19-3336; Rev 3; 1/11 KIT ATION EVALU E L B A AVAIL 16µA IQ, 1.2A PWM Step-Down DC-DC Converters The MAX1556/MAX1556A/MAX1557 are low-operatingcurrent (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 pinselectable 1.8V, 2.5V, and 3.3V outputs; and is also adjustable. The MAX1557 delivers up to 600mA; has pinselectable 1V, 1.3V, and 1.5V outputs; and is also adjustable. The MAX1556/MAX1556A/MAX1557 contain a low-onresistance 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 fixedfrequency 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. The MAX1556/MAX1556A/MAX1557 are available in a tiny 10-pin TDFN (3mm x 3mm) package. Applications Features o Up to 97% Efficiency o 95% Efficiency at 1mA Load Current o Low 16µA Quiescent Current o 1MHz PWM Switching o Tiny 3.3µH Inductor o Selectable 3.3V, 2.5V, 1.8V, 1.5V, 1.3V, 1.2V, 1.0V, and Adjustable Output o 1.2A Guaranteed Output Current (MAX1556/MAX1556A) o Voltage Positioning Optimizes Load-Transient Response o o o o Low 27µA Quiescent Current in Dropout Low 0.1µA Shutdown Current No External Schottky Diode Required Analog Soft-Start with Zero Overshoot Current o Small, 10-Pin, 3mm x 3mm TDFN Package Ordering Information PART ACQ MAX1556ETB/V+T -40°C to +85°C 10 TDFN-EP* AWS MAX1556AETB+ -40°C to +85°C 10 TDFN-EP* AUJ MAX1557ETB+ -40°C to +85°C 10 TDFN-EP* ACR MAX1556ETB+ Cell Phones and Smart Phones Digital Cameras and Camcorders *EP = Exposed paddle. +Denotes a lead(Pb)-free/RoHS-compliant package. /V denotes an automotive qualified part. T = Tape and reel. Hand-Held Instruments Pin Configuration Typical Operating Circuit INPUT 2.6V TO 5.5V OUTPUT 0.75V TO VIN INP IN VOLTAGE SELECT TOP VIEW LX MAX1556 MAX1556A MAX1557 PGND D1 OUT D2 ON OFF TOP MARK -40°C to +85°C 10 TDFN-EP* PDAs and Palmtop Computers Portable MP3 and DVD Players TEMP RANGE PIN-PACKAGE SS IN 1 10 D1 GND 2 9 INP SS 3 OUT 4 SHDN 5 MAX1556 MAX1556A MAX1557 8 LX 7 PGND 6 D2 SHDN GND TDFN ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1556/MAX1556A/MAX1557 General Description MAX1556/MAX1556A/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 Soldering Temperature (reflow) .......................................+260°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 2) PARAMETER CONDITIONS Input Voltage Undervoltage-Lockout Threshold Quiescent Supply Current Shutdown Supply Current VIN rising and falling, 35mV hysteresis (typ) V 2.55 25 Dropout 27 42 TA = +25°C 0.1 1 TA = +85°C 0.1 0.75 -0.25 +0.75 +1.75 300mA load -0.75 0 +0.75 600mA load -1.5 -0.75 0 1200mA load, MAX1556 -2.75 -2.25 -1.25 1200mA load, MAX1556A TA = -40°C to +85°C (Note 3) -2.25 -0.75 -1.5 +1.5 -2.25 +0.50 1200mA load, MAX1556 -4.0 -1.0 1200 600 V mA TA = +25°C 0.01 TA = +85°C 0.01 For preset output voltages µA % 600mA load MAX1557 µA +2.25 300mA load MAX1556/MAX1556A D1 = D2 = GND MAX1556/MAX1557 VIN No load No load 2 V 16 Output Accuracy FB Threshold Accuracy UNITS 5.5 2.35 SHDN = GND 2.20 MAX No switching, D1 = D2 = GND TA = 0°C to +85°C (Note 3) OUT Bias Current TYP 2.6 Output Voltage Range Maximum Output Current MIN 0.1 µA 3 4.5 D1 = D2 = GND, VOUT = 0.75V at 300mA (typ), TA = 0°C to +85°C MAX1556/MAX1557 No load -0.50 +0.75 +1.75 300mA load -1.2 0 +1.2 600mA load -1.75 -0.75 +0.25 1200mA load, MAX1556 only -3.25 -2.25 -1.25 D1 = D2 = GND, VOUT = 0.75V at 300mA (typ), TA = -40°C to +85°C MAX1556/MAX1557 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 _______________________________________________________________________________________ % 16µA IQ, 1.2A PWM DC-DC Step-Down Converters (VIN = VINP = VSHDN = 3.6V, TA = -40°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 2) PARAMETER CONDITIONS MAX1556, D1 = IN, D2 = GND; MAX1556A D1 = D2 = IN Line Regulation MAX1557, D1 = IN, D2 = GND MAX1556/MAX1556A p-Channel On-Resistance MAX1557 n-Channel On-Resistance p-Channel Current-Limit Threshold LX Leakage Current TYP VIN = 2.6V to 3.6V -0.37 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 MAX 0.27 VIN = 2.6V 0.33 % 0.35 0.7 0.48 MAX1556/MAX1556A 1.5 1.8 2.1 MAX1557 0.8 1.0 1.2 20 35 45 MAX1556/MAX1556A 1.8 MAX1557 1.0 VIN = 5.5V, LX = GND or IN 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 0.42 VIN = 3.6V n-Channel Zero Crossing Threshold RMS LX Output Current MIN 0 % 1.1 MHz kΩ 0.9 1 130 200 300 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 2: All units are 100% production tested at TA = +25°C. Limits over the operating range are guaranteed by design. Note 3: For the MAX1556, 3.3V output accuracy is specified with a 4.2V input. _______________________________________________________________________________________ 3 MAX1556/MAX1556A/MAX1557 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.) 