k GND R2 = R1 V -1) ( VOUT REF VREF = 1.5V * OPTIONAL, SEE TEXT FOR VALUE Figure 3. Adjustable Output Circuit __________________Design Procedure Setting the Output Voltage The MAX608's output voltage is preset to 5V (FB = 0V), or it can be adjusted from 16.5V down to 3V using external resistors R1 and R2, configured as shown in Figure 3. For adjustable-output operation, select feedback resistor R1 in the 10k to 500k range. R2 is given by: VOUT -1 R2 = (R1) ----- VREF ( ) where VREF equals 1.5V. OUT must always be connected to the circuit output. Figure 2 shows various circuit configurations for preset/ adjustable operation. Determining RSENSE Shutdown Mode Use the theoretical output current curves shown in Figures 4a-4d to select R SENSE . They are derived using the minimum (worst-case) current-limit comparator threshold value over the extended temperature range (-40C to +85C). No tolerance was included for RSENSE. The voltage drop across the diode is assumed to be 0.5V, and the drop across the power switch rDS(ON) and coil resistance is assumed to be 0.3V. When SHDN is high, the MAX608 enters shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference), and V OUT falls to a diode drop below V IN (due to the DC path from the input to the output). In shutdown mode, the supply current drops to less than 5A. SHDN is a TTL/CMOS logic-level input. Connect SHDN to GND for normal operation. Practical inductor values range from 10H to 300H. 22H is a good choice for most applications. In applications with large input/output differentials, the IC's output-current capability will be much less when the inductance value is too low, because the IC will always operate in discontinuous mode. If the inductor value is too low, the 8 Determining the Inductor (L) _______________________________________________________________________________________ 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller 3.5 1.5 RSENSE = 25m RSENSE = 35m 1.0 RSENSE = 50m 0.5 RSENSE = 100m MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) VOUT = 3.3V L = 22H MAX608 2.0 VOUT = 5V L = 22H 3.0 RSENSE = 20m 2.5 RSENSE = 25m 2.0 RSENSE = 35m 1.5 1.0 RSENSE = 50m 0.5 RSENSE = 100m 0 0 2.0 2.5 3.0 INPUT VOLTAGE (V) 2 3.5 Figure 4a. Maximum Output Current vs. Input Voltage (VOUT = 3.3V) 3.5 VOUT = 12V L = 22H 3.0 RSENSE = 20m RSENSE = 25m 2.5 RSENSE = 35m 2.0 1.5 1.0 RSENSE = 50m 0.5 5 Figure 4b. Maximum Output Current vs. Input Voltage (VOUT = 5V) MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) 3.5 3 4 INPUT VOLTAGE (V) VOUT = 15V L = 22H 3.0 RSENSE = 20m RSENSE = 25m 2.5 RSENSE = 35m 2.0 1.5 1.0 RSENSE = 50m 0.5 RSENSE = 100m 0 2 4 6 8 INPUT VOLTAGE (V) 10 RSENSE = 100m 0 12 2 4 6 8 10 12 INPUT VOLTAGE (V) 14 16 Figure 4c. Maximum Output Current vs. Input Voltage (VOUT = 12V) Figure 4d. Maximum Output Current vs. Input Voltage (VOUT = 15V) current will ramp up to a high level before the current-limit comparator can turn off the switch. The minimum on-time for the switch (tON(min)) is approximately 2s; select an inductor that allows the current to ramp up to ILIM. Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use with high switching frequencies. Make sure the inductor's saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current rating set by R SENSE. However, it is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). For highest efficiency, use a coil with low DC resistance, preferably under 20m. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 1 lists inductor suppliers and specific recommended inductors. The standard operating circuits use a 22H inductor. If a different inductance value is desired, select L such that: VIN(max) x 2s L -----------------ILIM Larger inductance values tend to increase the start-up time slightly, while smaller inductance values allow the coil current to ramp up to higher levels before the switch turns off, increasing the ripple at light loads. _______________________________________________________________________________________ 9 MAX608 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller Power Transistor Selection Use an N-channel MOSFET power transistor with the MAX608. Use logic-level or low-threshold N-FETs to ensure the external N-channel MOSFET (N-FET) is turned on completely and that start-up occurs. N-FETs provide the highest efficiency because they do not draw any DC gate-drive current. When selecting an N-FET, some important parameters to consider are the total gate charge (Qg), on-resistance (rDS(ON)), reverse transfer capacitance (CRSS), maximum drain to source voltage (VDS max), maximum gate to source voltage (VGS max), and minimum threshold voltage (VTH min). Qg takes into account all capacitances associated with charging the gate. Use the typical Qg value for best results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be 50nC or less. With larger numbers, the EXT pins may not be able to adequately drive the gate. The EXT rise/fall time varies with different capacitive loads as shown in the Typical Operating Characteristics. The two most significant losses contributing to the N-FET's power dissipation are I2R losses and switching losses. Select a transistor with low r DS(ON) and low CRSS to minimize these losses. Determine the maximum required gate-drive current from the Qg specification in the N-FET data sheet. Select an N-FET with a BVDSS > VOUT, BVGSS > VOUT, and a minimum VTH of 0.5V below the minimum input voltage. When using a power supply that decays with time (such as a battery), the N-FET transistor will operate in its linear region when the voltage at EXT approaches the threshold voltage of the FET, dissipating excessive power. Prolonged operation in this mode may damage the FET. To avoid this condition, make sure VEXT is above the VTH of the FET, or use a voltage detector (such as the MAX8211) to put the IC in