19-1760; Rev 1; 2/02 KIT ATION EVALU E L B AVAILA 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches Features ♦ ±1% Output Accuracy ♦ 95% Efficiency ♦ Internal PMOS and NMOS Switches 90mΩ On-Resistance at VIN = 4.5V 110mΩ 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 ♦ 600µA (max) Operating Supply Current ♦ <1µA Shutdown Supply Current ♦ Programmable Constant-Off-Time Operation ♦ 1MHz (max) Switching Frequency ♦ 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 ♦ 16-Pin QSOP Package Ordering Information TEMP RANGE PIN-PACKAGE MAX1742EEE PART -40°C to +85°C 16 QSOP MAX1842EEE -40°C to +85°C 16 QSOP Typical Configuration Applications 5V or 3.3V to Low-Voltage Conversion CPU I/O Ring Chipset Supplies Notebook and Subnotebook Computers INPUT 3V TO 5.5V 2.2µF Pin Configuration appears at end of data sheet. IN 10Ω OUTPUT 1.1V TO VIN LX MAX1742 FB MAX1842 VCC PGND 470pF GND SHDN COMP TOFF FBSEL REF 1µF SS 0.01µF Idle Mode is a trademark of Maxim Integrated Products. ________________________________________________________________ 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 MAX1742/MAX1842 General Description The MAX1742/MAX1842 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 90mΩ PMOS power switch and 70mΩ NMOS synchronous-rectifier switch easily deliver continuous load currents up to 1A. The MAX1742/MAX1842 produce a preset 2.5V, 1.8V, or 1.5V output voltage or an adjustable output from 1.1V to VIN. They achieve efficiencies as high as 95%. The MAX1742/MAX1842 use a unique current-mode, constant-off-time, 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. Both devices are designed for continuous output currents up to 1A. The MAX1742 uses a peak current limit of 1.3A (min) and is suitable for applications requiring small external component size and high efficiency. The MAX1842 has a higher current limit of 3.1A (min) and is intended for applications requiring an occasional burst of output current up to 2.7A. Both devices also feature 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 MAX1742/MAX1842 are available in 16pin QSOP packages. For similar devices that provide continuous output currents up to 2A and 3A, refer to the MAX1644 and MAX1623 data sheets. MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches ABSOLUTE MAXIMUM RATINGS VCC, IN to GND ........................................................-0.3V to +6V Continuous Power Dissipation (TA = +70°C) IN to VCC .............................................................................±0.3V SSOP (derate 16.7mW/°C above +70°C; GND to PGND.....................................................................±0.3V part mounted on 1 in.2 of 1oz. copper) ...............................1W 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 Lead Temperature (soldering, 10s) ................................ +300°C ESD Protection .....................................................................±2kV 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 VOUT VIN = 3V to 5.5V, ILOAD = 0 to 1A for MAX1742, ILOAD = 0 to 2.5A for MAX1842, VFB = VOUT FBSEL = unconnected 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 FBSEL = REF FBSEL = GND Adjustable Output Voltage Range TYP 3.0 FBSEL = VCC Preset Output Voltage MIN VIN = VCC = 3V to 5.5V, ILOAD = 0, FBSEL = GND VREF VIN V AC Load Regulation Error 2 % DC Load Regulation Error 0.4 % Dropout Voltage VDO Reference Voltage VREF Reference Load Regulation ∆VREF IREF = -1µA to +10µA PMOS Switch On-Resistance RON, P ILX = 0.5A NMOS Switch On-Resistance 2 RON, N VIN = VCC = 3V, ILOAD = 1A 250 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 ILX = 0.5A _______________________________________________________________________________________ mV V mV mΩ 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842 ELECTRICAL CHARACTERISTICS (continued) (VIN = VCC = 3.3V, FBSEL = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Current-Limit Threshold SYMBOL ILIMIT CONDITIONS MIN TYP MAX MAX1742 1.3 1.5 1.7 MAX1842 3.1 3.6 4.1 RMS LX Output Current Idle Mode Current Threshold Switching Frequency No-Load Supply Current Shutdown Supply Current PMOS Switch Off-Leakage Current 3.1 IIM f IIN + ICC 0.1 0.3 0.5 0.3 0.6 0.9 1 MHz 350 600 µA <1 5 µA 15 µA (Note 2) VFB = 1.2V SHDN = GND TSHDN Hysteresis = 15°C Undervoltage Lockout Threshold VUVLO VIN falling, hysteresis = 90mV IFB Off-Time Default Period tOFF 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 VFB = 1.2V FB = GND On-Time Period tON (Note 2) SS Source Current ISS SS Sink Current ISS I SHDN VIL SHDN Input High Threshold VIH 4 ✕ tOFF VSS = 1V V SHDN = 0 to VCC µs 5 6 100 -1 -4 FBSEL = REF µA µA 1 µA 0.8 V 2.0 FBSEL = GND µs µs 0.4 4 FBSEL Input Current FBSEL Logic Thresholds °C 2.6 tOFF SHDN Input Low Threshold 160 A 2.5 Off-Time Startup Period SHDN Input Current A MAX1842 Thermal Shutdown Threshold FB Input Bias Current A MAX1742 ICC(SHDN) SHDN = GND IIN UNITS V +4 µA 0.2 0.9 FBSEL = unconnected 0.7 ✕ VCC - 0.2 FBSEL = VCC VCC - 0.2 1.3 0.7 ✕ VCC +0.2 V _______________________________________________________________________________________ 3 MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 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 Preset Output Voltage SYMBOL VIN VOUT Adjustable Output Voltage Range Reference Voltage PMOS Switch On-Resistance NMOS Switch On-Resistance Current-Limit Threshold Idle Mode Current Threshold No-Load Supply Current CONDITIONS ILOAD = 0 to 1A for MAX1742, ILOAD = 0 to 2.5A for MAX1842, VFB = VOUT RON, N ILIMIT IIM ILX = 0.5A UNITS V 3.0 5.5 2.475 2.575 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 VREF ILX = 0.5A MAX VIN = 3V to 5.5V, FBSEL = VCC VIN = VCC = 3V to 5.5V, ILOAD = 0, FBSEL = GND RON, P MIN VIN = 4.5V 200 VIN = 3V 250 VIN = 4.5V 150 VIN = 3V MAX1742 2.9 4.3 0.05 0.55 MAX1842 0.2 1.0 VFB = 1.2V IFB VFB = 1.2V Off-Time Default Period tOFF RTOFF = 110kΩ 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. 4 1.8 MAX1742 IIN + ICC mΩ 200 1.2 MAX1842 FB Input Bias Current V _______________________________________________________________________________________ 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742 EFFICIENCY vs. OUTPUT CURRENT (VIN = 3.3V, L = 3.9µH) 80 75 70 85 80 75 VOUT = 1.8V, RTOFF = 43kΩ, f = 869kHz 90 85 80 75 70 65 65 60 60 60 VOUT = 1.5V, RTOFF = 56kΩ, f = 833kHz 55 VOUT = 1.5V, RTOFF = 100kΩ, f = 692kHz 0.01 0.1 1 0.001 0.01 1100 MAX1742 toc04 0.4 0.3 900 FREQUENCY (kHz) 0.1 0 -0.1 VIN = 3.3V, VOUT = 1.5V VIN = 5V, VOUT = 2.5V, L = 6µH 1000 VIN = 5V, VOUT = 1.5V, L = 6µH 800 700 600 500 400 VIN = 5V, VOUT = 1.5V, L = 6µH 300 -0.3 200 -0.4 VIN = 3.3V, VOUT = 1.5V, L = 3.9µH 100 -0.5 0.01 0.1 OUTPUT CURRENT (A) 1 MAX1742 SWITCHING FREQUENCY vs. OUTPUT CURRENT 0.5 0.001 0.1 0.01 OUTPUT CURRENT (A) MAX1742 NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT -0.2 50 0.001 1 0.1 OUTPUT CURRENT (A) OUTPUT CURRENT (A) 0.2 VIN = 3.3V, VOUT = 1.8V, L = 10µH, RTOFF = 160kΩ 55 50 50 0.001 VIN = 5V, VOUT = 1.8V, L = 15µH, RTOFF = 240kΩ 95 65 55 MAX1742 toc03 90 70 100 1 MAX1742 toc05 VOUT = 1.8V, RTOFF = 75kΩ, f = 833kHz EFFICIENCY (%) 85 NORMALIZED OUTPUT ERROR (%) EFFICIENCY (%) 90 VOUT = 2.5V, RTOFF = 36kΩ, f = 456kHz 95 EFFICIENCY (%) VOUT = 2.5V, RTOFF = 47kΩ, f = 926kHz 95 100 MAX1742 toc01 100 MAX1742 EFFICIENCY vs.OUTPUT CURRENT (fPWM = 270kHz) MAX1742 toc02 MAX1742 EFFICIENCY vs. OUTPUT CURRENT (VIN = 5.0V, L = 6.0µH) 0 0 0.2 0.4 0.6 0.8 1.0 OUTPUT CURRENT (A) _______________________________________________________________________________________ 5 MAX1742/MAX1842 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.) MAX1742 STARTUP AND SHUTDOWN MAX1742 