19-1457; Rev 3; 8/05 KIT ATION EVALU LE B A IL A AV 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches The MAX1644 constant-off-time, PWM step-down DCDC converter is ideal for use in applications such as PC cards, CPU daughter cards, and desktop computer bus-termination boards. The device features internal synchronous rectification for high efficiency and reduced component count. It requires no external Schottky diode. The internal 0.10Ω PMOS power switch and 0.10Ω NMOS synchronous-rectifier switch easily deliver continuous load currents up to 2A. The MAX1644 produces a preset +3.3V or +2.5V output voltage or an adjustable output from +1.1V to VIN. It achieves efficiencies as high as 95%. The MAX1644 uses a unique current-mode, constantoff-time, PWM control scheme, which includes an Idle Mode™ to maintain high efficiency during light-load operation. The programmable constant-off-time architecture sets switching frequencies up to 350kHz, allowing the user to optimize performance trade-offs between efficiency, output switching noise, component size, and cost. The device also features an adjustable soft-start to limit surge currents during start-up, a 100% duty cycle mode for low-dropout operation, and a lowpower shutdown mode that disconnects the input from the output and reduces supply current below 1µA. The MAX1644 is available in a 16-pin SSOP package. Applications Features ♦ ±1% Output Accuracy ♦ 95% Efficiency ♦ Internal PMOS and NMOS Switches 70mΩ On-Resistance at VIN = +4.5V 100mΩ On-Resistance at VIN = +3V ♦ Output Voltage +3.3V or +2.5V Pin-Selectable +1.1V to VIN Adjustable ♦ +3V to +5.5V Input Voltage Range ♦ 360µA (max) Operating Supply Current ♦ < 1µA Shutdown Supply Current ♦ Programmable Constant-Off-Time Operation ♦ 350kHz (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 SSOP Package Ordering Information +5V to +3.3V/+2.5V Conversion CPU I/O Supply PART +3.3V PC Card and CardBus Applications Notebook and Subnotebook Computers Desktop Bus-Termination Boards CPU Daughter Card Supply TEMP RANGE PIN-PACKAGE MAX1644EAE 40°C to +85°C 16 SSOP MAX1644EAE+ 40°C to +85°C 16 SSOP +Denotes lead-free package. Pin Configuration Typical Operating Circuit TOP VIEW INPUT +3V TO +5.5V IN LX MAX1644 FB VCC OUTPUT +1.1V TO VIN PGND 16 LX IN 2 15 PGND 14 LX LX 3 IN 4 MAX1644 GND COMP 6 REF SS TOFF 7 13 PGND 12 VCC SS 5 FBSEL SHDN COMP TOFF SHDN 1 11 FBSEL 10 REF 9 FB 8 GND RTOFF SSOP Idle Mode is a trademark of Maxim Integrated Products, Inc. A "+" SIGN WILL REPLACE THE FIRST PIN INDICATOR ON LEAD-FREE PACKAGES. ________________________________________________________________ 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 MAX1644 General Description MAX1644 2A, Low-Voltage, Step-Down Regulator 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) ............................1.2W Operating Temperature Range ...........................-40°C to +85°C All Other Pins to GND.................................-0.3V to (VCC + 0.3V) LX Current (Note 1)...........................................................±3.75A 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 Preset Output Voltage SYMBOL VOUT Adjustable Output Voltage Range DC Load Regulation Error Reference Voltage VDO TYP ILOAD = 0 to 2A, VFB = VOUT 3.366 VIN = VCC = 3V to 5.5V, FBSEL = VCC 2.500 2.525 2.550 VIN = VCC = 3V to 5.5V, FBSEL = REF 1.089 1.100 1.111 VREF VIN FBSEL = GND 1 FBSEL = REF, VCC, or unconnected 2 FBSEL = GND 0.2 FBSEL = REF, VCC, or unconnected 0.4 VIN = VCC = 3V, ILOAD = 1A, FBSEL = VCC VREF V % mV 1.100 1.111 V 0.5 1 mV VIN = 4.5V 70 150 VIN = 3V 100 200 RON, P ILX = 0.5A NMOS Switch On-Resistance RON, N ILX = 0.5A VIN = 4.5V 70 150 VIN = 3V 100 200 ILIMIT 2.5 2.9 IIM 0.25 mΩ mΩ 3.3 A 2.5 A 0.45 0.65 A RMS LX Output Current 350 kHz No-Load Supply Current IIN + ICC VFB = 1.2V 240 360 µA Shutdown Supply Current ICC(SHDN) SHDN = GND <1 3 µA PMOS Switch Off-Leakage Current IIN SHDN = GND 15 µA Thermal Shutdown Threshold 2 V % 200 1.089 PMOS Switch On-Resistance f V 