MAX1708EEE+TG05

19-2068; Rev 1; 11/10 High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter Features ♦ On-Chip 5A Power MOSFET ♦ 5V, 2A Output from a 3.3V Input ♦ Fixed 3.3V or 5V Output Voltage or Adjustable (2.5V to 5.5V) ♦ Input Voltage Range Down to 0.7V ♦ Low Power Consumption 1mW Quiescent Power 1µA Current in Shutdown Mode ♦ Low-Noise, Constant Frequency Operation (600kHz) ♦ Synchronizable Switching Frequency (350kHz to 1000kHz) ♦ Small QSOP Package ________________________Applications Routers, Servers, Workstations, Card Racks Local 2.5V to 3.3V or 5V Conversion Local 3.3V to 5V Conversion Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX1708EEE+ -40°C to +85°C 16 QSOP +Denotes a lead(Pb)-free/RoHS-compliant package. 3.6V or 5V RF PAs in Communications Handsets Typical Operating Circuit TOP VIEW INPUT 1V TO 5V 2.2μH OFF ON SYNC OR INTERNAL Pin Configuration ONA LX MAX1708 CLK GND SS/LIM REF OUTPUT 3.3V, 5V, OR ADJ UP TO 2A + ONB 1 16 CLK ONA 2 15 3.3/5 LX 3 LX 4 14 PGND MAX1708 LX 5 12 PGND GND 6 11 FB SS/LIM 7 OUT 13 PGND 10 OUT REF 8 9 GND QSOP ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1708 General Description The MAX1708 sets a new standard of space savings for high-power, step-up DC-DC conversion. It delivers up to 10W at a fixed (3.3V or 5V) or adjustable (2.5V to 5.5V) output, using an on-chip power MOSFET from a 0.7V to 5V supply. Fixed-frequency PWM operation ensures that the switching noise spectrum is constrained to the 600kHz fundamental and its harmonics, allowing easy postfiltering for noise reduction. External clock synchronization capability allows for even tighter noise spectrum control. Quiescent power consumption is less than 1mW to extend operating time in battery-powered systems. Two control inputs (ONA, ONB) allow simple push-on, push-off control through a single momentary push-button switch, as well as conventional on/off logic control. The MAX1708 also features programmable soft-start and current limit for design flexibility and optimum performance with batteries. The maximum RMS switch current rating is 5A. For a device with a higher (10A) switch current rating, refer to the MAX1709 data sheet. MAX1708 High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS ONA, ONB, OUT, SS/LIM, 3.3/5 to GND ...............-0.3V to +6.0V LX to PGND ...........................................................-0.3V to +6.0V FB, CLK, REF to GND.............................. -0.3V to (VOUT + 0.3V) PGND to GND .......................................................-0.3V to +0.3V Continuous Power Dissipation (TA = +70°C) QSOP (derate 8.30mW/°C above +70°C). ..................667mW 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 Soldering Temperature (reflow) .......................................+260°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 (VOUT = VCLK = 3.6V, ONA = ONB = FB = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Output Voltage CONDITIONS VFB < 0.1V (Note 1) 3.3/5 = GND, I SW = 0.5A 3.3/5 = OUT, I SW = 0.5A Load Regulation Measured between 0.5A < I SW < 1.5A (Note 2) FB Regulation Voltage (VFB) I SW = 0.5A FB Input Current VFB = 1.5V Output Voltage Adjust Range MIN TYP MAX 3.26 3.34 3.42 4.90 5.05 5.20 -0.40 -0.60 %/A 1.215 1.240 1.265 V 1 200 nA 5.5 V 2.5 V Output Undervoltage Lockout Rising and falling (Note 3) 2.0 2.3 V Frequency in Startup Mode VOUT =1.5V 40 400 kHz Minimum Startup Voltage I OUT < 1mA, TA = +25°C (Note 4) 1.1 V 4 5.0 μA 0.9 Minimum Operating Voltage (Note 5) Soft-Start Pin Current VSS/LIM = 1V 0.7 OUT Supply Current VFB = 1.5V (Note 6) 200 300 μA OUT Leakage Current In Shutdown V ONB = 3.6V 0.1 2 μA LX Leakage Current VLX = V ONB = VOUT = 5.5V 1 25 μA 30 80 m 3.2 N-Channel Switch On-Resistance N-Channel Current Limit V SS/LIM = open 4.5 5.3 7.0 SS/LIM = 150k to GND 1.80 3.00 3.85 1.245 1.260 1.275 V 10 mV 5 mV RMS Switch Current 5 Reference Voltage IREF = 0mA Reference Load Regulation -1μA
