KIT EVALUATION E BL LA AVAI 19-1986; Rev 1; 3/02 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package The MAX1843 uses a unique current-mode, constant-offtime, PWM control scheme, which includes Idle Mode™ to maintain high efficiency during light-load operation. The programmable constant-off-time architecture sets switching frequencies up to 1MHz, allowing the user to optimize performance trade-offs between efficiency, output switching noise, component size, and cost. The MAX1843 features an adjustable 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 MAX1843 is available in a 28pin QFN package with an exposed backside pad. Features ♦ ±1% Output Accuracy ♦ Up to 1MHz Switching Frequency ♦ 95% Efficiency ♦ Internal PMOS/NMOS Switches 90mΩ/70mΩ On-Resistance at VIN = +4.5V 110mΩ/80mΩ On-Resistance at VIN = +3V ♦ Output Voltage +2.5V, +1.8V, or +1.5V Pin Selectable +1.1V to VIN Adjustable ♦ +3V to +5.5V Input Voltage Range ♦ 350µA Operating Supply Current ♦ <1µA Shutdown Supply Current ♦ Programmable Constant-Off-Time Operation ♦ Idle Mode Operation at Light Loads ♦ Thermal Shutdown ♦ Adjustable Soft-Start Inrush Current Limiting ♦ 100% Duty Cycle During Low-Dropout Operation ♦ Output Short-Circuit Protection ♦ 28-Pin QFN Package Applications 5V or 3.3V to Low-Voltage Conversion CPU I/O Ring Chipset Supplies Notebook and Subnotebook Computers OUTPUT +1.1V TO VIN PIN-PACKAGE 28 QFN N.C. SHDN N.C. LX N.C. LX N.C. TOP VIEW 28 27 26 25 24 23 22 21 PGND MAX1843 FB IN 2 20 PGND PGND LX 3 19 LX LX VCC 2.2µF TEMP RANGE -40°C to +85°C N.C. 1 IN 10Ω PART MAX1843EGI Pin Configuration Typical Configuration INPUT +3V TO +5.5V Ordering Information 470pF GND SHDN COMP TOFF IN 4 18 LX MAX1843 FBSEL N.C. 5 REF 1µF SS 17 PGND SS 6 16 VCC 0.01µF COMP 7 Idle Mode is a trademark of Maxim Integrated Products. N.C. 12 13 14 REF FB 11 GND 10 N.C. 9 N.C. 8 TOFF 15 FBSEL QFN ________________________________________________________________ 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 MAX1843 General Description The MAX1843 constant-off-time, pulse-width modulated (PWM) step-down DC-DC converter is ideal for use in 5V and 3.3V to low-voltage conversion necessary in notebook and subnotebook computers. This device features an internal PMOS power switch and internal synchronous rectification for high efficiency and reduced component count. An external Schottky diode is not required. The internal 90mΩ power switch and 70mΩ NMOS synchronous-rectifier switch easily deliver continuous load currents up to 2.7A. The MAX1843 produces a preset +2.5V, +1.8V, or +1.5V output voltage or an adjustable output from +1.1V to VIN. It achieves efficiencies as high as 95%. MAX1843 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package ABSOLUTE MAXIMUM RATINGS VCC, IN to GND ........................................................-0.3V to +6V Continuous Power Dissipation (TA = +70°C) IN to VCC .............................................................................±0.3V 28-Pin QFN (derate 20mW/°C above +70°C, part mounted GND to PGND.....................................................................±0.3V on 1in2 of 1oz copper)......................................................1.6W Operating Temperature Range ...........................-40°C to +85°C All Other Pins to GND.................................-0.3V to (VCC + 0.3V) LX Current (Note 1).............................................................±4.7A Storage Temperature Range .............................-65°C to +150°C REF Short Circuit to GND Duration ............................Continuous Junction Temperature ......................................................+150°C ESD Protection .....................................................................±2kV Lead Temperature (soldering, 10s) ................................ +300°C Note 1: LX has internal clamp diodes to PGND and IN. Applications that forward bias these diodes should take care not to exceed the IC’s package power dissipation limits. 