19-3336; Rev 0; 7/04 KIT ATION EVALU E L B AVAILA 16µA IQ, 1.2A PWM Step-Down DC-DC Converters The MAX1556/MAX1557 are low-operating-current (16µA), fixed-frequency step-down regulators. High efficiency, low-quiescent operating current, low dropout, and minimal (27µA) quiescent current in dropout make these converters ideal for powering portable devices from 1-cell Li-ion or 3-cell alkaline/NiMH batteries. The MAX1556 delivers up to 1.2A; has pin-selectable 1.8V, 2.5V, and 3.3V outputs; and is also adjustable. The MAX1557 delivers up to 600mA; has pin-selectable 1V, 1.3V, and 1.5V outputs; and is also adjustable. The MAX1556/MAX1557 contain a low-on-resistance internal MOSFET switch and synchronous rectifier to maximize efficiency and dropout performance while minimizing external component count. A proprietary topology offers the benefits of a high fixed-frequency operation while still providing excellent efficiency at both light and full loads. A 1MHz PWM switching frequency keeps components small. Both devices also feature an adjustable soft-start to minimize battery transient loading. Features ♦ Up to 97% Efficiency ♦ 95% Efficiency at 1mA Load Current ♦ Low 16µA Quiescent Current ♦ 1MHz PWM Switching ♦ Tiny 3.3µH Inductor ♦ Selectable 3.3V, 2.5V, 1.8V, 1.5V, 1.3V, 1.0V, and Adjustable Output ♦ 1.2A Guaranteed Output Current (MAX1556) ♦ Voltage Positioning Optimizes Load-Transient Response ♦ Low 27µA Quiescent Current in Dropout ♦ Low 0.1µA Shutdown Current ♦ No External Schottky Diode Required ♦ Analog Soft-Start with Zero Overshoot Current ♦ Small, 10-Pin, 3mm x 3mm TDFN Package The MAX1556/MAX1557 are available in a tiny 10-pin TDFN (3mm x 3mm) package. Applications PDAs and Palmtop Computers Ordering Information TEMP RANGE PIN-PACKAGE MAX1556ETB -40°C to +85°C 10 TDFN-EP* (T1033-1) ACQ MAX1557ETB -40°C to +85°C 10 TDFN-EP* (T1033-1) ACR Cell Phones and Smart Phones Digital Cameras and Camcorders Portable MP3 and DVD Players Hand-Held Instruments TOP MARK PART *EP = Exposed paddle. Pin Configuration Typical Operating Circuit INPUT 2.6V TO 5.5V OUTPUT 0.75V TO VIN INP LX MAX1556/ MAX1557 PGND D1 OFF 1 GND 2 3 OUT OUT 4 SS SHDN 5 D2 ON IN SS IN VOLTAGE SELECT TOP VIEW MAX1556/ MAX1557 10 D1 9 INP 8 LX 7 PGND 6 D2 SHDN GND TDFN ________________________________________________________________ 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 MAX1556/MAX1557 General Description MAX1556/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters ABSOLUTE MAXIMUM RATINGS IN, INP, OUT, D2, SHDN to GND ..........................-0.3V to +6.0V SS, D1 to GND .............................................-0.3V to (VIN + 0.3V) PGND to GND .......................................................-0.3V to +0.3V LX Current (Note 1)...........................................................±2.25A Output Short-Circuit Duration.....................................Continuous Continuous Power Dissipation (TA = +70°C) 10-Pin TDFN (derate 24.4mW/°C above +70°C) .......1951mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Note 1: LX has internal clamp diodes to GND 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 = VINP = VSHDN = 3.6V, TA = - 40°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS Input Voltage Undervoltage-Lockout Threshold Quiescent Supply Current Shutdown Supply Current UNITS V 2.35 2.55 V No switching, D1 = D2 = GND 16 25 Dropout 27 42 TA = +25°C 0.1 1 TA = +85°C 0.1 VIN rising and falling, 35mV hysteresis (typ) SHDN = GND 2.20 0.75 Output Accuracy TA = -40°C to +85°C (Note 2) No load -0.25 300mA load 600mA