IREF
50μA 4 Reference Supply Rejection 2.5V
VOUT
5.5V 0.2 Input Low Level (Note 7) 2 UNITS ONA, ONB, 3.3/5, 1.2V < V OUT < 5.5V 0.2
V OUT CLK, 2.7V < VOUT < 5.5V 0.2
V OUT _______________________________________________________________________________________ A ARMS V High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter (VOUT = VCLK = 3.6V, ONA = ONB = FB = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Input High Level Logic Input Current CONDITIONS MIN ONA, ONB, 3.3/5, 1.2V < V OUT < 5.5V 0.8
CLK, 2.7 V< VOUT < 5.5V 0.8
VONA, VONB, VCLK, V3.3/5 = 0V, 5.5V Internal Oscillator Frequency Maximum Duty Cycle External Clock Frequency Range V OUT V OUT (Note 8) CLK Rise/Fall Time (Note 8) MAX UNITS V -1 1 μA 520 600 680 kHz 82 88 94 % 1000 kHz 50 Ns UNITS 350 CLK Pulse Width TYP 100 ns ELECTRICAL CHARACTERISTICS (VOUT = VCLK = 3.6V, ONA = ONB = FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 9) PARAMETER CONDITIONS VFB < 0.1V, VIN = 2.4V (Note 1) Output Voltage FB Regulation Voltage I SW = 0.5A FB Input Current (VFB) VFB = 1.5V 3.3/5 = GND, I SW = 0.5A 3.3/5 = OUT, I SW = 0.5A Load Regulation Measured between 0.5A < I SW < 1.5A (Note 2) Soft-Start Pin Current VSS/LIM = 1V OUT Leakage Current in Shutdown V ONB = 3.6V OUT Supply Current VFB = 1.5V (Note 6) MIN MAX 3.24 3.45 4.90 5.22 1.20 1.28 V 200 nA -0.60 %/A 5.2 μA 2 μA 300 μA 80 m
3.2 N-Channel Switch On-Resistance N-Channel Current Limit Reference Voltage SS/LIM = unconnected 4.5 7.5 SS/LIM = 150k
to GND 1.8 4.0 IREF = 0mA 1.24 1.28 V A V _______________________________________________________________________________________ 3 MAX1708 ELECTRICAL CHARACTERISTICS (continued) MAX1708 High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter ELECTRICAL CHARACTERISTICS (continued) (VOUT = VCLK = 3.6V, ONA = ONB = FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 9) PARAMETER Input Low Level (Note 7) Input High Level Logic Input Current CONDITIONS MIN ONA, ONB, 3.3/5, 1.2V < V OUT < 5.5V CLK, 2.7V < VOUT < 5.5V ONA, ONB, 3.3/5, 1.2V < V OUT < 5.5V 0.8
VOUT CLK, 2.7V < VOUT < 5.5V VONA, VONB, VCLK, V3.3/5 = 0V, 5.5V 0.8
VOUT Internal Oscillator Frequency Maximum Duty Cycle External Clock Frequency Range CLK Pulse Width (Note 8) CLK Rise/Fall Time (Note 8) MAX 0.2
VOUT 0.2
VOUT UNITS V V -1 1 μA 500 700 kHz 80 95 % 350 1000 kHz 100 ns 50 Ns Note 1: Output voltage is specified at 0.5A switch current ISW, which is equivalent to approximately 0.5A ✕ (VIN / VOUT) of load current. Note 2: Load regulation is measured by forcing specified switch current and straight-line calculation of change in output voltage in external feedback mode. Note that the equivalent load current is approximately ISW ✕ (VIN / VOUT). Note 3: Until undervoltage lockout is reached, the device remains in startup mode. Do not apply full load until this voltage is reached. Note 4: Startup is tested with Figure 1’s circuit. Output current is measured when both the input and output voltages are applied. Note 5: Minimum operating voltage. The MAX1708 is bootstrapped and will operate down to a 0.7V input once started. Note 6: Supply current is measured from the output voltage (3.3V) to the OUT pin. This correlates directly with actual input supply current but is reduced in value according to the step-up ratio and efficiency. Note 7: ONA and ONB inputs have approximately 0.15V hysteresis. Note 8: Guaranteed by design, not production tested. Note 9: Specifications to -40°C are guaranteed by design, not production tested. 