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 VIN = +3V to +5.5V FBSEL = unconnected VOUT TYP 3.0 FBSEL = VCC Preset Output Voltage MIN MAX UNITS 5.5 V TA = +25°C to +85°C 2.500 2.525 2.550 TA = 0°C to +85°C 2.487 2.525 2.563 TA = +25°C to +85°C 1.500 1.515 1.530 TA = 0°C to +85°C 1.492 1.515 1.538 TA = +25°C to +85°C 1.800 1.818 1.836 TA = 0°C to +85°C 1.791 1.818 1.845 TA = +25°C to +85°C 1.089 1.100 1.111 TA = 0°C to +85°C 1.084 1.100 1.117 V ILOAD = 0 to 2.5A FBSEL = REF VFB = VOUT FBSEL = GND Adjustable Output Voltage Range VIN = VCC = +3V to +5.5V, FBSEL = GND VREF VIN AC Load Regulation Error 2 % DC Load Regulation Error 0.4 % Dropout Voltage VDO Reference Voltage VREF VIN = VCC = +3V, ILOAD = 1A 250 mV TA = +25°C to +85°C 1.089 1.100 1.111 TA = 0°C to +85°C 1.084 1.100 1.117 0.5 2 VIN = +4.5V 90 200 VIN = +3V 110 250 VIN = +4.5V 70 150 VIN = +3V 80 200 3.6 4.1 A 3.1 A Reference Load Regulation ∆VREF IREF = -1µA to +10µA PMOS Switch On-Resistance RON,P ILX = 0.5A NMOS Switch On-Resistance RON,N ILX = 0.5A Current-Limit Threshold ILIMIT 3.1 RMS LX Output Current 2 V _______________________________________________________________________________________ V mV mΩ mΩ 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package (VIN = VCC = +3.3V, FBSEL = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS Idle-Mode Current Threshold Switching Frequency No-Load Supply Current Shutdown Supply Current Thermal Shutdown Threshold Undervoltage Lockout f 0.6 0.9 A 1 MHz 350 600 µA SHDN = GND, includes PMOS leakage <1 15 µA TSHDN Hysteresis = 15°C VUVLO VIN falling, hysteresis = 90mV 160 2.6 2.7 V 0 60 250 nA RTOFF = 110kΩ 0.9 1.00 1.1 RTOFF = 30.1kΩ 0.24 0.30 0.37 RTOFF = 499kΩ 3.8 4.5 5.2 FB = GND tON SS Source Current ISS ISS ISHDN SHDN Logic Input Low Voltage VIL SHDN Logic Input High Voltage VIH IFB µs 4 tOFF (Note 2) µs 0.4 µs 4 VSS = 1V 5 6 µA 1 µA 0.8 V 4 µA 100 VSHDN = 0 to VCC µA -1 2.0 VFBSEL = 0 to VCC V -4 FBSEL = GND 0.2 FBSEL = REF FBSEL Logic Thresholds °C 2.5 VFB = 1.2V On-Time FBSEL Input Current 0.3 (Note 2) UNITS VFB = 1.2V Off-Time Startup Period SHDN Input Current MAX IIN + ICC tOFF SS Sink Current TYP IIN + ICC FB Input Bias Current Off-Time MIN 0.9 1.3 0.7VCC - 0.2 FBSEL = unconnected 0.7VCC + 0.2 V 3.1 ARMS VCC - 0.2 FBSEL = VCC Maximum Output RMS Current ELECTRICAL CHARACTERISTICS (VIN = VCC = +3.3V, FBSEL = GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 3) PARAMETER Input Voltage SYMBOL CONDITIONS VIN, VCC Preset Output Voltage VOUT Adjustable Output Voltage Range VIN = +3V to +5.5V, ILOAD = 0 to 2.5A, VFB = VOUT VREF PMOS Switch On-Resistance RON,P ILX = 0.5A MAX UNITS V 3.0 5.5 FBSEL = VCC 2.475 2.756 FBSEL = unconnected 1.485 1.545 FBSEL = REF 1.782 1.854 FBSEL = GND 1.078 1.122 VREF VIN V 1.078 1.122 V VIN = VCC = +3V to +5.5V, FBSEL = GND Reference Voltage MIN VIN = +4.5V 200 VIN = +3V 250 V mΩ _______________________________________________________________________________________ 3 MAX1843 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (VIN = VCC = +3.3V, FBSEL = GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 3) PARAMETER SYMBOL NMOS Switch On-Resistance RON,N Current-Limit Threshold ILIMIT CONDITIONS ILX = 0.5A MIN No-Load Supply Current IIN + ICC VFB = 1.2V FB Input Bias Current IFB VFB = 1.2V Off-Time tOFF RTOFF = 110kΩ MAX VIN = +4.5V 150 VIN = +3V 200 Idle-Mode Current Threshold UNITS mΩ 2.9 4.3 A 0.2 1.0 A 600 µA 0 300 nA 0.85 1.15 µs Note 2: Recommended operating frequency, not production tested. Note 3: Specifications from 0°C to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) EFFICIENCY vs. OUTPUT CURRENT (VIN = +3.3V, L = 1.5µH) EFFICIENCY vs. OUTPUT CURRENT (VIN = +5.0V, L = 2.5µH) 80 75 VOUT = +1.8V, RTOFF = 75kΩ, fPWM = 910kHz 65 60 50 0.001 0.01 0.1 1 MAX1843 toc04 VOUT = +2.5V, RTOFF = 39kΩ, fPWM = 610kHz 90 85 80 75 VOUT = +1.8V, RTOFF = 43kΩ, fPWM = 1050kHz 70 65 60 VOUT = +1.5V, RTOFF = 1OOkΩ, fPWM = 770kHz 55 VOUT = +2.5V, RTOFF = 56kΩ, fPWM = 1000kHz 55 50 