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 0.1 10,000 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 ILOAD = 750mA VOUT AC-COUPLED 10mV/div 14 12 VLX 2V/div 0 10 8 6 2 1.803 0 4.0 4.5 INPUT VOLTAGE (V) 5.0 5.5 5.5 HEAVY-LOAD SWITCHING WAVEFORMS 16 1.804 3.5 5.0 SUPPLY CURRENT vs. INPUT VOLTAGE ILX 4 3.0 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 90 EFFICIENCY (%) VIN = 5V 100 MAX1556/7 toc02 90 80 100 MAX1556/7 toc01 100 VIN = 4.2V EFFICIENCY vs. LOAD CURRENT WITH 1.8V OUTPUT EFFICIENCY vs. LOAD CURRENT WITH 2.5V OUTPUT MAX1556/7 toc03 EFFICIENCY vs. LOAD CURRENT WITH 3.3V OUTPUT OUTPUT VOLTAGE (V) MAX1556/MAX1556A/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 500mA/div 0 1 2 3 4 5 6 400ns INPUT VOLTAGE (V) _______________________________________________________________________________________ 16µA IQ, 1.2A PWM DC-DC Step-Down Converters EXTERNAL FEEDBACK SWITCHING WAVEFORMS LIGHT-LOAD SWITCHING WAVEFORMS SOFT-START/SHUTDOWN WAVEFORMS MAX1556/7 toc10b MAX1556/7 toc10 MAX1556/7 toc11 VOUT VLX VOUT 2V/div 0 200mA/div ILX 500mA/div 0 IIN 500mA/div 0 200mA/div ILX 0 0 ILX VIN = 5V, VOUT = 3.3V, IOUT = 500mA 2µs/div 4µs/div 100µs/div LOAD TRANSIENT SOFT-START RAMP TIME vs. CSS MAX1556/7 toc13 MAX1556/7 toc12 10 SOFT-START RAMP TIME (ms) 1V/div 0 CSS = 470pF RLOAD = 4Ω 0 2V/div VLX 5V/div 0 VSHDN 20mV/div AC-COUPLED 50mV/div AC-COUPLED VOUT 1 500mA/div 0 IOUT IOUTMIN = 20mA 0.1 0 500 1000 1500 2000 20µs/div 2500 CSS (pF) BODE PLOT LINE TRANSIENT MAX1556/7 toc14 MAX1556/7 toc15 4V 3.5V 10mV/div AC-COUPLED VOUT 500mA/div 0 IOUT 200mA/div ILX 0 40µs/div 240 30 210 20 180 10 150 0 120 90 -10 0dB -20 60 PHASE MARGIN = 53° -30 30 -40 0 -50 IOUTMIN = 180mA 20µs/div GAIN (dB) 50mV/div AC-COUPLED VOUT VIN MAX1556/7 toc16 40 PHASE (DEGREES) LOAD TRANSIENT -30 COUT = 22µF, RLOAD = 4Ω -60 0.1 1 10 100 -60 1000 FREQUENCY (kHz) _______________________________________________________________________________________ 5 MAX1556/MAX1556A/MAX1557 Typical Operating Characteristics (continued) (VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.) MAX1556/MAX1556A/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 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 D2 MAX1556 VOUT 0 MAX1556A VOUT MAX1557 VOUT 0 Adjustable (VFB = 0.75V) from 0.75V to VIN 3.3V Adjustable (VFB = 0.75V) from 0.75V to VIN 0 1 3.3V 1.5V 1.5V 1 0 2.5V 1.2V 1.3V 1 1 1.8V 2.5V 1.0V D1 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/MAX1556A/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 MAX1556A MAX1557 SS Figure 1. Functional Diagram L1 3.3µH INPUT 2.6V TO 5.5V INP R1 100Ω C4 0.47µF C1 10µF LX OUTPUT 0.75V TO VIN 1.2A INP C4 10µF C2 22µF MAX1556 MAX1556A IN L2 4.7µH INPUT 2.6V TO 5.5V VOLTAGE SELECT OUT SS ON OFF SHDN GND Figure 2. MAX1556 Typical Application Circuit MAX1557 PGND D1 D2 C5 22µF IN PGND VOLTAGE SELECT LX OUTPUT 0.75V TO VIN 600mA 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/MAX1556A/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, MAX1556A, 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/MAX1556A/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 gain-control 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/MAX1556A/ 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/MAX1556A/MAX1557 function with a low input-to-output voltage difference by operating at 100% duty cycle. In this state, the high-side p-channel 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/MAX1556A/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 MOSFET is always