19-2198; Rev 0; 10/01 KIT EVALUATION AVAILABLE 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches Applications Features ♦ ±1.5% Output Accuracy ♦ 94% Efficiency ♦ Internal PMOS/NMOS Switches 45mΩ/55mΩ On-Resistance at VIN = +4.5V 50mΩ/55mΩ On-Resistance at VIN = +3V ♦ Output Voltages +3.3V, +2.5V, or +1.5V Pin Selectable (MAX1831) +2.5V, +1.8V, or +1.5V Pin Selectable (MAX1830) +1.1V to VIN Adjustable (Both Devices) ♦ +3V to +5.5V Input Voltage Range ♦ 750µA (max) Operating Supply Current ♦ <1µA Shutdown Supply Current ♦ Programmable Constant-Off-Time Operation ♦ Up to 1MHz Switching Frequency ♦ Idle Mode Operation at Light Loads ♦ Thermal Shutdown ♦ Internal Digital Soft-Start Inrush Current Limiting ♦ 100% Duty Cycle During Low-Dropout Operation ♦ Output Short-Circuit Protection ♦ 16-Pin QSOP Package Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX1830EEE -40°C to +85°C 16 QSOP MAX1831EEE -40°C to +85°C 16 QSOP 5V or 3.3V to Low-Voltage Conversion CPU I/O Supplies Chipset Supplies Notebook and Subnotebook Computers Typical Configuration INPUT +3V TO +5.5V IN LX OUTPUT +1.1V TO VIN MAX1830 FB MAX1831 VCC PGND GND Pin Configuration appears at end of data sheet. SHDN COMP TOFF FBSEL REF Idle Mode is a trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1830/MAX1831 General Description The MAX1830/MAX1831 constant-off-time, pulse-widthmodulated (PWM) step-down DC-DC converters are ideal for use in 5V and 3.3V to low-voltage conversion necessary in notebook and subnotebook computers. These devices feature internal synchronous rectification for high efficiency and reduced component count. They require no external Schottky diode. The internal 45mΩ PMOS power switch and 55mΩ NMOS synchronous-rectifier switch easily deliver continuous load currents up to 3A. The MAX1830 produces preset +2.5V, +1.8V, or +1.5V output voltage or an adjustable output from +1.1V to VIN. The MAX1831 produces preset +3.3V, +2.5V, and +1.5V output voltages and an adjustable output from +1.1V to VIN. They achieve efficiencies as high as 94%. The MAX1830/MAX1831 use a unique current-mode, constant-off-time, PWM control scheme, which includes Idle Mode™ to maintain high efficiency during lightload operation. The programmable constant-off-time architecture sets switching frequencies up to 1MHz, allowing the user to optimize performance tradeoffs between efficiency, output switching noise, component size, and cost. Both devices are designed for continuous output currents up to 3A, an internal digital soft-start to limit surge currents during startup, a 100% duty cycle mode for low-dropout operation, and a low-power shutdown mode that disconnects the input from the output and reduces supply current below 1µA.The MAX1830/MAX1831 are available in 16-pin QSOP packages. For similar devices that provide continuous output currents of 1A to 3A, refer to the MAX1644, MAX1623, and MAX1742/MAX1842/MAX1843 data sheets. MAX1830/MAX1831 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches ABSOLUTE MAXIMUM RATINGS VCC, IN, SHDN to GND ............................................-0.3V to +6V IN to VCC .............................................................................±0.3V GND to PGND.....................................................................±0.3V COMP, FB, TOFF, FBSEL, REF to GND .....-0.3V to (VCC + 0.3V) LX Current (Note 1) ...............................................................5.1A REF Short Circuit to GND Duration ............................Continuous ESD Protection .....................................................................±2kV Continuous Power Dissipation (TA = +70°C) 16-Pin QSOP (derate 14mW/°C above +70°C; part mounted on 1in2 of 1oz copper) .............................1.12W Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range .............................