load +1.75 -0.75 0 +0.75 -1.5 -0.75 0 1200mA load, MAX1556 only -2.75 -2.25 -1.25 No load -0.75 -1.5 +1.5 600mA load -2.25 +0.50 -4.0 -1.0 1200 MAX1557 600 FB Threshold Accuracy D1 = D2 = GND, VOUT = 0.75V at 300mA (typ), TA = -40°C to +85°C µA µA V % mA TA = +25°C 0.01 TA = +85°C 0.01 For preset output voltages D1 = D2 = GND, VOUT = 0.75V at 300mA (typ), TA = 0°C to +85°C +2.25 300mA load MAX1556 D1 = D2 = GND VIN +0.75 1200mA load, MAX1556 only 2 MAX 5.5 TA = 0°C to +85°C (Note 2) OUT Bias Current TYP 2.6 Output Voltage Range Maximum Output Current MIN 0.1 µA 3 4.5 No load -0.50 +0.75 +1.75 300mA load -1.2 0 +1.2 +0.25 600mA load -1.75 -0.75 1200mA load, MAX1556 only -3.25 -2.25 No load -1.25 +2.25 300mA load -1.75 +1.50 600mA load -2.75 +0.25 1200mA load, MAX1556 only -4.25 -1.00 _______________________________________________________________________________________ -1.25 % 16µA IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1557 ELECTRICAL CHARACTERISTICS (continued) (VIN = VINP = VSHDN = 3.6V, TA = - 40°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS MAX1556, D1 = IN, D2 = GND Line Regulation MAX1557, D1 = IN, D2 = GND MAX1556 p-Channel On-Resistance MAX1557 n-Channel On-Resistance p-Channel Current-Limit Threshold VIN = 3.6V to 5.5V 0.33 VIN = 2.6V to 3.6V -0.1 VIN = 3.6V to 5.5V 0.09 VIN = 3.6V 0.19 VIN = 2.6V 0.23 VIN = 3.6V 0.35 VIN = 2.6V 0.42 VIN = 3.6V 0.27 VIN = 2.6V 0.33 MAX 0.35 0.7 0.48 1.5 1.8 2.1 MAX1557 0.8 1.0 1.2 20 35 45 MAX1556 1.8 MAX1557 1.0 TA = +25°C 0.1 TA = +85°C 0.1 Maximum Duty Cycle 10 100 SS Output Impedance ∆VSS / ISS for ISS = 2µA SS Discharge Resistance SHDN = GND, 1mA sink current Ω Ω A mA ARMS µA % Minimum Duty Cycle Internal Oscillator Frequency UNITS % MAX1556 VIN = 5.5V, LX = GND or IN LX Leakage Current TYP -0.37 n-Channel Zero Crossing Threshold RMS LX Output Current MIN VIN = 2.6V to 3.6V 0 % 0.9 1 1.1 MHz 130 200 300 kΩ 90 200 Ω Thermal-Shutdown Threshold +160 °C Thermal-Shutdown Hysteresis 15 °C LOGIC INPUTS (D1, D2, SHDN) Input-Voltage High 2.6V ≤ VIN ≤ 5.5V 1.4 V Input-Voltage Low Input Leakage 0.4 TA = +25°C 0.1 TA = +85°C 0.1 1 V µA Note 1: All units are 100% production tested at TA = +25°C. Limits over the operating range are guaranteed by design. Note 2: For the MAX1556, 3.3V output accuracy is specified with a 4.2V input. _______________________________________________________________________________________ 3 Typical Operating Characteristics (VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT WITH 2.5V OUTPUT EFFICIENCY (%) 70 60 80 70 40 VIN = 2.6V 60 1 10 100 1000 70 VIN = 5V VIN = 3.6V VIN = 3V 60 1 10 100 1000 10,000 0.1 1 10 10,000 LOAD CURRENT (mA) LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT WITH 1.0V OUTPUT (MAX1557) OUTPUT VOLTAGE vs. LOAD CURRENT OUTPUT VOLTAGE vs. INPUT VOLTAGE WITH 600mA LOAD 70 VIN = 3V VIN = 2.6V 1.81 1.80 1.79 TA = +25°C 1.78 1.77 TA = +85°C 1.76 50 1 10 200 TA = -40°C 1.809 TA = +25°C 1.808 1.807 1.806 TA = +85°C 1.805 600 800 1000 1200 TA = +25°C 1.783 1.782 TA = +85°C 2.5 3.0 3.5 4.0 MAX1556/7 toc09 16 ILOAD = 750mA VOUT AC-COUPLED 10mV/div 14 12 VLX 2V/div 0 10 8 6 ILX 500mA/div 0 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 5.0 5.5 5.5 HEAVY-LOAD SWITCHING WAVEFORMS 2 1.803 5.0 SUPPLY CURRENT vs. INPUT VOLTAGE 4 1.804 4.5 INPUT VOLTAGE (V) 18 SUPPLY CURRENT (µA) 1.810 TA = -40°C 1.784 MAX1556/7 toc08 1.811 400 20 MAX1556/7 toc07 1.812 1.785 LOAD CURRENT (mA) LOAD CURRENT (mA) OUTPUT VOLTAGE vs. INPUT VOLTAGE WITH NO LOAD 1.786 1.779 0 1000 100 1.787 1.780 1.74 0.1 1.788 1.781 1.75 40 1.789 