LOAD-TRANSIENT RESPONSE MAX1742 toc07 MAX1742 toc06 IINPUT 0A 1A/div VOUTPUT AC-COUPLED, 50mV/div VSHDN 0V 5V/div VOUTPUT 0V 1V/div IL 0.5A/div 0V 0V VSS 2V/div 10µs/div 1ms/div MAX1742 LINE-TRANSIENT RESPONSE SUPPLY CURRENT vs. SUPPLY VOLTAGE MAX1742 toc09 NO-LOAD SUPPLY CURRENT, IIN + ICC (µA) 500 VINPUT 2V/div 0V VOUTPUT 20mV/div AC-COUPLED 450 90 NO LOAD 400 80 350 70 300 60 250 50 200 40 150 30 100 20 SHUTDOWN 50 10 0 20µs/div IOUT = 1A, VOUT = 1.5V, RTOFF = 100kΩ, L = 6µH 0 0 1 2 3 4 5 VIN (V) OFF-TIME vs. RTOFF MAX1742 toc10 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 50 100 150 200 250 300 350 400 450 500 RTOFF (kΩ) 6 100 _______________________________________________________________________________________ 6 SHUTDOWN SUPPLY CURRENT, IIN + ICC (nA) MAX1742 toc08 tOFF (µs) MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1842 EFFICIENCY vs. OUTPUT CURRENT (VIN = 5.0V, L = 2.5µH) 80 75 VOUT = 1.8V, RTOFF = 75kΩ, fPWM = 910kHz 70 65 60 50 0.001 0.01 0.1 1 75 VOUT = 1.8V, RTOFF = 43kΩ, fPWM = 1050kHz 70 55 50 0.001 VOUT = 2.5V, RTOFF = 56kΩ, fPWM = 1000kHz 0.01 0.1 1 10 OUTPUT CURRENT (A) OUTPUT CURRENT (A) MAX1842 EFFICIENCY vs. OUTPUT CURRENT (fPWM = 270kHz) MAX1842 NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT 85 80 75 VIN = 3.3V, VOUT = 1.8V, L = 4.7µH, RTOFF = 160kΩ 70 65 60 0.1 MAX1842 toc14 MAX1842 toc13 VIN = 5V, VOUT = 1.8V, L = 5.6µH, RTOFF = 240kΩ 90 EFFICIENCY (%) 80 60 10 100 95 85 65 VOUT = 1.5V, RTOFF = 1OOkΩ, fPWM = 770kHz 55 VOUT = 2.5V, RTOFF = 39kΩ, fPWM = 610kHz 90 EFFICIENCY (%) 85 VIN = 3.3V, VOUT = 1.5V, L = 1.5µH NORMALIZED OUTPUT ERROR (%) EFFICIENCY (%) 90 95 MAX1842 toc12 VOUT = 2.5V, RTOFF = 47kΩ, fPWM = 1070kHz 95 100 MAX1842 toc11 100 MAX1842 EFFICIENCY vs. OUTPUT CURRENT (VIN = 3.3V, L = 1.5µH) 0 -0.1 -0.2 VIN = 5V, VOUT = 1.5V, L = 2.5µH -0.3 55 50 0.001 0.01 0.1 1 10 0.01 0.1 1 10 IOUT (A) OUTPUT CURRENT (A) MAX1842 SWITCHING FREQUENCY vs. OUTPUT CURRENT MAX1842 STARTUP AND SHUTDOWN VIN = 5V, VOUT = 2.5V, L = 2.5µH 1000 MAX1842 toc15 MAX1842 toc16 1200 FREQUENCY (kHz) -0.4 0.001 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 OUTPUT CURRENT (A) 2.5 3.0 1ms/div ROUT = 0.5Ω, RTOFF = 56kΩ VIN = 3.3V, VOUT = 1.5V _______________________________________________________________________________________ 7 MAX1742/MAX1842 Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1842 LINE-TRANSIENT RESPONSE MAX1842 LOAD-TRANSIENT RESPONSE MAX1842 toc18 MAX1842 toc17 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 Pin Description 8 PIN NAME FUNCTION 1 SHDN 2, 4 IN Supply Voltage Input—for the internal PMOS power switch. 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. 5 SS Soft-Start. Connect a capacitor from SS to GND to limit inrush current during startup. 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 Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal MOSFETs. Drive high or connect to VCC for normal operation. 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. See Figure 1. Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch. _______________________________________________________________________________________ 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches The MAX1742/MAX1842 synchronous, current-mode, constant-off-time, PWM DC-DC converters step 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. Both devices deliver up to 1A of continuous output current; the MAX1842 delivers bursts of output current up to 2.7A (see the Extended Current Limit section). 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 MAX1742/MAX1842 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. 