-65°C to +150°C Junction Temperature ......................................................+150°C Lead Temperature (soldering, 10s) ................................ +300°C Note 1: LX has internal clamp diodes to PGND and IN. Applications that forward bias the diode should take care not to exceed the IC’s package power dissipation. 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 (VIN = VCC = +3.3V, FBSEL = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Input Voltage SYMBOL CONDITIONS VIN, VCC VOUT VIN = +3.7V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT MAX UNITS 5.5 V FBSEL = VCC 2.487 2.525 2.563 FBSEL = unconnected 1.492 1.515 1.538 FBSEL = REF (MAX1830) 1.791 1.818 1.845 FBSEL = GND 1.084 1.100 1.117 FBSEL = REF (MAX1831) 3.283 3.333 3.383 VCC = VIN = +3V to +5.5V, ILOAD = 0, FBSEL = GND Adjustable Output Voltage Range TYP 3.0 VIN = +3V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT Preset Output Voltage MIN VIN VREF AC Load Regulation Error 2 DC Load Regulation Error 0.4 Dropout Voltage VDO Reference Voltage VREF Reference Load Regulation ∆VREF Current-Limit Threshold ILIMIT Maximum Output RMS Current Idle Mode Current Threshold VCC = VIN = +3V, ILOAD = 3A ILX = 0.5A NMOS Switch On-Resistance RON, N ILX = 0.5A Switching Frequency No-Load Supply Current 2 f IIN + ICC 1.089 1.100 1.111 V 0.5 2 mV 4.0 4.8 5.4 A 3.4 A A 0.6 1.0 VIN = 4.5V 45 90 VIN = 3V 50 110 VIN = 4.5V 55 95 VIN = 3V 55 100 (Note 2) VFB = 1.2V % mV 0.2 RON, P % 330 IOUT(RMS) (Note 4) PMOS Switch On-Resistance V 150 IREF = -1µA to +10µA IIM V 325 _______________________________________________________________________________________ mΩ 1 MHz 750 µA 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches (VIN = VCC = +3.3V, FBSEL = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Shutdown Supply Current TYP MAX SHDN = GND, into VCC and IN pins; LX = 0 or 3.3V CONDITIONS MIN 0.2 20 SHDN = GND, into IN with LX = 0 0.2 20 20 SHDN = GND, into IN with LX = 3.3V 0.1 Thermal Shutdown Threshold TSHDN Hysteresis = 15°C 165 Undervoltage Lockout Threshold VUVLO VIN falling, hysteresis = 90mV FB Input Bias Current Off-Time IFB tOFF tON ISHDN SHDN Logic Input Low Voltage VIL SHDN Logic Input High Voltage VIH 2.6 2.8 V 70 300 nA RTOFF = 110kΩ 0.85 1.00 1.15 µs RTOFF = 44kΩ 0.3 0.4 0.5 µs RTOFF = 440kΩ 3.0 3.9 5.0 µs (Note 2) 4 ✕ tOFF µs 3 ✕ 256 cycles 0.40 SHDN = 0 or VCC µs -0.5 0.5 µA 0.8 V 5 µA 0.2 V 1.3 V 2.0 FBSEL Input Current V -5 FBSEL = GND FBSEL = REF FBSEL Logic Thresholds °C 0 Soft-Start Time (Note 3) SHDN Input Current µA 1.8 VFB = 1.2V Startup Off-Time Minimum On-Time UNITS 0.9 FBSEL = unconnected 0.7 ✕ VCC - 0.2 0.7 ✕ VCC + 0.2 V FBSEL = VCC VCC - 0.2 VCC + 0.2 V ELECTRICAL CHARACTERISTICS (VIN = VCC = +3.3V, FBSEL = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 5) PARAMETER Input Voltage SYMBOL CONDITIONS VIN, VCC VIN = +3V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT Preset Output Voltage VOUT VIN = +3.7V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT MIN TYP MAX UNITS V 3.0 5.5 FBSEL = VCC 2.475 2.575 FBSEL = unconnected 1.485 1.545 FBSEL = REF (MAX1830) 1.782 1.854 FBSEL = GND 1.078 1.122 FBSEL = REF (MAX1831) 3.267 3.399 V _______________________________________________________________________________________ 3 MAX1830/MAX1831 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (VIN = VCC = +3.3V, FBSEL = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 5) PARAMETER SYMBOL CONDITIONS MIN VCC = VIN = +3V to +5.5V, ILOAD = 0, FBSEL = GND Adjustable Output Voltage Range TYP MAX UNITS VREF VIN V Reference Voltage VREF 1.078 1.122 