OUTPUT VOLTAGE (V) VIN = 5V TA = -45°C 1.82 OUTPUT VOLTAGE (V) 80 VIN = 3.6V 1.83 MAX1556/7 toc06 1.84 MAX1556/7 toc04 90 2.5 1000 100 LOAD CURRENT (mA) 100 60 VIN = 2.6V 40 0.1 10,000 80 50 40 0.1 4 90 50 50 EFFICIENCY (%) VIN = 5V VIN = 3.6V VIN = 3V MAX1556/7 toc05 EFFICIENCY (%) VIN = 3.6V 80 90 EFFICIENCY (%) VIN = 5V 100 MAX1556/7 toc02 90 VIN = 4.2V 100 MAX1556/7 toc01 100 EFFICIENCY vs. LOAD CURRENT WITH 1.8V OUTPUT MAX1556/7 toc03 EFFICIENCY vs. LOAD CURRENT WITH 3.3V OUTPUT OUTPUT VOLTAGE (V) MAX1556/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 0 1 2 3 4 5 6 400ns INPUT VOLTAGE (V) _______________________________________________________________________________________ 16µA IQ, 1.2A PWM DC-DC Step-Down Converters (VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.) SOFT-START/SHUTDOWN WAVEFORMS MAX1556/7 toc10 MAX1556/7 toc11 VOUT 1V/div 0 CSS = 470pF RLOAD = 4Ω 2V/div VLX 0 ILX 500mA/div 0 IIN 500mA/div 0 200mA/div 0 ILX SOFT-START RAMP TIME (ms) 5V/div 0 VSHDN 20mV/div AC-COUPLED VOUT SOFT-START RAMP TIME vs. CSS 10 MAX1556/7 toc12 LIGHT-LOAD SWITCHING WAVEFORMS 1 0.1 4µs/div 0 100µs/div 500 1000 1500 2000 2500 CSS (pF) LOAD TRANSIENT LOAD TRANSIENT MAX1556/7 toc13 MAX1556/7 toc14 50mV/div AC-COUPLED VOUT 50mV/div AC-COUPLED VOUT 500mA/div 500mA/div 0 0 IOUT IOUT IOUTMIN = 20mA IOUTMIN = 180mA 20µs/div 20µs/div BODE PLOT MAX1556/7 toc15 4V VIN 10mV/div AC-COUPLED 200mA/div ILX GAIN (dB) 3.5V VOUT MAX1556/7 toc16 40 210 20 180 10 150 0 120 -10 90 0dB -20 40µs/div 60 PHASE MARGIN = 53° -30 30 -40 0 -50 0 240 30 PHASE (DEGREES) LINE TRANSIENT -30 COUT = 22µF, RLOAD = 4Ω -60 0.1 1 10 100 -60 1000 FREQUENCY (kHz) _______________________________________________________________________________________ 5 MAX1556/MAX1557 Typical Operating Characteristics (continued) 16µA IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1557 Pin Description PIN NAME 1 IN 2 GND FUNCTION Supply Voltage Input. Connect to a 2.6V to 5.5V source. Ground. Connect to PGND. Soft-Start Control. Connect a 1000pF capacitor (CSS) from SS to GND to eliminate input-current overshoot during startup. CSS is required for normal operation of the MAX1556/MAX1557. For greater than 22µF total output capacitance, increase CSS to COUT / 22,000 for soft-start. SS is internally discharged through 200Ω to GND in shutdown. 3 SS 4 OUT Output Sense Input. Connect to the output of the regulator. D1 and D2 select the desired output voltage through an internal feedback resistor-divider. The internal feedback resistor-divider remains connected in shutdown. 5 SHDN Shutdown Input. Drive SHDN low to enable low-power shutdown mode. Drive high or connect to IN for normal operation. 6 D2 7 PGND OUT Voltage-Select Input. See Table 1. 8 LX Inductor Connection. Connected to the drains of the internal power MOSFETs. High impedance in shutdown mode. 9 INP Supply Voltage, High-Current Input. Connect to a 2.6V to 5.5V source. Bypass with a 10µF ceramic capacitor to PGND. 10 D1 OUT Voltage-Select Input. See Table 1. EP — Exposed Paddle. Connect to ground plane. EP also functions as a heatsink. Solder to circuit-board ground plane to maximize thermal dissipation. Power Ground. Connect to GND. Table 1. Output-Voltage-Select Truth Table D1 D2 MAX1556 VOUT MAX1557 VOUT 0 0 0 1 3.3V 1.5V 1 0 2.5V 1.3V 1 1 1.8V 1.0V Adjustable from 0.75V to VIN A zero represents D_ being driven low or connected to GND. A 1 represents D_ being driven high or connected to IN. Detailed Description The MAX1556/MAX1557 synchronous step-down converters deliver a guaranteed 1.2A/600mA at output voltages from 0.75V to V IN . They use a 1MHz PWM current-mode control scheme with internal compensation, allowing for tiny external components and a fast transient response. At light loads the MAX1556/MAX1557 automatically switch to pulse-skipping mode to keep the quiescent supply current as low as 16µA. Figures 2 and 3 show the typical application circuits. 