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 MAX1742/MAX1842s’ 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 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 (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 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 MAX1742/MAX1842 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). _______________________________________________________________________________________ 9 MAX1742/MAX1842 _______________Detailed Description MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches INPUT CIN = 10µF (MAX1742) CIN = 33µF (MAX1842) L IN 10Ω 2.2µF OUTPUT COUT = 47µF (MAX1742) COUT = 150µF (MAX1842) LX MAX1742 FB 470pF VCC PGND SHDN COMP FBSEL GND VOUT = 2.5V, FBSEL = VOUT = 1.8V, FBSEL = VOUT = 1.5V, FBSEL = REF 1µF TOFF SS VCC REF FLOATING 0.01µF RTOFF Figure 1. Typical Circuit 0.01µF FBSEL SS FB FEEDBACK SELECTION COMP REF 470pF VIN 10Ω VCC MAX1742 MAX1842 Gm IN 10µF CURRENT SENSE SKIP REF 2.2µF PWM LOGIC AND DRIVERS SUMMING COMPARATOR LX VOUT COUT SHDN REF VIN 3.0V TO 5.5V REF TIMER 1µF GND TOFF CURRENT SENSE PGND RTOFF NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS. Figure 2. Functional Diagram 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 syn- 10 chronous 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 ______________________________________________________________________________________ 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches Power Dissipation Power dissipation in the MAX1742/MAX1842 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 __________________Design Procedure For typical applications, use the recommended component values in Tables 1 or 2. 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. VOUT (V) fPWM (kHz) L (µH) RTOFF (kΩ) 5 3.3 850 5.6 39 5 2.5 1070 5.6 47 5 1.8 910 5.6 75 5 1.5 770 5.6 100 3.3 2.5 610 3.9 39 3.3 1.8 1050 3.9 43 3.3 1.5 1000 3.9 56 Table 2. MAX1842 Recommended Component Values (Continuous Output Current = 1A, Burst Output Current = 2.7A) VIN (V) VOUT (V) fPWM (kHz) L (µH) RTOFF (kΩ) 5 3.3 800 2.2 39 5 2.5 1180 2.2 47 5 1.8 850 2.2 75 5 1.5 715 2.2 100 3.3 2.5 570 1.5 39 3.3 1.8 985 1.5 43 3.3 1.5 940 1.5 56 MAXIMUM RECOMMENDED OPERATING FREQUENCY vs. INPUT VOLTAGE 1400 MAX1842 fig03 Junction-to-ambient thermal resistance, θJA, is highly dependent on the amount of copper area immediately surrounding the IC leads. The MAX1742 evaluation kit has 0.5in2 of copper area and a thermal resistance of 80°C/W with no forced airflow. Airflow over the board significantly reduces the junction-to-ambient thermal resistance. For heatsinking purposes, evenly distribute the copper area connected at the IC among the highcurrent pins. VIN (V) VOUT = 1.5V 1200 OPERATING FREQUENCY (kHz) Thermal Resistance Table 1. MAX1742 Recommended Component Values (IOUT = 1A) 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 ______________________________________________________________________________________ 11 MAX1742/MAX1842 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. MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches Table 3. Output Voltage Programming FBSEL FB OUTPUT VOLTAGE (V) VCC Output voltage 2.5 Unconnected Output voltage 1.5 REF Output voltage 1.8 GND Resistive divider PIN Adjustable VOUT LX MAX1742 MAX1842 R2 FB R1 R1 = 50kΩ R2 = R1(VOUT / VREF - 1) VREF = 1.1V Figure 4. Adjustable Output Voltage 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 MAX1742/MAX1842 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 3. 