V Current-Limit Threshold ILIMIT 3.9 5.4 A Idle Mode Current Threshold IIM 0.14 1.0 A PMOS Switch On-Resistance RON, P ILX = 0.5A NMOS Switch On-Resistance RON, N ILX = 0.5A IIN + ICC VFB = 1.2V FB Input Bias Current IFB VFB = 1.2V Off-Time tOFF RTOFF = 110kΩ No-Load Supply Current VIN = 4.5V 90 VIN = +3V 110 VIN = 4.5V 95 VIN = +3V mΩ 100 750 µA 0 360 nA 0.8 1.2 µs Not production tested. Soft-start time is measured with respect to the number of cycles on LX. This is a metal migration limit. Maximum output current may be limited by thermal capability to a lower value than this. Specifications from 0°C to -40°C are guaranteed by design, not production tested. Note 2: Note 3: Note 4: Note 5: Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) VOUT = 3.3V RTOFF = 39kΩ 80 VOUT = 1.8V RTOFF = 75kΩ 75 70 VOUT = 1.5V RTOFF = 82kΩ 65 85 80 75 70 65 60 60 55 55 50 0.001 50 0.001 0.01 0.1 OUTPUT CURRENT (A) 4 90 EFFICIENCY (%) 85 95 1 10 100 95 90 EFFICIENCY (%) 90 100 EFFICIENCY vs. OUTPUT CURRENT (fPWM = 300kHz) MAX1830 toc02 95 MAX1830 toc01 100 EFFICIENCY vs. OUTPUT CURRENT (VIN = 3.3V, L = 1.5µH) VOUT = 1.5V VOUT = 2.5V RTOFF = 62kΩ RTOFF = 39kΩ 85 VOUT = 1.8V VIN = 3.3V RTOFF = 160kΩ L = 2.5µH 80 75 70 65 VOUT = 1.8V RTOFF = 51kΩ MAX1830 toc03 EFFICIENCY vs. OUTPUT CURRENT (VIN = 5.0V, L = 1.5µH) EFFICIENCY (%) MAX1830/MAX1831 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches 60 VOUT = 1.8V VIN = 5.0V RTOFF = 220kΩ L = 5.2µH 55 0.01 0.1 OUTPUT CURRENT (A) 1 10 50 0.001 0.01 0.1 OUTPUT CURRENT (A) _______________________________________________________________________________________ 1 10 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches SWITCHING FREQUENCY vs. OUTPUT CURRENT NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT 0 -0.05 -0.10 -0.15 VIN = 3.3V VOUT = 1.5V L = 1.5µH VIN = 5.0V VOUT = 1.5V L = 1.5µH VIN = 5.0V, VOUT = 1.5V, L = 1.5µH 1000 IIN 1A/div 800 VSHDN 5V/div 600 VIN = 3.3V, VOUT = 1.5V, L = 1.5µH 400 VOUT 1V/div 200 -0.20 -0.25 0.001 MAX1830 toc05 MAX1830 toc04 0.05 STARTUP AND SHUTDOWN MAX1830 toc06 1200 FREQUENCY (kHz) NORMALIZED OUTPUT ERROR (%) 0.10 0 0.01 0.1 1 0 10 0.5 OUTPUT CURRENT (A) 1.0 1.5 2.0 OUTPUT CURRENT (A) 2.5 3.0 400µs/div VIN = 3.3V, VOUT = 1.5V, ROUT = 0.5Ω, RTOFF = 82kΩ, L = 1.5µH LINE-TRANSIENT RESPONSE LOAD-TRANSIENT RESPONSE MAX1830 toc08 MAX1830 toc07 VIN 2V/div VOUT 50mV/div AC-COUPLE VOUT 50mV/div AC-COUPLED IOUT 2A/div 0 10µs/div VIN = 3.3V, VOUT = 1.5V, RTOFF = 82kΩ, L = 1.5µH, IOUT = 0.1A TO 3A 20µs/div VOUT = 1.8V, IOUT = 1A, RTOFF = 75kΩ, L = 1.5µH _______________________________________________________________________________________ 5 MAX1830/MAX1831 Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831 Pin Description PIN NAME FUNCTION 1, 3, 14, 16 LX Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch. Connect the inductor from this node to the output filter capacitor and load. 2, 4 IN Supply Voltage Input for the internal PMOS power switch 5 SHDN Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal MOSFETs. Drive high or connect to VCC for normal operation. 6 COMP Integrator Compensation. Connect a capacitor from COMP to VCC for integrator compensation. See Integrator Amplifier section. 7 TOFF Off-Time Select Input. Sets the PMOS power switch off-time during constant-off-time operation. Connect a resistor from TOFF to GND to adjust the PMOS switch off-time. 8 FB 9 GND Analog Ground 10 REF Reference Output. Bypass REF to GND with a 1µF capacitor. 11 FBSEL 12 VCC 13, 15 PGND Feedback Input for both preset-output and adjustable-output operating modes. Connect directly to output for fixed-voltage operation or to a resistive divider for adjustable operating modes. Feedback Select Input. Selects output voltage. See Table 3 for programming instructions. Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCC with a 10Ω and 2.2µF lowpass filter (Figure 1). Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch. _______________Detailed Description The MAX1830/MAX1831 synchronous, current-mode, constant-off-time, PWM DC-DC converters step down input voltages of +3V to +5.5V to preset output voltages, or to an adjustable output voltage from +1.1V to VIN. The MAX1830 has preset outputs +2.5V, +1.8V, and +1.5V. The MAX1831 has preset outputs of +3.3V, +2.5V or +1.5V. Both devices deliver up to 3A of continuous output current. Internal switches composed of a 45mΩ PMOS power switch and a 55mΩ NMOS synchronous rectifier switch improve efficiency, reduce component count, and eliminate the need for an external Schottky diode. The MAX1830/MAX1831 optimize efficiency by operating in constant-off-time mode under heavy loads and in Maxim’s proprietary Idle Mode under light loads. A single resistor-programmable constant-off-time control sets switching frequencies up to 1MHz, allowing the user to optimize performance trade-offs in efficiency, switching noise, component size, and cost. Under lowdropout conditions, the device operates in a 100% duty-cycle mode, where the PMOS switch remains continuously on. Idle Mode enhances light-load efficiency by skipping cycles, thus reducing transition and gatecharge losses. 6 When power is drawn from a regulated supply, constantoff-time PWM architecture essentially provides constantfrequency operation. This architecture has the inherent advantage of quick response to line and load transients. The MAX1830/MAX1831 current-mode, constant-offtime PWM architecture regulates the output voltage by changing the PMOS switch on-time relative to the constant off-time. Increasing the on-time increases the peak inductor current and the amount of energy transferred to the load per pulse. Modes of Operation The current through the PMOS switch determines the mode of operation: constant-off-time mode (for load currents greater than half the Idle Mode threshold), or Idle Mode (for load currents less than half the Idle Mode threshold). Current sense is achieved through a proprietary architecture that eliminates current-sensing I2R losses. Constant-Off-Time Mode Constant-off-time operation occurs when the current through the PMOS switch is greater than the Idle Mode threshold current (which corresponds to a load current of half the Idle Mode threshold). In this mode, the regu- _______________________________________________________________________________________ 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches IN CIN 22µF, 6.3V X5R 10Ω 2.2µF LX MAX1830 FB MAX1831 VCC PGND 470pF SHDN COMP MAX1830/MAX1831 L 1.5µF INPUT OUTPUT SUMIDA CDRH-6D28 COUT 120µF, 4V Panasonic SP GND FBSEL REF 1µF TOFF MAX1830 VOUT = 2.5V, FBSEL = VOUT = 1.8V, FBSEL = VOUT = 1.5V, FBSEL = VCC REF FLOATING MAX1831 VOUT = 2.5V, FBSEL = VOUT = 3.3V, FBSEL = VOUT = 1.5V, FBSEL = VCC REF FLOATING RTOFF Figure 1. Typical Circuit FBSEL FB DIGITAL SOFT-START FEEDBACK SELECTION MAX1830 MAX1831 COMP REF 470pF VIN 10Ω VCC Gm CIN CERAMIC CURRENT SENSE SKIP REF 2.2µF PWM LOGIC AND DRIVERS SUMMING COMPARATOR VOUT LX COUT SHDN REF VIN 3.0V TO 5.5V IN REF TIMER 1µF GND TOFF CURRENT SENSE PGND RTOFF NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS. Figure 2. Functional Diagram lation comparator turns the PMOS switch on at the end of each off-time, keeping the device in continuous-conduction mode. The PMOS switch remains on until the output is in regulation or the current limit is reached. When the PMOS switch turns off, it remains off for the programmed off-time (t OFF ). To