6 Control Scheme During PWM operation the converters use a fixed-frequency, current-mode control scheme. The heart of the current-mode PWM controller is an open-loop, multipleinput comparator that compares the error-amp voltage feedback signal against the sum of the amplified current-sense signal and the slope-compensation ramp. At the beginning of each clock cycle, the internal high-side p-channel MOSFET turns on until the PWM comparator trips. During this time the current in the inductor ramps up, sourcing current to the output and storing energy in the inductor’s magnetic field. When the p-channel turns off, the internal low-side n-channel MOSFET turns on. Now the inductor releases the stored energy while the current ramps down, still providing current to the output. The output capacitor stores charge when the inductor current exceeds the load and discharges when the inductor current is lower than the load. Under overload conditions, when the inductor current exceeds the current limit, the high-side MOSFET is turned off and the low-side MOSFET remains on until the next clock cycle. _______________________________________________________________________________________ 16µA IQ, 1.2A PWM DC-DC Step-Down Converters BIAS CURRENT-LIMIT COMPARATOR VCS CURRENT SENSE MAX1556/MAX1557 IN SHDN SHORT-CIRCUIT PROTECTION CLOCK 1MHz 0.675V PWM COMPARATOR INP PWM AUTO SKIP CONTROL LX SLOPE COMP PGND SKIP-OVER ENTER SKIP/ SR OFF ZERO-CROSS DETECT ERROR AMPLIFIER OUT REFERENCE 1.25V GND D1 OUTPUT VOLTAGE SELECTOR D2 MAX1556 MAX1557 SS Figure 1. Functional Diagram L1 3.3µH INPUT 2.6V TO 5.5V INP R1 100Ω C1 10µF LX VOLTAGE SELECT INP D1 VOLTAGE SELECT OUT SHDN GND Figure 2. MAX1556 Typical Application Circuit C5 22µF MAX1557 IN D2 SS LX OUTPUT 0.75V TO VIN 600mA PGND PGND ON OFF L2 4.7µH INPUT 2.6V TO 5.5V C4 10µF C2 22µF MAX1556 IN C4 0.47µF OUTPUT 0.75V TO VIN 1.2A C3 1000pF D1 OUT D2 SS ON OFF SHDN C6 1000pF GND Figure 3. MAX1557 Typical Application Circuit _______________________________________________________________________________________ 7 Load-Transient Response/ Voltage Positioning The MAX1556/MAX1557 match the load regulation to the voltage droop seen during transients. This is sometimes called voltage positioning. The load line used to achieve this behavior is shown in Figures 4 and 5. There is minimal overshoot when the load is removed and minimal voltage drop during a transition from light load to full load. Additionally, the MAX1556 and MAX1557 use a wide-bandwidth feedback loop to respond more quickly to a load transient than regulators using conventional integrating feedback loops (see Load Transient in the Typical Operating Characteristics). The MAX1556/MAX1557 use of a wide-band control loop and voltage positioning allows superior load-transient response by minimizing the amplitude and duration of overshoot and undershoot in response to load transients. Other DC-DC converters, with high gaincontrol loops, use external compensation to maintain tight DC load regulation but still allow large voltage droops of 5% or greater for several hundreds of microseconds during transients. For example, if the load is a CPU running at 600MHz, then a dip lasting 100µs corresponds