shutdown mode once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input voltages can also cause this condition. The MAX608's maximum allowed switching frequency during normal operation is 300kHz. However, at startup, the maximum frequency can be 500kHz, so the maximum current required to charge the N-FET's gate is f(max) x Qg(typ). Use the typical Qg number from the transistor data sheet. For example, the MMFT3055EL has a Qg(typ) of 7nC (at VGS = 5V), therefore the current required to charge the gate is: 10 IGATE (max) = (500kHz) (7nC) = 3.5mA. Figure 2a's application circuit uses a 4-pin MMFT3055EL surface-mount N-FET that has 150m on-resistance with 4.5V VGS, and a guaranteed VTH of less than 2V. Figure 2c's application circuit uses an Si6426DQ logic-level NFET with a threshold voltage (VTH) of 1V. Diode Selection The MAX608's high switching frequency demands a high-speed rectifier. Schottky diodes such as the 1N5817-1N5822 are recommended. Make sure the Schottky diode's average current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds V OUT . For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes such as the MUR105 or EC11FS1 can be used instead. At heavy loads and high temperatures, the benefits of a Schottky diode's low forward voltage may outweigh the disadvantage of high leakage current. Capacitor Selection Output Filter Capacitor The primary criterion for selecting the output filter capacitor (C4) is low effective series resistance (ESR). The product of the peak inductor current and the output filter capacitor's ESR determines the amplitude of the ripple seen on the output voltage. Two OS-CON 100F, 16V output filter capacitors in parallel with 35m of ESR each typically provide 75mV ripple when stepping up from 2V to 5V at 500mA (Figure 2a). Smaller-value and/or higherESR capacitors are acceptable for light loads or in applications that can tolerate higher output ripple. Since the output filter capacitor's ESR affects efficiency, use low-ESR capacitors for best performance. See Table 1 for component selection. Input Bypass Capacitors The input bypass capacitor (C1) reduces peak currents drawn from the voltage source and also reduces noise caused by the switching action of the MAX608 at the voltage source. The input voltage source impedance determines the size of the capacitor required at the OUT input. As with the output filter capacitor, a low-ESR capacitor is recommended. For output currents up to 1A, 150F (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC with a 0.1F ceramic capacitor (C2) placed as close as possible to the OUT and GND pins. Reference Capacitor Bypass REF with a 0.1F capacitor (C3). REF can source up to 100A of current for external loads. ______________________________________________________________________________________ 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller Surface Mount Through Hole INDUCTORS CAPACITORS Sumida CD54 series CDR125 series Coiltronics CTX20 series Coilcraft DO3316 series DO3340 series Matsuo 267 series Sprague 595D series AVX TPS series Sanyo OS-CON series Sumida RCH855 series RCH110 series Sanyo OS-CON series Nichicon PL series Feed-Forward Capacitor When adjusting the output voltage, it may be necessary to parallel a 47pF to 220pF capacitor across R2, as shown in Figures 2 and 3. Choose the lowest capacitor value that insures stability; high capacitance values may degrade line regulation. __________Applications Information Starting Up Under Load The Typical Operating Characteristics show the Start-Up Voltage vs. Load Current graphs for 5V and 12V output voltages. These graphs depend on the type of power switch used. The MAX608 is not designed to start up under full load with low input voltages. Layout Considerations Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting GND, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). Also, minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. Place input bypass capacitor C2 as close as possible to OUT and GND. If an external resistor divider is used (Figures 2 and 3), the trace from FB to the resistors must be extremely short. TRANSISTORS Siliconix Si9410DY Si4410DY Si6426DQ Si6946DQ Motorola MTP3055EL MTD20N03HDL MMFT3055ELT1 DIODES Central Semiconductor CMPSH-3 CMPZ5240 Nihon EC11 FS1 series (highspeed silicon) Motorola MBRS1100T3 MMBZ5240BL Motorola 1N5817-1N5822 MUR105 (high-speed silicon) SUPPLIER PHONE FAX AVX USA: (803) 448-9411 (803) 448-1943 Central Semiconductor USA: (516) 435-1110 (516) 435-1824 Coilcraft USA: (708) 639-6400 (708) 639-1469 Coiltronics USA: (407) 241-7876 (407) 241-9339 Matsuo USA: (714) 969-2491 Japan: 81-6-337-6450 (714) 960-6492 81-6-337-6456 Motorola USA: (800) 521-6274 (602) 952-4190 Nichicon USA: (708) 843-7500 (708) 843-2798 Nihon USA: (805) 867-2555 (805) 867-2556 Sanyo USA: (619) 661-6835 Japan: 81-7-2070-1005 (619) 661-1055 81-7-2070-1174 Siliconix USA: (800) 554-5565 (408) 970-3950 Sprague USA: (603) 224-1961 (603) 224-1430 Sumida USA: (708) 956-0666 Japan: 81-3-3607-5111 (708) 956-0702 81-3-3607-5144 ______________________________________________________________________________________ 11 MAX608 PRODUCTION MAX608 5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller ___________________Chip Topography EXT OUT CS 0.126" (3.200mm) GND AGND FB SHDN REF 0.080" (2.032mm) TRANSISTOR COUNT: 501 SUBSTRATE CONNECTED TO OUT ________________________________________________________Package Information DIM D 0-8 A 0.101mm 0.004in. e B A1 E 12 C H L Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.) A A1 B C E e H L INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016 DIM PINS D D D 8 14 16 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27 INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00 ______________________________________________________________________________________ 21-0041A