0.001 10 0.01 0.1 1 OUTPUT CURRENT (A) OUTPUT CURRENT (A) EFFICIENCY vs. OUTPUT CURRENT (fPWM = 270kHz) NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT 100 95 95 EFFICIENCY (%) 85 VIN = +5V, VOUT = +1.8V, L = 5.6µH, RTOFF = 240kΩ 90 MAX1843 toc05 EFFICIENCY (%) 90 70 100 85 80 75 VIN = +3.3V, VOUT = +1.8V, L = 4.7µH, RTOFF = 160kΩ 70 65 60 0.1 VIN = +3.3V, VOUT = +1.5V, L = 1.5µH 0 10 MAX1843 toc06 VOUT = +2.5V, RTOFF = 47kΩ, fPWM = 1070kHz NORMALIZED OUTPUT ERROR (%) 95 MAX1843 toc03 100 EFFICIENCY (%) MAX1843 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package -0.1 -0.2 VIN = +5V, VOUT = +1.5V, L = 2.5µH -0.3 55 50 0.001 0.01 0.1 IOUT (A) 4 1 10 -0.4 0.001 0.01 0.1 1 OUTPUT CURRENT (A) _______________________________________________________________________________________ 10 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package SUPPLY CURRENT vs. SUPPLY VOLTAGE 80 350 70 300 60 250 50 200 40 150 30 100 20 SHUTDOWN 50 10 0 1 2 3 4 5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 6 50 100 150 200 250 300 350 400 450 500 VIN (V) RTOFF (kΩ) SWITCHING FREQUENCY vs. OUTPUT CURRENT STARTUP AND SHUTDOWN MAX1843 toc08 MAX1843 toc07 1200 VIN = +5V, VOUT = +2.5V, L = 2.5µH 1000 FREQUENCY (kHz) 4.5 0 0 0 MAX1843 toc02 90 NO LOAD tOFF (µs) 450 400 5.0 100 SHUTDOWN SUPPLY CURRENT, IIN + ICC (nA) NO-LOAD SUPPLY CURRENT, IIN + ICC (µA) OFF-TIME vs. RTOFF MAX1843 toc01 500 0 IINPUT 1A/div 0 VSHDN 5V/div 0 VOUTPUT 1V/div 0 VSS 2V/div 800 600 VIN = +3.3V, VOUT = +1.5V, L = 1.5µH 400 VIN = +5V, VOUT = +1.5V, L = 2.5µH 200 0 0 0.5 1.0 1.5 2.0 2.5 3.0 1ms/div ROUT = 0.5Ω, RTOFF = 56kΩ VIN = +3.3V, VOUT = +1.5V OUTPUT CURRENT (A) LINE-TRANSIENT RESPONSE LOAD-TRANSIENT RESPONSE MAX1843 toc09 MAX1843 toc10 VINPUT 2V/div VOUTPUT 50mV/div 0 VOUTPUT 20mV/div AC-COUPLED IL 2A/div 10µs/div 20µs/div IOUT = 2.5A, VOUT = +1.5V RTOFF = 100kΩ, L = 2.2µH _______________________________________________________________________________________ 5 MAX1843 Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package MAX1843 Pin Description PIN NAME 1, 5, 10, 11, 12, 22, 24, 26, 28 FUNCTION N.C. 2, 4 IN Supply Voltage Input—for the internal PMOS power switch 3, 18, 19, 23, 25 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. 6 SS Soft-Start. Connect a capacitor from SS to GND to limit inrush current during startup. 7 COMP Integrator Compensation. Connect a capacitor from COMP to VCC for integrator compensation. See Integrator Amplifier section. 8 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. 9 FB 13, backside pad GND Analog Ground. Connect exposed backside pad to pin 13. 14 REF Reference Output. Bypass REF to GND with a 1µF capacitor. 15 FBSEL 16 VCC 17, 20, 21 PGND Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch. 27 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. Not internally connected. 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 2 for programming instructions. Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCC with a 10Ω and 2.2µF lowpass filter. See Figure 1. Detailed Description The MAX1843 synchronous, current-mode, constant-offtime, PWM DC-DC converter steps down input voltages of +3V to +5.5V to a preset output voltage of +2.5V, +1.8V, or +1.5V, or to an adjustable output voltage from +1.1V to VIN. It delivers up to 2.7A of output current. Internal switches composed of a 0.09Ω PMOS power switch and a 0.07Ω NMOS synchronous-rectifier switch improve efficiency, reduce component count, and eliminate the need for an external Schottky diode. The MAX1843 optimizes efficiency by operating in constant-off-time mode under heavy loads and in Maxim’s proprietary idle mode under light loads. A single resistorprogrammable 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 low-dropout 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 gate-charge 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 MAX1843’s current-mode, constant-off-time PWM architecture regulates the output voltage by changing the PMOS switch on-time relative to the constant offtime. 