to 60,000 CPU clock cycles. Voltage positioning on the MAX1556/MAX1557 allows up to 2.25% (typ) of load-regulation voltage shift but has no further transient droop. Thus, during load transients, the voltage delivered to the CPU remains within spec more effectively than with other regulators that might have tighter initial DC accuracy. In summary, a 2.25% load regulation with no transient droop is much better than a converter with 0.5% load regulation and 5% or more of voltage droop during load transients. Load-transient variation can be seen only with an oscilloscope (see the Typical Operating Characteristics), while DC load regulation read by a voltmeter does not show how the power supply reacts to load transients. Dropout/100% Duty-Cycle Operation The MAX1556/MAX1557 function with a low input-to-output voltage difference by operating at 100% duty cycle. In this state, the high-side p-channel MOSFET is always on. This is particularly useful in battery-powered applications with a 3.3V output. The system and load might 8 1.0 CHANGE IN OUTPUT VOLTAGE (%) As the load current decreases, the converters enter a pulse-skip mode in which the PWM comparator is disabled. At light loads, efficency is enhanced by a pulse-skip mode in which switching occurs only as needed to service the load. Quiescent current in skip mode is typically 16µA. See the Light-Load Switching Waveforms and Load Transient graphs in the Typical Operating Characteristics. 0.5 0 VIN = 3.6V VIN = 5.5V -0.5 -1.0 VIN = 2.6V -1.5 -2.0 -2.5 0 200 400 600 800 1000 1200 LOAD CURRENT (mA) Figure 4. MAX1556 Voltage-Positioning Load Line 1.0 0.8 CHANGE IN OUTPUT VOLTAGE (%) MAX1556/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 0.6 0.4 VIN = 3.6V 0.2 VIN = 5.5V 0 -0.2 VIN = 2.6V -0.4 -0.6 -0.8 -1.0 0 200 400 600 LOAD CURRENT (mA) Figure 5. MAX1557 Voltage-Positioning Load Line operate normally down to 3V or less. The MAX1556/ MAX1557 allow the output to follow the input battery voltage as it drops below the regulation voltage. The quiescent current in this state rises minimally to only 27µA (typ), which aids in extending battery life. This dropout/100% duty-cycle operation achieves long battery life by taking full advantage of the entire battery range. The input voltage required to maintain regulation is a function of the output voltage and the load. The difference between this minimum input voltage and the output voltage is called the dropout voltage. The dropout voltage is therefore a function of the on-resistance of the internal p-channel MOSFET (RDS(ON)P ) and the inductor resistance (DCR). VDROPOUT = IOUT x (RDS(ON)P + DCR) _______________________________________________________________________________________ 16µA IQ, 1.2A PWM DC-DC Step-Down Converters MANUFACTURER PART VALUE (µH) DCR (mΩ) ISAT (mA) SIZE (mm) SHIELDED Taiyo Yuden LMNP04SB3R3N 3.3 36 1300 5 x 5 x 2.0 Yes Taiyo Yuden LMNP04SB4R7N 4.7 50 1200 5 x 5 x 2.0 Yes TOKO D52LC 3.5 73 1340 5 x 5 x 2.0 Yes TOKO D52LC 4.7 87 1140 5 x 5 x 2.0 Yes Sumida CDRH3D16 4.7 50 1200 3.8 x 3.8 x 1.8 Yes TOKO D412F 4.7 100* 1200* 4.8 x 4.8 x 1.2 Yes Murata LQH32CN 4.7 97 790 2.5 x 3.2 x 2.0 No Sumitomo CXL180 4.7 70* 1000* 3.0 x 3.2 x 1.7 No Sumitomo CXLD140 4.7 100* 800* 2.8 x 3.2 x 1.5 No *Estimated based upon similar-valued prototype inductors. (RDS(ON)P) is given in the Electrical Characteristics. DCR for a few recommended inductors is listed in Table 2. Soft-Start The MAX1556/MAX1557 use soft-start to eliminate inrush current during startup, reducing transients at the input source. Soft-start is particularly