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 50kΩ for R1. R2 is given by the equation: V R2 = R1 OUT − 1 VREF Programming the Switching Frequency and Off-Time The MAX1742/MAX1842 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. R TOFF 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 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 where VREF is typically 1.1V. 12 ______________________________________________________________________________________ 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches SHDN V × t OFF IPEAK = IOUT + OUT 2 × L 0 1.8V 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 MAX1742/MAX1842 requires a minimum output ripple voltage of VRIPPLE ≥ 1% x VOUT. The minimum ESR of the output capacitor should be: ESR > 1% × MAX1742/MAX1842 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: L t OFF Stable operation requires the correct output filter capacitor. When choosing the output capacitor, ensure that: t COUT ≥ OFF 33µFV / µs for the MAX1742 VOUT t COUT ≥ OFF 79µFV / µs for the MAX1842 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 VSS (V) 0.7V 0 ILIMIT ILIMIT (A) 0 t Figure 5. Soft-Start Current Limit over Time decreases stability. Choose the capacitor values that result in optimal performance. 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: 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. ______________________________________________________________________________________ 13 The constant-current source stops charging once the voltage across the soft-start capacitor reaches 1.8V (Figure 5). 2.70 For applications requiring occasional short bursts of high output current (up to 2.7A), the MAX1842 provides a higher current-limit threshold. When using the MAX1842, choose external components capable of withstanding its higher peak current limit. The MAX1842 is capable of delivering large output currents for limited durations, and its thermal characteristics allow it to operate at continuously higher output currents. Figure 6 shows its maximum recommended continuous output current versus ambient temperature. Figure 7 shows the maximum recommended burst current versus the output current duty cycle at high temperatures. Figure 7 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. 2.65 OUTPUT CURRENT (A) Extended Current Limit (MAX1842) 2.60 2.55 2.50 2.45 2.40 2.35 2.30 25 Circuit Layout and Grounding 14 65 75 85 TA = +85°C TA = +55°C MAX1842 fig07 2.6 Good layout is necessary to achieve the MAX1742/ MAX1842s’ 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. The following points are in order of decreasing importance: 55 MAXIMUM RECOMMENDED BURST CURRENT vs. BURST CURRENT DUTY CYCLE Frequency Variation with Output Current where RPMOS is the resistance of the internal MOSFETs (90mΩ typ). 45 Figure 6. MAX1842 Maximum Recommended Continuous Output Current vs. Temperature 2.7 ∆fPWM = -IOUT x RPMOS / (VIN x tOFF) 35 TEMPERATURE (°C) Note that if the thermal limitations of the MAX1842 are exceeded, it will enter thermal shutdown to prevent destructive failure. The operating frequency of the MAX1742/MAX1842 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: MAX1842 fig06 MAX1842 MAXIMUM RECOMMENDED CONTINUOUS OUTPUT CURRENT vs. TEMPERATURE BURST CURRENT (A) MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 2.5 2.4 2.3 IOUT IS A 100Hz SQUARE WAVE FROM 1A TO THE BURST CURRENT 2.2 0 20 40 60 80 100 DUTY CYCLE (%) Figure 7. MAX1842 Maximum Recommended Burst Current vs. Burst Current Duty Cycle 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. ______________________________________________________________________________________ 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 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 TOP VIEW 16 LX SHDN 1 IN 2 15 PGND LX 3 14 LX IN 4 SS 5 MAX1742 MAX1842 COMP 6 TRANSISTOR COUNT: 3662 12 VCC 11 FBSEL TOFF 7 Chip Information 13 PGND 10 REF FB 8 9 GND QSOP ______________________________________________________________________________________ 15 MAX1742/MAX1842 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. 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.) QSOP.EPS MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 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. 16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.