4 _______________________________________________________________________________________ High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter 60 40 20 60 VIN = 1.2V 40 VOUT = 3.3V VIN = 3.3V, VOUT = 5V, IOUT =1A 86 0 1 10 100 1000 10,000 0.1 10 1 100 1000 350 10,000 LINE REGULATION (VOUT = 5V) -0.5 1.0 0.5 0 -0.5 -1.0 -1.0 -1.5 -1.5 0.3 0.2 VOUT REGULATION (%) VOUT REGULATION (%) 1.5 10 100 0 -0.1 IOUT = 1A 1000 10,000 -0.3 0.1 1 10 1000 100 10,000 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) INPUT VOLTAGE (V) LINE REGULATION (VOUT = 3.3V) NO LOAD INPUT CURRENT vs. INPUT VOLTAGE NO LOAD INPUT CURRENT vs. INPUT VOLTAGE INPUT CURRENT (mA) 0.5 0.4 0.3 0.2 0.1 0 VOUT = 5V, VIN INCREASING 15 10 VOUT = 3.3V, VIN INCREASING 60 VOUT = 5V, VIN DECREASING 50 40 30 20 5 -0.1 1.75 2.00 2.25 2.50 INPUT VOLTAGE (V) 2.75 VOUT = 3.3V, VIN DECREASING 10 IOUT = 500mA PLOT NORMALIZED TO VIN = 2.5V 0 0 3.00 MAX1708 toc05b 20 INPUT CURRENT (mA) IOUT = 1A 70 MAX1708 toc05a 25 MAX1708 toc04b 0.8 1.50 IOUT = 500mA PLOT NORMALIZED TO VIN = 3.3V PLOT NORMALIZED TO IOUT = 500mA -2.0 1 0.1 -0.2 PLOT NORMALIZED TO IOUT = 500mA -2.0 950 MAX1708 toc04a 2.0 MAX1708 toc03a 0 -0.3 850 LOAD REGULATION (VIN = 2.5V, VOUT = 3.3V) 0.5 -0.2 750 LOAD REGULATION (VIN = 3.3V, VOUT = 5V) 1.0 0.6 650 SWITCHING FREQUENCY (kHz) 1.5 0.7 550 OUTPUT CURRENT (mA) 2.0 0.1 450 OUTPUT CURRENT (mA) MAX1708 toc03b 0.1 88 87 VOUT = 5V VOUT REGULATION (%) MAX1708 toc01b 89 20 0 VOUT REGULATION (%) EFFICIENCY vs. SWITCHING FREQUENCY 90 EFFICIENCY (%) VIN = 2.5V VIN = 2.5V 80 EFFICIENCY (%) EFFICIENCY (%) MAX1708 toc01a VIN = 3.3V 80 EFFICIENCY vs. OUTPUT CURRENT 100 MAX1708 toc02 EFFICIENCY vs. OUTPUT CURRENT 100 0 1 2 3 4 INPUT VOLTAGE (V) 5 6 0 1 2 3 4 5 6 INPUT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX1708 Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) STARTUP VOLTAGE vs. LOAD CURRENT (VOUT = 3.3V) 1.0 TA = +25°C 0.8 TA = +85°C 2.0 0.6 0.4 1.5 TA = -40°C TA = +25°C 1.0 TA = +85°C 0.5 PLOT NORMALIZED TO 25°C FREQUENCY CHANGE (%) 1.2 TA = -40°C STARTUP VOLTAGE (V) 1.4 2 MAX1708 toc06b 1.6 STARTUP VOLTAGE (V) 2.5 MAX1708 toc06a 1.8 SWITCHING FREQUENCY vs. TEMPERATURE 1 0 -1 0.2 VOUT = 3.3V 0 10 100 1000 LOAD CURRENT (mA) 10,000 -2 1 NOISE vs. FREQUENCY SWITCH CURRENT LIMIT (A) RESOLUTION = 1kHz 3000 2500 2000 1500 1000 1000 10,000 -40 -15 10 35 60 LOAD CURRENT (mA) TEMPERATURE (°C) SWITCH CURRENT LIMIT vs. SS/LIM RESISTANCE SWITCH CURRENT LIMIT vs. TEMPERATURE MAX1708 toc09 3500 100 5 MAX1708 toc08 4000 10 4 3 2 1 6.0 85 MAX1708 toc10 1 SWITCH CURRENT LIMIT (A) 0 5.5 5.0 4.5 500 VIN = 3.3V, VOUT = 5V 0 0 0.1 1 10 4.0 0 FREQUENCY (MHz) 50 100 150 200 250 300 -40 SS/LIM RESISTANCE (kΩ) 10 MAX1708 toc12 3.5V VIN 500mV/div 3V 5V 0 IL 4A 2A/div 2A VOUT 5V AC-COUPLED 50mV/div 0 VOUT 5V AC-COUPLED 50mV/div 1μs/div 35 TEMPERATURE (°C) MAX1708 toc11 VLX 5V/div -15 LINE TRANSIENT RESPONSE HEAVY SWITCHING WAVEFORM 6 MAX1708 toc07 STARTUP VOLTAGE vs. LOAD CURRENT (VOUT = 5V) NOISE (μV) MAX1708 High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter 100μs/div _______________________________________________________________________________________ 60 85 High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter STARTUP WITHOUT SOFT-START (CSS = 0) LOAD TRANSIENT RESPONSE MAX1708 toc14 MAX1708 toc13 5V VONA 5V/div 0 4A ISW 2A/div 2A 0 VOUT 5V AC-COUPLED 50mV/div 0 IIN 2A 1A/div 1A 2A 0 IOUT 1A 1A/div 0 VOUT 4V 2V/div 2V RL = 5Ω 2ms/div 40μs/div STARTUP WITH SOFT-START (CSS = 0.01μF) STARTUP WITH SOFT-START (CSS = 0.1μF) MAX1708 toc15 5V VONA 5V/div 0 5V VONA 5V/div 0 2A IIN 1A/div IIN 1A 1A/div 0 VOUT 2V/div 1A 0 4V 4V 2V MAX1708 toc16 VOUT 2V 2V/div 0 RL = 5Ω RL = 5Ω 0 2ms/div 2ms/div _______________________________________________________________________________________ 7 MAX1708 Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708 Pin Description 8 PIN NAME FUNCTION 1 ONB Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1). 