useful for higherimpedance input sources such as Li+ and alkaline cells. Connect the required soft-start capacitor from SS to GND. For most applications using a 22µF output capacitor, connect a 1000pF capacitor from SS to GND. If a larger output capacitor is used, then use the following formula to find the value of the soft-start capacitor: CSS = COUT 22000 Soft-start is implemented by exponentially ramping up the output voltage from 0 to VOUT(NOM) with a time constant equal to C SS times 200kΩ (see the Typical Operating Characteristics). Assuming three time constants to full output voltage, use the following formula to calculate the soft-start time: t SS = 600 x 103 x CSS thermal shutdown. In this mode the internal p-channel switch and the internal n-channel synchronous rectifier are turned off. The device resumes normal operation when the junction temperature falls below +145°C. Applications Information The MAX1556/MAX1557 are optimized for use with small external components. The correct selection of inductors and input and output capacitors ensures high efficiency, low output ripple, and fast transient response. Adjusting the Output Voltage The adjustable output is selected when D1 = D2 = 0 and an external resistor-divider is used to set the output voltage (see Figure 6). The MAX1556/MAX1557 have a defined line- and load-regulation slope. The load regulation is for both preset and adjustable outputs and is described in the Electrical Characteristics table and Figures 4 and 5. The impact of the line-regulation slope can be reduced by applying a correction factor to the feedback resistor equation. First, calculate the correction factor, k, by plugging the desired output voltage into the following formula: V − 0.75V k = 1.06 x 10−2 V x OUTPUT 3.6V Shutdown Mode Connecting SHDN to GND or logic low places the MAX1556/MAX1557 in shutdown mode and reduces supply current to 0.1µA. In shutdown, the control circuitry and the internal p-channel and n-channel MOSFETs turn off and LX becomes high impedance. Connect SHDN to IN or logic high for normal operation. Thermal Shutdown k represents the shift in the operating point at the feedback node (OUT). Select the lower feedback resistor, R3, to be ≤35.7kΩ to ensure stability and solve for R2: 0.75V − k V = OUTPUT R3 (R3 + R2) As soon as the junction temperature of the MAX1556/MAX1557 exceeds +160°C, the ICs go into _______________________________________________________________________________________ 9 MAX1556/MAX1557 Table 2. Inductor Selection MAX1556/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters Inductor Selection A 4.7µH inductor with a saturation current of at least 800mA is recommended for the MAX1557 full-load (600mA) application. For the MAX1556 application with 1.2A full load, use a 3.3µH inductor with at least 1.34A saturation current. For lower full-load currents the inductor current rating can be reduced. For maximum efficiency, the inductor’s resistance (DCR) should be as low as possible. Please note that the core material differs among different manufacturers and inductor types and has an impact on the efficiency. See Table 2 for recommended inductors and manufacturers. OUTPUT R2 ERROR AMPLIFIER OUT R3 REFERENCE 1.25V Capacitor Selection Ceramic input and output capacitors are recommended for most applications. For best stability over a wide temperature range, use capacitors with an X5R or better dielectric due to their small size, low ESR, and low temperature coefficients. Output Capacitor The output capacitor COUT is required to keep the output