control the current under short-circuit conditions, the PMOS switch remains off for approximately 4 x tOFF when VOUT < VOUT(NOM) / 4. Idle Mode Under light loads, the devices improve efficiency by switching to a pulse-skipping Idle Mode. Idle Mode operation occurs when the current through the PMOS switch is less than the Idle Mode threshold current. Idle Mode forces the PMOS to remain on until the current through the switch reaches the Idle Mode threshold, thus minimizing the unnecessary switching that degrades efficiency under light loads. In Idle Mode, the _______________________________________________________________________________________ 7 Table 1. Recommended Component Values (IOUT = 3.0A) VIN (V) VOUT (V) fPWM (kHz) L (µH) RTOFF (kΩ) 5 3.3 800 2.2 39 5 2.5 865 2.2 56 5 1.8 850 2.2 75 5 1.5 860 2.2 82 5 1.1 625 2.2 130 3.3 2.5 570 1.5 39 3.3 1.8 850 1.5 51 3.3 1.5 860 1.5 62 3.3 1.1 680 1.5 100 100% Duty-Cycle Operation When the input voltage drops near the output voltage, the duty cycle increases until the PMOS MOSFET is on continuously. The dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal PMOS switch and parasitic resistance in the inductor. The PMOS switch remains on continuously as long as the current limit is not reached. Internal Digital Soft-Start Circuit Soft-start allows a gradual increase of the current-limit level at startup to reduce input-surge currents. The MAX1830/MAX1831 contain internal digital soft-start circuits, controlled by a counter, a digital-to-analog converter (DAC), and the current-limit comparator. At power-on or in shutdown mode, the soft-start counter is reset to zero. When the MAX1830/MAX1831 are enabled or powered up, its counter starts counting LX switching cycles, and the DAC begins incrementing the comparison voltage applied to the current-limit comparator. The DAC ramps up the internal current limit in four 25% steps, as the count increases to 256 cycles. As a result, the main output capacitor charges up relatively slowly. The exact time of the output rise depends on nominal switching frequency, output capacitance, and the load current, and is typically 1ms. Shutdown Drive SHDN to a logic-level low to place the MAX1830/MAX1831 in low-power shutdown mode and 8 MAXIMUM RECOMMENDED OPERATING FREQUENCY vs. INPUT VOLTAGE 1400 MAX1830/MAX1831 device operates in discontinuous conduction. Currentsense circuitry monitors the current through the NMOS synchronous switch, turning it off before the current reverses. This prevents current from being pulled from the output filter through the inductor and NMOS switch to ground. As the device switches between operating modes, no major shift in circuit behavior occurs. VOUT = 1.5V 1200 OPERATING FREQUENCY (kHz) MAX1830/MAX1831 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches 1000 VOUT = 1.8V 800 VOUT = 2.5V 600 400 VOUT = 3.3V 200 0 2.6 3.1 3.6 4.1 4.6 5.1 5.6 VIN (V) Figure 3. Maximum Recommended Operating Frequency vs. Input Voltage reduce supply current to less than 1µA. In shutdown, all circuitry and internal MOSFETs turn off, and the LX node becomes high impedance. Drive SHDN to a logic-level high or connect to VCC for normal operation. Summing Comparator Three signals are added together at the input of the summing comparator (Figure 2): an output-voltage error signal relative to the reference voltage, an integrated output-voltage error correction signal, and the sensed PMOS switch current. The integrated error signal is provided by a transconductance amplifier with an external capacitor at COMP. This integrator provides high DC accuracy without the need for a high-gain amplifier. Connecting a capacitor at COMP modifies the overall loop response (see Integrator