2 ONA On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1). 3, 4, 5 LX 6, 9 GND Drain of N-Channel Power Switch. Connect pins 3, 4, and 5 together with wide traces. Connect an external Schottky diode from LX to V OUT. (Figure 1) Ground 7 SS/LIM Soft-Start and/or Current-Limit Input. Connect a capacitor from SS/LIM to GND to control the rate at which the device reaches current limit (soft-start). To reduce the current limit from the preset values, connect a resistor from SS/LIM to GND (see Design Procedure). During shutdown, SS/LIM is internally pulled to GND to discharge the soft-start capacitor. 8 REF Voltage Reference Output. Bypass with a 0.22μF capacitor to GND. Maximum REF load is 50μA. 10 OUT Output Voltage Sense Input. The device is powered from OUT. Bypass with a 0.1μF capacitor to PGND with less than 5mm trace length. Connect a 2
series resistor from the output filter capacitor (0.1μF) to OUT (Figure 1). 11 FB DC-DC Converter Feedback Input. Connect FB to GND for internally set output voltage (see 3.3/5 pin description). Connect a resistor-divider from the output to set the output voltage in the 2.5V to 5.5V range. FB regulates to 1.24V (Figure 4). 12, 13, 14 PGND 15 3.3/5 Output Voltage Selection Input. When FB is connected to GND, the regulator uses internal feedback to set the output voltage. 3.3/5 = low sets output to 3.3V; 3.3/5 = high sets output to 5V. If an external divider is used at FB, connect 3.3/5 to ground. 16 CLK Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Drive CLK with an external clock for external synchronization. Power Ground, Source of N-Channel Power MOSFET Switch. Connect pins 12, 13, and 14 together with wide traces. _______________________________________________________________________________________ High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter D1 VOUT +5V VIN KEEP TRACES SHORT AND WIDE C1 150μF GND MAX1708 L1 2.2μH LX LX LX R2 2Ω ONA ONB ON/OFF CONTROL C2 150μF GND CLK R1 3.3/5 C3 MAX1708 SS/LIM OUT C5 0.1μF C4 0.22μF FB REF GND KEEP TRACES SHORT AND WIDE GND PGND PGND PGND Figure 1. Standard Operating Circuit _______________Detailed Description The MAX1708 step-up converter offers high efficiency and high integration for high-power applications. It operates with an input voltage as low as 0.7V and is suitable for single- to 3-cell battery inputs, as well as 2.5V or 3.3V regulated supply inputs. The output voltage is preset to 3.3V or 5.0V or can be adjusted with external resistors for voltages between 2.5V to 5.5V. The MAX1708 internal N-channel MOSFET switch is rated for 5A (RMS value) and can deliver loads to 2A, depending on input and output voltages. For flexibility, the current limit and soft-start rate are independently programmable. A 600kHz switching frequency allows for a small inductor to be used. The switching frequency is also synchronizable to an external clock ranging from 350kHz to 1MHz. ONA, ONB The logic levels at ONA and ONB turn the MAX1708 on or off. When ONA = 1 or ONB = 0, the device is on. When ONA = 0 and ONB = 1, the device is off (Table 1). Logic high on-control can be implemented by connecting ONB high and using ONA for shutdown. Implement inverted single-line on/off control by grounding ONA and toggling ONB. Implement momentary pushbutton on/off as described in the Applications