3.333 IREF = -1µA to +10µA Switching Frequency UNITS 5.5 3.300 ΔVREF Idle Mode Current Threshold MAX VIN = VCC = 4V to 5.5V, FBSEL = unconnected Reference Load Regulation Current-Limit Threshold MIN 3.0 VIN = VCC = 3V to 5.5V, ILOAD = 0, FBSEL = GND or REF AC Load Regulation Error Dropout Voltage CONDITIONS VIN, VCC TSHDN (Note 2) Hysteresis = 15°C 150 _______________________________________________________________________________________ °C 2A, Low-Voltage, Step-Down Regulator 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 Undervoltage Lockout Threshold VUVLO FB Input Bias Current IFB Off-Time Default Period tOFF CONDITIONS MIN TYP MAX UNITS VIN falling, hysteresis = 40mV 2.5 2.6 2.7 V VFB = 1.2V RTOFF = 150kΩ RTOFF = 30.1kΩ 0 1.13 0.20 80 1.33 0.33 200 1.53 nA 4.3 5.6 RTOFF = 499kΩ Off-Time Start-Up Period On-Time Period SS Source Current SS Sink Current SHDN Input Current tOFF tON ISS ISS I SHDN SHDN Input Low Threshold VIL SHDN Input High Threshold FBSEL Input Current VIH 4 · tOFF FB = GND VSS = 1V 0.4 3.5 100 V SHDN = 0 to VCC -0.5 5 µs µs µA µA 6.5 0.5 µA 0.8 V +5 0.2 1.3 µA 2.0 V -5 FBSEL = GND FBSEL = REF FBSEL Logic Thresholds µs 0.9 FBSEL = unconnected FBSEL = VCC 0.7 · VCC - 0.2 VCC - 0.2 V 0.7 · VCC + 0.2 Maximum Output RMS Current 5.8 ARMS 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 Preset Output Voltage VOUT VIN = 3.0V to 5.5V, ILOAD = 0, FBSEL = GND or REF Adjustable Output Voltage Reference Voltage VIN = VCC = 4V to 5.5V, ILOAD = 0 to 2A, FBSEL = unconnected VIN = 3V to 5.5V, FBSEL = VCC VFB = VOUT VIN = 3V to 5.5V, FBSEL = REF VREF UNITS 3.0 5.5 V 3.276 3.390 2.48 1.08 2.57 1.12 VREF VIN V 1.08 V V 2.3 IIM 0.2 0.7 A 0 1.03 360 250 1.63 µA nA µs ILX = 0.5A NMOS Switch On-Resistance RON, N ILX = 0.5A No-Load Supply Current FB Input Bias Current Off-Time Default Period MAX ILIMIT RON, P Idle Mode Current Threshold TYP 1.12 150 200 150 200 3.5 PMOS Switch On-Resistance Current-Limit Threshold MIN IIN + ICC IFB tOFF VIN = 4.5V VIN = 3V VIN = 4.5V VIN = 3V VFB = 1.2V VFB = 1.2V RTOFF = 150kΩ mΩ mΩ A Note 2: Recommended operating frequency, not production tested. Note 3: Specifications from 0°C to -40°C are guaranteed by design, not production tested. _______________________________________________________________________________________ 3 MAX1644 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) 80 EFFICIENCY (%) 60 50 40 70 50 40 30 30 20 20 10 10 0 0.001 0.01 0.1 1 VIN = 5V, VOUT = 1.5V, L = 6.0μH, RTOFF = 270kΩ 60 VIN = 3.3V, VOUT = 1.5V, L = 4.7μH, RTOFF = 200kΩ 0.01 OUTPUT CURRENT (A) SWITCHING FREQUENCY vs. OUTPUT CURRENT 250 200 100 MAX1644-06 4.0 3.0 VIN = 3.3V, VOUT = 1.5V, L = 4.7μH, RTOFF = 200kΩ -0.3 D E -0.4 -0.6 -0.7 -0.8 -1.0 0.0001 0.001 0.01 2.5 2.0 0.5 0 0.5 1.0 1.5 0 2.0 IOUT = 0 0.10 0.09 400 0.08 350 0.07 300 0.06 SHDN = VIN = VCC 250 0.05 200 0.04 UNDERVOLTAGE LOCKOUT 150 100 0.03 0.02 SHDN = GND 50 0.01 0 0 0 1 2 3 4 5 6 SHUTDOWN SUPPLY CURRENT IIN + ICC (μA) MAX1644-07 450 200 300 400 500 600 STARTUP AND SHUTDOWN TRANSIENTS SUPPLY CURRENT vs. SUPPLY VOLTAGE 500 100 RTOFF (kΩ) OUTPUT CURRENT (A) VSHDN 5V/div IIN 1A/div VOUT 2V/div VSS 1V/div MAX1644-09 0 0 0 0 0 VIN = 5.0V, VOUT = 3.3V, IOUT = 2A 2ms/div SUPPLY VOLTAGE 4 0.1 1 A: VIN = 3.3V, VOUT = 1.5V, L = 4.7μH, RTOFF = 200kΩ, FBSEL = GND B: VIN = 3.3V, VOUT = 1.5V, L = 4.7μH, RTOFF = 200kΩ, FBSEL = REF C: VIN = 5V, VOUT = 3.3V, L = 6.0μH, RTOFF = 120kΩ, FBSEL = OPEN D: VIN = 5V, VOUT = 1.5V, L = 6.0μH, RTOFF = 270kΩ, FBSEL = GND E: VIN = 5V, VOUT = 1.5V, L = 6.0μH, RTOFF = 270kΩ, FBSEL = REF 1.0 0 C -0.5 1.5 50 B -0.2 OUTPUT CURRENT (A) 3.5 VIN = 5V, VOUT = 1.5V, L = 6.0μH, RTOFF = 270kΩ 150 10 OFF-TIME vs. RTOFF MAX1644-04 VIN = 5V, VOUT = 3.3V, L = 6.0μH, RTOFF = 120kΩ 1 4.5 tOFF (μs) SWITCHING FREQUENCY (kHz) 300 0.1 OUTPUT CURRENT (A) 350 A -0.1 -0.9 0 0.001 10 0 DC LOAD-REGULATION ERROR (%) 90 VIN = 5V, VOUT = 3.3V, L = 6.0μH, RTOFF = 120kΩ 70 MAX1644-02 90 EFFICIENCY (%) 100 MAX1644-01 100 80 DC LOAD-REGULATION ERROR vs. OUTPUT CURRENT