Amplifier). _______________________________________________________________________________________ 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches PIN OUTPUT VOLTAGE (V) FBSEL FB MAX1830 MAX1831 VCC Output voltage 2.5 2.5 Unconnected Output voltage 1.5 1.5 REF Output voltage 1.8 3.3 GND Resistive divider Adjustable VOUT LX MAX1830 MAX1831 R2 FB R1 R1 = 30kΩ R2 = R1(VOUT / VREF - 1) VREF = 1.1V Figure 4. Adjustable Output Voltage Synchronous Rectification In a step-down regulator without synchronous rectification, an external Schottky diode provides a path for current to flow when the inductor is discharging. Replacing the Schottky diode with a low-resistance NMOS synchronous switch reduces conduction losses and improves efficiency. The NMOS synchronous-rectifier switch turns on following a short delay after the PMOS power switch turns off, thus preventing cross conduction or “shoot through.” In constant-off-time mode, the synchronous-rectifier switch turns off just prior to the PMOS power switch turning on. While both switches are off, inductor current flows through the internal-body diode of the NMOS switch. The internal-body diode’s forward voltage is relatively high. An external Schottky diode from PGND to LX can improve efficiency. Thermal Resistance Junction-to-ambient thermal resistance, θJA, is highly dependent on the amount of copper area immediately surrounding the IC leads. The MAX1830/MAX1831 evaluation kit has 0.7in2 of copper area and a thermal resistance of +71°C/W with no forced airflow. Airflow over the board significantly reduces the junction-toambient thermal resistance. For heatsinking purposes, evenly distribute the copper area connected at the IC among the high-current pins. Power Dissipation Power dissipation in the MAX1830/MAX1831 is dominated by conduction losses in the two internal power switches. Power dissipation due to supply current in the control section and average current used to charge and discharge the gate capacitance of the internal switches (i.e., switching losses) is approximately: PDS = C x VIN2 x fPWM where C = 5nF and f PWM is the switching frequency in PWM mode. This number is reduced when the switching frequency decreases as the part enters Idle Mode. Combined conduction losses in the two power switches are approximated by: PD = IOUT2 x RPMOS where RPMOS is the on-resistance of the PMOS switch. The junction-to-ambient thermal resistance required to dissipate this amount of power is calculated by: θJA = (TJ,MAX - TA,MAX) / PD(TOT) where: θJA = junction-to-ambient thermal resistance TJ,MAX = maximum junction temperature TA,MAX = maximum ambient temperature PD(TOT) = total losses Design Procedure For typical applications, use the recommended component values in Table 1. For other applications, take the following steps: 1) Select the desired PWM-mode switching frequency; 1MHz is a good starting point. See Figure 3 for maximum operating frequency. 2) Select the constant off-time as a function of input voltage, output voltage, and switching frequency. 3) Select RTOFF as a function of off-time. 4) Select the inductor as a function of output voltage, off-time, and peak-to-peak inductor current. Setting the Output Voltage The output of the MAX1830/MAX1831 is selectable between one of three preset output voltages. For a preset output voltage, connect FB to the output voltage and connect FBSEL as indicated in Table 2. For an adjustable output voltage, connect FBSEL to GND and connect FB to a resistive divider between the output _______________________________________________________________________________________ 9 MAX1830/MAX1831 Table 2. Output Voltage Programming MAX1830/MAX1831 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches voltage and ground (Figure 4). Regulation is maintained for adjustable output voltages when VFB = VREF. Use 30kΩ for R1. R2 is given by the equation: V R2 = R1 OUT − 1 VREF where VREF is typically 1.1V. Programming the Switching Frequency and Off-Time The MAX1830/MAX1831 feature a programmable PWM mode-switching frequency, which is set by the input and output voltage and the value of RTOFF, connected from TOFF to GND. RTOFF sets the PMOS power switch off-time in PWM mode. Use the following equation to select the off-time according to your desired switching frequency in PWM mode: t OFF = (VIN – VOUT − VPMOS ) fPWM ( VIN − VPMOS + VNMOS ) where: tOFF = the programmed off-time VIN = the input voltage VOUT = the output voltage VPMOS = the voltage drop across the internal PMOS power switch VNMOS = the voltage drop across the internal NMOS synchronous-rectifier switch fPWM = switching frequency in PWM mode Select RTOFF according to the formula: RTOFF = (tOFF - 0.07µs) (110kΩ / 1.00µs) Recommended values for RTOFF range from 36kΩ to 430kΩ for off-times of 0.4µs to 4µs. Inductor Selection The key inductor parameters must be specified: inductor value (L) and peak current (IPEAK). The following equation includes a constant, denoted as LIR, which is the ratio of peak-to-peak inductor AC current (ripple current) to maximum DC load current. A higher value of LIR allows smaller inductance but results in higher losses and ripple. A good compromise between size and losses is found at approximately a 25% ripple-current to load-current ratio (LIR = 0.25), which corresponds to a peak-inductor current 1.125 times the DC load current: 10 L = VOUT × t OFF IOUT × LIR where: IOUT = maximum DC load current LIR = ratio of peak-to-peak AC inductor current to DC load current, typically 0.25 The peak-inductor current at full load is 1.125 x IOUT if the above equation is used; otherwise, the peak current is calculated by: IPEAK = IOUT + VOUT × t OFF 2 × L Choose an inductor with a saturation current at least as high as the peak-inductor current. The inductor you select should exhibit low losses at your chosen operating frequency. Capacitor Selection The input-filter capacitor reduces peak currents and noise at the voltage source. Use a low-ESR and lowESL capacitor located no further than 5mm from IN. Select the input capacitor according to the RMS input ripple-current requirements and voltage rating: IRIPPLE = ILOAD ( VOUT VIN − VOUT ) VIN where IRIPPLE = input RMS current ripple. The output-filter capacitor affects the output-voltage ripple, output load-transient response, and feedback-loop stability. For stable operation, the MAX1830/MAX1831 require a minimum output ripple voltage of VRIPPLE ≥ 1% ✕ VOUT. The minimum ESR of the output capacitor should be: ESR > 1% × L t OFF Stable operation requires the correct output-filter capacitor. When choosing the output capacitor, ensure that: t COUT ≥ OFF 79µFV / µs VOUT ______________________________________________________________________________________ 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831 MAX1830/MAX1831 MAXIMUM RECOMMENDED CONTINUOUS OUTPUT CURRENT vs. TEMPERATURE MAXIMUM RECOMMENDED BURST CURRENT vs. BURST CURRENT DUTY CYCLE 3.50 3.40 3.40 3.20 BURST CURRENT (A) OUTPUT CURRENT (A) 3.30 3.10 3.00 2.90 2.80 TA = +55°C 3.20 TA = +85°C 3.00 2.80 2.70 2.60 2.60 2.50 IOUT IS A 100Hz SQUARE WAVE FROM 1A TO THE BURST CURRENT 0.7IN2 OF 1-OZ COPPER 2.40 2.40 25 35 45 55 65 75 85 0 Figure 5. Maximum Recommended Continuous Output Current vs. Temperature High-Current Thermal Considerations High ambient temperatures can limit the maximum current or duty factor of the output current, depending on the total copper, are connected to the