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, of idle mode), 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. _______________________________________________________________________________________ 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package IN CIN 33µF 10Ω 2.2µF OUTPUT LX COUT 150µF MAX1843 FB 470pF VCC PGND SHDN COMP FBSEL MAX1843 L INPUT GND REF 1µF TOFF SS VOUT = +2.5V, FBSEL = VOUT = +1.8V, FBSEL = VOUT = +1.5V, FBSEL = VCC REF FLOATING 0.01µF RTOFF Figure 1. Typical Circuit 0.01µF FBSEL SS FB FEEDBACK SELECTION COMP REF 470pF VIN 10Ω VCC CIN CERAMIC CURRENT SENSE gm SKIP REF 2.2µF PWM LOGIC AND DRIVERS SUMMING COMPARATOR VOUT LX COUT SHDN REF VIN +3.0V TO +5.5V IN MAX1843 REF CURRENT SENSE TIMER 1µF GND TOFF PGND RTOFF NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS. Figure 2. Functional Diagram 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 regulation 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 (tOFF). To control the current under short-circuit conditions, the PMOS switch remains off for approximately 4 x tOFF when VOUT < VOUT(NOM) / 4. _______________________________________________________________________________________ 7 MAX1843 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package Idle Mode Under light loads, this device improves 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 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. 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. Shutdown Drive SHDN to a logic-level low to place the MAX1843 in low-power shutdown mode and 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 the Integrator Amplifier section). Synchronous Rectification 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. Thermal Resistance Junction-to-ambient thermal resistance, θJA, is highly dependent on the amount of copper area immediately surrounding the IC leads. The MAX1843 EV kit has 1in2 of copper area and a thermal resistance of 50°C/W with no forced airflow. Airflow over the board significantly reduces the junction-to-ambient thermal resistance. For heatsinking purposes, it is essential to connect the exposed backside pad to a large analog ground plane. Power Dissipation Power dissipation in the MAX1843 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 = 2.5nF and fPWM 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 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. 8 _______________________________________________________________________________________ 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package MAX1843 MAXIMUM RECOMMENDED OPERATING FREQUENCY vs. INPUT VOLTAGE MAX1843 fig03 1400 VOUT = +1.5V FREQUENCY (kHz) 1200 VOUT LX 1000 MAX1843 R2 VOUT = +1.8V 800 VOUT = +2.5V 600 FB 400 VOUT = +3.3V R1 R2 = R1(VOUT / VREF - 1) VREF = 1.1V 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 Figure 4. Adjustable Output Voltage Table 1. Recommended Component Values (IOUT = 2.7A) Table 2. Output Voltage Programming VIN (V) VOUT (V) fPWM (kHz) L (µH) RTOFF (kΩ) FBSEL FB OUTPUT VOLTAGE (V) 5 3.3 800 2.2 39 VCC Output voltage 2.5 5 2.5 1180 2.2 47 Unconnected Output voltage 1.5 5 1.8 850 2.2 75 5 1.5 715 2.2 100 REF Output voltage 1.8 3.3 2.5 570 1.5 39 GND Adjustable 3.3 1.8 985 1.5 43 Resistive divider 3.3 1.5 940 1.5 56 PIN 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. 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 MAX1843 is selectable between one of three preset output voltages: +2.5V, +1.8V, and +1.5V. 