EFFICIENCY vs. OUTPUT CURRENT MAX1644-03 EFFICIENCY vs. OUTPUT CURRENT SUPPLY CURRENT ICC (μA) MAX1644 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches _______________________________________________________________________________________ 10 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644-10 4V VIN 3V MAX1644-11 LOAD-TRANSIENT RESPONSE (FBSEL = REF) LINE-TRANSIENT RESPONSE 2A IL 0 VOUT = 1.5V, IOUT = 2A VIN = 3.3V, VOUT = 1.5V VOUT 50mV/div VOUT 20mV/div 20μs/div 20μs/div Pin Description 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 output filter capacitor and load. 5 SS Soft-Start. Connect a capacitor from SS to GND to limit inrush current during start-up. 6 COMP Integrator Compensation. Connect a capacitor from COMP to VCC for integrator compensation. See the 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 Feedback Select Input. Selects AC load-regulation error and output voltage. See Table 2 for programming instructions. 12 VCC Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCC with a 10Ω and 2.2µF lowpass filter. See Figure 1. 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 resistor-divider for adjustable operating modes. Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch. _______________________________________________________________________________________ 5 MAX1644 Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1644 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches _______________Detailed Description The MAX1644 synchronous, current-mode, constant-offtime, PWM DC-DC converter steps down input voltages of +3V to +5.5V to a preset output voltage of either +3.3V or +2.5V, or to an adjustable output voltage from +1.1V to VIN. The device delivers up to 2A of continuous load current. Internal switches composed of a 0.1Ω PMOS power switch and a 0.1Ω NMOS synchronous-rectifier switch improve efficiency, reduce component count, and eliminate the need for an external Schottky diode. The MAX1644 optimizes performance 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 350kHz, 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 permanently 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 MAX1644’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 0.2A) or Idle Mode (for load currents less than 0.2A). 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 (0.4A, which corresponds to a load current of 0.2A). In this mode, the regulation comparator turns the PMOS switch on at the end of each offtime, 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 pro- 6 grammed off-time (tOFF). If the output falls dramatically out of regulation—approximately VFB / 4—the PMOS switch remains off for approximately four times tOFF. The NMOS synchronous rectifier turns on shortly after the PMOS switch turns off, and it remains on until shortly before the PMOS switch turns back on. Idle Mode Under light loads, the 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 0.4A, thus minimizing the unnecessary switching that degrades efficiency under light loads. In Idle Mode, the device operates in discontinuous conduction. Current-sense 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 MAX1644 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 1): 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). _______________________________________________________________________________________ 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644 0.01μF FBSEL SS FB FEEDBACK SELECTION COMP REF 470pF VIN 10Ω VCC IN MAX1644 10μF CURRENT SENSE Gm SKIP REF 2.2μF PWM LOGIC AND DRIVERS SUMMING COMPARATOR LX VOUT COUT SHDN REF VIN 3.0V TO 5.5V REF CURRENT SENSE TIMER 1μF GND TOFF PGND RTOFF NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS. Figure 1. 