on. This is particularly useful in battery-powered applications with a 3.3V output. The system and load might operate normally down to 3V or less. The MAX1556/MAX1556A/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 (R DS(ON)P) and the inductor resistance (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. VDROPOUT = IOUT x (RDS(ON)P + DCR) RDS(ON)P is given in the Electrical Characteristics. DCR for a few recommended inductors is listed in Table 2. Soft-Start The MAX1556/MAX1556A/MAX1557 use soft-start to eliminate inrush current during startup, reducing transients at the input source. Soft-start is particularly useful for higher-impedance 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 Thermal Shutdown As soon as the junction temperature of the MAX1556/MAX1556A/MAX1557 exceeds +160°C, the ICs go into 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/MAX1556A/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 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 Shutdown Mode Connecting SHDN to GND or logic low places the MAX1556/MAX1556A/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. The MAX1556/MAX1556A/MAX1557 offer preset output voltages of 1.0V, 1.2V, 1.3V, 1.5V, 1.8V, 2.5V, and 3.3V as well as an adjustable output using external resistors. Whenever possible, the preset outputs (set by D1 and D2) should be used. With external resistor feedback, noise coupling to FB can cause alternate LX pulse to terminate early resulting in an inductor current waveform with alternate large and small current pulses. See the External Feedback Switching Waveforms graph in Typical Operating Characteristics section. Note that external feedback and the alternating large-small pulse waveform do not impact loop stability and have no harmful effect on regulation or reliability. 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 _______________________________________________________________________________________ 9 MAX1556/MAX1556A/MAX1557 Table 2. Inductor Selection MAX1556/MAX1556A/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 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: k = 1.06 x 10 −2 ⎛V − 0.75V ⎞ V x ⎜ OUTPUT ⎟ 3.6V ⎠ ⎝ OUTPUT R2 ERROR AMPLIFIER OUT R3 REFERENCE 1.25V 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) 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/MAX1556A 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. 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 10 SS Figure 6. Adjustable Output Voltage 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 and MAX1556A, 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. Soft-Start Capacitor The soft-start capacitor, CSS, is required for proper operation of the MAX1556/MAX1556A/MAX1557. The recommended value of CSS is discussed in the SoftStart section. Soft-start times for various soft-start capacitors are shown in the Typical Operating Characteristics. PCB Layout and Routing Due to fast-switching waveforms and high-current paths, careful PCB 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 as possible to the IC. 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. ______________________________________________________________________________________ 16µA IQ, 1.2A PWM DC-DC Step-Down Converters PROCESS: BiCMOS Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO. 10 TDFN T1033-1 21-0137 90-0003 ______________________________________________________________________________________ 11 MAX1556/MAX1556A/MAX1557 Chip Information MAX1556/MAX1556A/MAX1557 16µA IQ, 1.2A PWM DC-DC Step-Down Converters Revision History REVISION NUMBER REVISION DATE DESCRIPTION 0 7/04 Initial release 1 3/08 Adding MAX1556A as a new version 2 6/10 Added soldering temperature, added TOC for external feedback switching waveforms, and added paragraph discussing noise coupling when using external feedback resistors 3 1/11 Added automotive qualified part PAGES CHANGED — 1–12 1, 2, 5, 6, 9, 10, 11 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. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2011 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.