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 voltage and ground (Figure 4). Regulation is maintained for adjustable output voltages when VFB = VREF. Use a resistor in the 10kΩ to 50kΩ range for R1. R2 is given by the equation: V R2 = R1 OUT VREF − 1 where VREF is typically 1.1V. _______________________________________________________________________________________ 9 MAX1843 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package Programming the Switching Frequency and Off-Time The MAX1843 features 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 offtime in PWM mode. Use the following equation to select the off-time according to the 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 f PWM = 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: 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 10 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: I PEAK = 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: I RIPPLE = I LOAD 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 MAX1843 requires a minimum output ripple voltage of VRIPPLE ≥ 1% x 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 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. Choose the capacitor values that result in optimal performance. ______________________________________________________________________________________ 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package VSS = 4µA × t CSS The soft-start current limit varies with the voltage on the soft-start pin, SS, according to the equation: V − 0.7V SSILIMIT = SS × ILIMIT 1.1V where ILIMIT is the current threshold from the Electrical Characteristics. The constant-current source stops charging once the voltage across the soft-start capacitor reaches 1.8V (Figure 5). Frequency Variation with Output Current The operating frequency of the MAX1843 is determined primarily by t OFF (set by R TOFF), V IN, and V OUT 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 (90mΩ typ). MAX1843 Soft-Start Soft-start allows a gradual increase of the internal current limit to reduce input surge currents at startup and at exit from shutdown. A timing capacitor, CSS, placed from SS to GND sets the rate at which the internal current limit is changed. Upon power-up, when the device comes out of undervoltage lockout (2.6V typ) or after the SHDN pin is pulled high, a 4µA constant-current source charges the soft-start capacitor and the voltage on SS increases. When the voltage on SS is less than approximately 0.7V, the current limit is set to zero. As the voltage increases from 0.7V to approximately 1.8V, the current limit is adjusted from 0 to the current-limit threshold (see the Electrical Characteristics). The voltage across the soft-start capacitor changes with time according to the equation: SHDN 0 1.8V VSS (V) 0.7V 0 ILIMIT ILIMIT (A) 0 t Figure 5. Soft-Start Current-Limit Over Time Circuit Layout and Grounding Good layout is necessary to achieve the MAX1843’s intended output power level, high efficiency, and low noise. Good layout includes the use of ground planes, careful component placement, and correct routing of traces using appropriate trace widths. 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) Ground planes are 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. For heat dissipation, connect the exposed backside pad to a large analog ground plane, preferably on a surface of the board that receives good airflow. If the ground plane is located on the IC surface, make use of the N.C. pins adjacent to GND to lower thermal resistance to the ground plane. If the ground is located elsewhere, use several vias to lower thermal resistance. Typical applications use multiple ground planes to minimize thermal resistance. Avoid large AC currents through the analog ground plane. Chip Information TRANSISTOR COUNT: 3662 ______________________________________________________________________________________ 11 Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) 32L QFN .EPS MAX1843 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Synchronous Rectification in QFN Package 12 ______________________________________________________________________________________ 2.7A, 1MHz, Low-Voltage, Step-Down Regulator with Internal Synchronous Rectification in QFN Package 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. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1843 Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)