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 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. Thermal Resistance Junction-to-ambient thermal resistance, θJA, is highly dependent on the amount of copper area immediately surrounding the IC leads. The MAX1644 evaluation kit has 0.5 in.2 of copper area and a thermal resistance of 60°C/W with no airflow. Airflow over the IC significantly reduces the junction-to-ambient 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 MAX1644 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 are less than 30mW at 300kHz. 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 · RON The junction-to-ambient thermal resistance required to dissipate this amount of power is calculated by: θJA = (TJ,MAX - TA,MAX) / PD where: θJA = junction-to-ambient thermal resistance TJ,MAX = maximum junction temperature TA,MAX = maximum ambient temperature _______________________________________________________________________________________ 7 MAX1644 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches __________________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; 300kHz is a good starting point. 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. Table 1. Recommended Component Values (IOUT = 2A, fPWM = 300kHz) VIN (V) VOUT (V) L (µH) RTOFF (kΩ) 5 3.3 6.0 120 5 2.5 6.8 180 5 1.8 6.8 240 5 1.5 6.0 270 3.3 2.5 3.3 82 3.3 1.8 4.7 180 3.3 1.5 4.7 200 Table 2. Output Voltage and AC LoadRegulation Selection PIN OUTPUT VOLTAGE (V) AC LOADREGULATION ERROR (%) FBSEL FB VCC Output Voltage 2.5 2 Unconnected Output Voltage 3.3 2 REF Resistor Divider Adjustable 2 GND Resistor Divider Adjustable 1 Setting the Output Voltage The output of the MAX1644 is selectable between one of two preset output voltages: (2.5V or 3.3V) with a 2% AC load-regulation error, or an adjustable output voltage from the reference voltage (nominally 1.1V) up to VIN with a 1% or 2% AC load-regulation error. For a preset output voltage, connect FB to the output voltage, and connect FBSEL to VCC (2.5V output voltage) or 8 VOUT LX MAX1644 R2 FB R1 = 50kΩ R2 = R1(VOUT / VREF - 1) VREF = 1.1V R1 Figure 2. Adjustable Output Voltage leave unconnected (3.3V output voltage). Internal resistor-dividers divide down the output voltage, regulating the divided voltage to the internal reference voltage. For output voltages other than 2.5V or 3.3V, or for tighter AC load regulation, connect FBSEL to GND (1% regulation) or to REF (2% regulation), and connect FB to a resistor divider between the output voltage and ground (Figure 2). Regulation is maintained for adjustable output voltages when VFB equals VREF. Use 50kΩ for R1. R2 is given by the equation: ⎛V ⎞ R2 = R1 ⎜ OUT − 1⎟ V ⎝ REF ⎠ where VREF is typically 1.1V. Programming the Switching Frequency and Off-Time The MAX1644 features a programmable PWM mode switching frequency, which is set by the input and output voltage and the value of R TOFF, 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 your desired switching frequency in PWM mode (IOUT > 0.2A): t OFF = where: (VIN – VOUT − VPMOS ) fPWM ( VIN − VPMOS + VNMOS ) tOFF = the programmed off-time VIN = the input voltage VOUT = the output voltage VNMOS = the voltage drop across the internal PMOS power switch VPMOS = the voltage drop across the internal NMOS synchronous-rectifier switch _______________________________________________________________________________________ 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches Inductor Selection Three key inductor parameters must be specified: inductor value (L), peak current (IPEAK), and DC resistance (RDC). The following equation includes a constant, denoted as LIR, which is the ratio of peakto-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 loadcurrent ratio (LIR = 0.25), which corresponds to a peak inductor current 1.125 times higher than the DC load current: L = VOUT × tOFF 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 · IOUT if the above equation is used; otherwise, the peak current is calculated by: IPEAK = IOUT + 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: ( VOUT VIN − VOUT ESR > 1% × L tOFF Stable operation requires the correct output filter capacitor. When choosing the output capacitor, ensure that: COUT ≥ (tOFF / VOUT) ✕ (64µFV / µs) With an AC load regulation setting of 1%, the COUT requirement doubles, and the minimum ESR of the output capacitor is halved. 