Information section. Both inputs have approximately 0.15V of hysteresis. Switching Frequency The MAX1708 switches at the fixed-frequency internal oscillator rate (600kHz) or can be synchronized to an external clock. Connect CLK to OUT for internal clock operation. Apply a clock signal to CLK to synchronize to an external clock. The MAX1708 will synchronize to a new external clock rate in two cycles and will take approximately 40μs to revert to its internal clock frequency once the external clock pulses stop and CLK is driven high. Table 2 summarizes oscillator operation. Operation The MAX1708 switches at a constant frequency (600kHz) and modulates the MOSFET switch pulse width to control the power transferred per cycle and regulate the voltage across the load. In low-noise applications, the fundamental and the harmonics generated by the fixed switching frequency are easily filtered out. Figure 2 shows the simplified functional diagram for the MAX1708. Figure 3 shows the simplified PWM con- _______________________________________________________________________________________ 9 MAX1708 High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter Table 1. On/Off Logic Control Table 2. Selecting Switching Frequency ONA 0 ONB 0 MAX1708 On 0 1 Off 1 0 On 1 1 On CLK 0 1 External clock (350kHz−1000kHz) MAX1708 IC POWER PWM CONTROLLER 2.15V EN ONA 1.26V DUAL MODE FB STARTUP Q OSCILLATOR D EN ON RDY REFERENCE EN CLK FB 3.3/5 GND Synchronized PWM UNDERVOLTAGE LOCKOUT OUT ONB REF MODE Not allowed PWM 600kHz OSCILLATOR LX OSC N PGND FB OUT Figure 2. Simplified Functional Diagram troller functional diagram. The MAX1708 enters synchronized current-mode PWM when a clock signal (350kHz < fCLK < 1MHz) is applied to CLK. For wireless or noise-sensitive applications, this ensures that switching harmonics are predictable and kept outside the IF frequency band(s). High-frequency operation permits low-magnitude output ripple voltage and minimum inductor and filter capacitor size. Switching losses will increase at higher frequencies (see MAX1708 IC Power Dissipation). Setting the Output Voltage ⎛V ⎞ R4 = R3 ⎜ OUT − 1⎟ ⎝ VFB ⎠ where VFB = 1.24V. Soft-Start/Current Limit Adjustment (SS/LIM) The soft-start pin allows the soft-start time to be adjusted by connecting a capacitor from SS/LIM to GND. Select capacitor C3 (see Figure 1): tSS = 4ms + [110 ✕ C3 (in μF)] where tSS is the time (in milliseconds) it takes output to reach its final value. The MAX1708 features Dual Mode™ operation. When FB is connected to ground, the MAX1708 generates a fixed output voltage of either 3.3V or 5V, depending on the logic applied to the 3.3/5 input (Figure 1). The output can be configured for other voltages, using two external resistors as shown in Figure 4. To set the output voltage externally, choose an R3 value that is large enough to minimize load at the output but small enough to minimize errors due to leakage and the time constant to FB. A value of R4 ≤ 50kΩ is required To improve efficiency or reduce inductor size at reduced load currents, the current limit can be reduced from its nominal value (see Electrical Characteristics). A resistor (R1 in Figure 1) between SS/LIM and ground reduces the current limit as follows: Dual Mode is a trademark of Maxim Integrated Products. where I1 is the desired current limit in amperes and R1 ≤ 312kΩ. ILIM = 5A, if R1 is omitted. 