voltage ripple small and to ensure regulation loop stability. COUT must have low impedance at the switching frequency. A 22µF ceramic output capacitor is recommended for most applications. If a larger output capacitor is used, then paralleling smaller capacitors is suggested to keep the effective impedance of the capacitor low at the switching frequency. Input Capacitor Due to the pulsating nature of the input current in a buck converter, a low-ESR input capacitor at INP is required for input voltage filtering and to minimize interference with other circuits. The impedance of the input capacitor CINP should be kept very low at the switching frequency. A minimum value of 10µF is recommended at INP for most applications. The input capacitor can be increased for better input filtering. IN Input Filter In all MAX1557 applications, connect INP directly to IN and bypass INP as described in the Input Capacitor section. No additional bypass capacitor is required at IN. For applications using the MAX1556, an RC filter between INP and IN keeps power-supply noise from entering the IC. Connect a 100Ω resistor between INP and IN, and connect a 0.47µF capacitor from IN to GND. SS Figure 6. Adjustable Output Voltage PC Board Layout and Routing Due to fast-switching waveforms and high-current paths, careful PC board layout is required. An evaluation kit (MAX1556EVKIT) is available to speed design. When laying out a board, minimize trace lengths between the IC, the inductor, the input capacitor, and the output capacitor. Keep these traces short, direct, and wide. Keep noisy traces, such as the LX node trace, away from OUT. The input bypass capacitors should be placed as close to the IC as possible. Connect GND to the exposed paddle and star PGND and GND together at the output capacitor. The ground connections of the input and output capacitors should be as close together as possible. Chip Information TRANSISTOR COUNT: 7567 PROCESS: BiCMOS Soft-Start Capacitor The soft-start capacitor, CSS, is required for proper operation of the MAX1556/MAX1557. The recommended value of CSS is discussed in the Soft-Start section. Soft-start times for various soft-start capacitors are shown in the Typical Operating Characteristics. 10 ______________________________________________________________________________________ 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 6, 8, &10L, DFN THIN.EPS D N PIN 1 INDEX AREA E E2 DETAIL A CL CL L A L e e PACKAGE OUTLINE, 6, 8, 10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm NUMBER OF LEADS SHOWN ARE FOR REFERENCE ONLY 21-0137 F 1 2 ______________________________________________________________________________________ 11 MAX1556/MAX1557 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. MAX1556/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 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. COMMON DIMENSIONS SYMBOL A MIN. MAX. 0.70 0.80 D 2.90 3.10 E 2.90 3.10 A1 L 0.00 0.05 k 0.40 0.20 0.25 MIN. A2 0.20 REF. PACKAGE VARIATIONS PKG. CODE N D2 E2 e JEDEC SPEC b [(N/2)-1] x e T633-1 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF T833-1 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF T1033-1 10 1.50±0.10 2.30±0.10 0.50 BSC MO229 / WEED-3 0.25±0.05 2.00 REF T1433-1 14 1.70±0.10 2.30±0.10 0.40 BSC ---- 0.20±0.03 2.40 REF T1433-2 14 1.70±0.10 2.30±0.10 0.40 BSC ---- 0.20±0.03 2.40 REF PACKAGE OUTLINE, 6, 8, 10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm 21-0137 F 2 2 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. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.