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. Setting the AC Loop Gain VOUT × tOFF 2 × L Choose an inductor with a saturation current at least as high as the peak inductor current. To minimize loss, choose an inductor with a low DC resistance. IRIPPLE = ILOAD The output filter capacitor affects the output voltage ripple, output load-transient response, and feedback loop stability. For stable operation, the MAX1644 requires a minimum output ripple voltage of VRIPPLE ≥ 2% · VOUT (with 2% load regulation setting). The minimum ESR of the output capacitor should be: The MAX1644 allows selection of a 1% or 2% AC loadregulation error when the adjustable output voltage mode is selected (Table 2). A 2% setting is automatically selected in preset output voltage mode (FBSEL connected to VCC or unconnected). A 2% load-regulation error setting reduces output filter capacitor requirements, allowing the use of smaller and less expensive capacitors. Selecting a 1% load-regulation error reduces transient load errors, but requires larger capacitors. ) VIN _______________________________________________________________________________________ 9 MAX1644 f PWM = switching frequency in PWM mode (IOUT > 0.2A) Select RTOFF according to the formula: RTOFF = (tOFF - 0.07µs) (150kΩ / 1.26µs) Recommended values for RTOFF range from 39kΩ to 470kΩ for off-times of 0.4µs to 4µs. MAX1644 2A Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches Soft-Start Soft-start allows a gradual increase of the internal current limit to reduce input surge currents at start-up and at exit from shutdown. A charging capacitor, C SS , 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 5µA 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 2.9A. The voltage across the soft-start capacitor changes with time according to the equation: VSS = 5μA × t CSS The soft-start current limit varies with the voltage on the soft-start pin, SS, according to the equation: ILIMIT = (VSS - 0.7V) · 2.7A/V, for VSS > 0.7V The constant-current source stops charging once the voltage across the soft-start capacitor reaches 1.8V (Figure 3). Circuit Layout and Grounding Good layout is necessary to achieve the MAX1644’s intended output power level, high efficiency, and low noise. Good layout includes the use of a ground plane, appropriate 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 together. 10 SHDN 0 1.8V VSS (V) 0.7V 0 2.9A ILIMIT (A) 0 t Figure 3. Soft-Start Current Limit over Time 2) Connect the input filter capacitor less than 5mm away from IN. The connecting copper trace carries large currents and must be at least 2mm wide, preferably 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. ___________________Chip Information TRANSISTOR COUNT: 1758 ______________________________________________________________________________________ 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches SSOP.EPS 2 1 INCHES E H MILLIMETERS DIM MIN MAX MIN MAX A 0.068 0.078 1.73 1.99 A1 0.002 0.008 0.05 0.21 B 0.010 0.015 0.25 0.38 C D 0.20 0.09 0.004 0.008 SEE VARIATIONS E 0.205 e 0.212 0.0256 BSC 5.20 MILLIMETERS INCHES D D D D D 5.38 MIN MAX MIN MAX 0.239 0.239 0.278 0.249 0.249 0.289 6.07 6.07 7.07 6.33 6.33 7.33 0.317 0.397 0.328 0.407 8.07 10.07 8.33 10.33 N 14L 16L 20L 24L 28L 0.65 BSC H 0.301 0.311 7.65 7.90 L 0.025 0∞ 0.037 8∞ 0.63 0∞ 0.95 8∞ N A C B e L A1 D NOTES: 1. D&E DO NOT INCLUDE MOLD FLASH. 2. MOLD FLASH OR PROTRUSIONS NOT TO EXCEED .15 MM (.006"). 3. CONTROLLING DIMENSION: MILLIMETERS. 4. MEETS JEDEC MO150. 5. LEADS TO BE COPLANAR WITHIN 0.10 MM. PROPRIETARY INFORMATION TITLE: PACKAGE OUTLINE, SSOP, 5.3 MM APPROVAL DOCUMENT CONTROL NO. 21-0056 REV. C 1 1 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 ____________________ 11 © 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc. MAX1644 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.)