10 R1 = 312kΩ × I1 ILIM ______________________________________________________________________________________ High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter PRODUCTION Surface mount INDUCTORS CAPACITORS Coiltronics UP2B-2R2 Sanyo 6TPC100M Motorola MBRD1035CTL Coilcraft DO3316P-222HC Panasonic EEFUE0J151R Central CMSH5-20 Table 4. Component Suppliers SUPPLIER DIODES PHONE FAX Central 631-435-1110 631-435-1824 Coilcraft 847-639-6400 847-639-1489 Coiltronics 561-241-7876 561-241-9339 Motorola 602-303-5454 602-994-6430 Panasonic 714-373-7939 714-373-7183 __________________Design Procedure Inductor Selection (L1) The MAX1708’s high switching frequency allows the use of a small-size inductor. Use a 2.2μH inductor for 600kHz operation. If the MAX1708 is synchronized at a different frequency, scale the inductor value with the inverse of frequency (L1 = 2.2μH ✕ 600kHz / fSYNC). The PWM design tolerates inductor values within ±25% of this calculated value, so choose the closest standard inductor value. For example, use 3.3μH for 350kHz and 1.5μH for 1MHz). Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use at high switching frequencies. Ensure the inductor’s saturation rating (the current at which the core begins to saturate and inductance falls) exceeds the internal current limit. Note that this current may be reduced through SS/LIM if less than the MAX1708’s full load current is needed (see Electrical Characteristics for ratings). For highest efficiency, use a coil with low DC resistance, preferably under 20mΩ. To minimize radiated noise, use a toroid, pot core, or shielded inductor. See Tables 3 and 4 for a list of recommended components and component suppliers. To calculate the maximum output current (in amperes), use the following equation: ⎛ ⎛V + VD − VIN ⎞ ⎞ IOUT(MAX) = D' ⎜ ILIM − D' ⎜ OUT ⎟⎟ 2 × ƒ × L1 ⎝ ⎠⎠ ⎝ where: VIN = input voltage VD = forward voltage drop of the Schottky diode at ILIM VOUT = output voltage D' = (VIN) / (VOUT + VD), neglecting switch voltage drop f = switching frequency L1 = inductor value ILIM = minimum value of switch current limit from Electrical Characteristics or set by R1 of Figure 1. VIN FB REF SLOPE COMP LX R Q N VOUT LX S MAX1708 R4 SS/LIM 12.5 (LIMITED TO 100mV) FB 11mΩ PGND OSCILLATOR Figure 3. Simplified PWM Controller Functional Diagram KEEP SHORT R3 Figure 4. Adjustable Output Voltage ______________________________________________________________________________________ 11 MAX1708 Table 3. Component Selection Guide High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter MAX1708 MAX1708 IC Power Dissipation μC 270kΩ MAX1708 ON/OFF ONB VDD I/O ONA I/O The major components of MAX1708 dissipated power are switch conductance loss (PSW), capacitive loss (PCAP), and switch transition loss (PTRAN). Numerical examples provided in brackets ({ }) correspond to the following condition: {VIN = 3.3V, VOUT = 5V, VD = 0.5V, IOUT = 2A} An important parameter to compute the power dissipated in the MAX1708 is the approximate peak switch current (ISW): 270kΩ 0.1μF Figure 5. Momentary Pushbutton On-Off Switch Diode Selection (D1) The MAX1708’s high switching frequency demands a high-speed rectifier. Use Schottky diodes (Table 3). The diode’s current rating must exceed the maximum load current, and its breakdown voltage must exceed VOUT. The diode must be placed within 10mm of the LX switching node and the output filter capacitor. The diode also must be able to dissipate the power calculated by the following equation: PDIODE = IOUT ✕ VD where IOUT is the average load current and VD is the diode forward voltage at the peak switch current. Capacitor Selection Input Bypass Capacitor (C1) A 150μF, low-ESR input capacitor will reduce peak currents and reflected noise due to inductor current ripple. Lower ESR allows for lower input ripple current, but combined ESR values up to 100mΩ are acceptable. Smaller ceramic capacitors may also be used for light loads or in applications that can tolerate higher input current ripple. Output Filter Capacitor (C2) The output filter capacitor ESR must be kept under 30mΩ for stable operation. Polymer capacitors of 150μF (Panasonic EEFUE0J151R) typically exhibit 10mΩ of ESR. This translates to approximately 35mV of output ripple at 3.5A switch current. Bypass the MAX1708 IC supply input (OUT) with a 0.1μF ceramic capacitor to GND and a 2Ω series resistor (R2, as shown in Figure 1). 