MAX1830/ MAX1831 and available airflow. Figure 5 shows the maximum recommended continuous output current vs. ambient temperature. Figure 6 shows the maximum recommended output current vs. the output current duty cycle at high temperatures. These figures are based on 0.7in2 of 1oz copper in free air. Figure 6 assumes that the output current is a square wave with a 100Hz frequency. The duty cycle is defined as the duration of the burst current divided by the period of the square wave. This figure shows the limitations for continuous bursts of output current. Note that if the thermal limitations of the MAX1830/ MAX1831 are exceeded, it enters thermal shutdown to prevent destructive failure. 40 60 80 100 Figure 6. Maximum Recommended Burst Current vs. Burst Current Duty Cycle Integrator Amplifier An internal transconductance amplifier fine tunes the output DC accuracy. A capacitor, CCOMP, from COMP to VCC compensates the transconductance amplifier. For stability, choose CCOMP = 470pF. A large capacitor value maintains a constant average output voltage but slows the loop response to changes in output voltage. A small capacitor value speeds up the loop response to changes in output voltage but decreases stability. 20 DUTY CYCLE (%) TEMPERATURE (°C) Frequency Variation with Output Current The operating frequency of the MAX1830/MAX1831 is determined primarily by tOFF (set by RTOFF), VIN, and VOUT as shown in the following formula: fPWM = (VIN − VOUT − VPMOS ) [tOFF (VIN − VPMOS + VNMOS )] However, as the output current increases, the voltage drop across the NMOS and PMOS switches increases and the voltage across the inductor decreases. This causes the frequency to drop. The change in frequency can be approximated with the following formula: ∆fPWM = -IOUT x RPMOS / (VIN x tOFF) where RPMOS is the resistance of the internal MOSFETs (50mΩ typ). Circuit Layout and Grounding Good layout is necessary to achieve the MAX1830/ MAX1831s’ intended output power level, high efficiency, and low noise. Good layout includes the use of a ground plane, careful component placement, and correct routing of traces using appropriate trace widths. ______________________________________________________________________________________ 11 MAX1830/MAX1831 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches The following points are in order of decreasing importance: 1) Minimize switched-current and high-current ground loops. Connect the input capacitor’s ground, the output capacitor’s ground, and PGND. Connect the resulting island to GND at only one point. 2) Connect the input filter capacitor less than 5mm away from IN. The connecting copper trace carries large currents and must be at least 1mm wide, preferably 2.5mm. 3) Place the LX node components as close together and as near to the device as possible. This reduces resistive and switching losses as well as noise. 4) A ground plane is essential for optimum performance. In most applications, the circuit is located on a multilayer board, and full use of the four or more layers is recommended. Use the top and bottom layers for interconnections and the inner layers for an uninterrupted ground plane. Avoid large AC currents through the ground plane. Pin Configuration ___________________Chip Information TRANSISTOR COUNT: 3662 TOP VIEW LX 1 16 LX IN 2 15 PGND LX 3 14 LX IN 4 SHDN 5 MAX1830 MAX1831 COMP 6 13 PGND 12 VCC 11 FBSEL 10 REF TOFF 7 9 FB 8 GND QSOP 12 ______________________________________________________________________________________ 3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches QSOP.EPS 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. 13 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1830/MAX1831 Package Information