12 I ISW = OUT {3.33A } D' VIN {0.6} D' = VOUT + VD PD = PSW + PCAP + PTRAN {0.472W} PSW = (1 - D') ISW2 ✕ RSW {0.353W} PCAP = (CDIO + CDSW + CGSW) (VOUT + VD)2f {0.045W} PTRAN = (VOUT + VD) ISW ✕ tSW ✕ f / 3 {0.073W} where: RSW = switch resistance {80mΩ} CDIO = catch-diode capacitance {500pF} CDSW = switch drain capacitance {1250pF} CGSW = switch gate capacitance {750pF} f = switching frequency {600kHz} tSW = switch turn-on or turn-off time {20ns} Applications Information Using a Momentary On/Off Switch A momentary pushbutton switch can be used to turn the MAX1708 on and off. As shown in Figure 5, when ONA is pulled low and ONB is pulled high, the device 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 microcontroller to exit reset. The controller issues a logic high to ONA, which guarantees that the device will stay on regardless of the subsequent switch state. To turn the regulator off, depress the switch long enough for the controller to read the switch status and pull ONA low. When the switch is released, ONB pulls high and the regulator turns off. ______________________________________________________________________________________ High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter ___________________ Chip Information SUBSTRATE: GND PROCESS: BiCMOS Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO. 16 QSOP E16+1 21-0055 90-0167 ______________________________________________________________________________________ 13 MAX1708 Layout Considerations Due to high inductor current levels and fast switching waveforms, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Connect PGND, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). In addition, minimize trace lengths to reduce stray capacitance and trace resistance, especially from the LX pins to the catch diode (D1) and output capacitor (C2) to PGND pins. If an external resistor-divider is used to set the output voltage (Figure 4), the trace from FB to the resistors must be extremely short and must be shielded from switching signals, such as CLK or LX. To optimize package power dissipation and minimize device heating under heavy loads, expand PC trace area connected to the three PGND pins as much as the layout can allow. This is best accomplished with a large PGND plane on the surface of the board. Also note that outer-layer ground plane area beneath the device provides little heat-sinking benefit. If an outer-layer ground plane is not feasible, the PGND pins should be connected to the inner-layer ground plane with multiple vias (at least three vias per pin is recommended). Since the purpose of these vias is to optimize thermal conductivity to the inner ground plane, be sure that the vias have no gaps in their connections to the ground plane. Refer to a layout example in the MAX1708EVKIT data sheet. MAX1708 High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter Revision History REVISION NUMBER REVISION DATE 0 7/01 1 11/10 DESCRIPTION Initial release PAGES CHANGED — Updated the N-Channel Current Limit parameter in the Electrical Characteristics, corrected the equation in the Setting the Output Voltage section 3, 10 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. 14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.