19-1400; Rev 0; 11/98 L MANUA ION KIT HEET T A U L EVA TA S WS DA FOLLO Low-Noise, 5.5V-Input, PWM Step-Down Regulator Features The MAX1692 is a low-noise, pulse-width-modulated (PWM), DC-DC step-down converter. It powers logic and transmitters in small wireless systems such as cellular phones, communicating PDAs, and handy-terminals. The device features an internal synchronous rectifier for high efficiency; it requires no external Schottky diode. Excellent noise characteristics and fixed-frequency operation provide easy post-filtering. The MAX1692 is ideally suited for Li-Ion battery applications. It is also useful for +3V or +5V fixed input applications. The device operates in one of four modes. Forced PWM mode operates at a fixed frequency regardless of the load. Synchronizable PWM mode allows an external switching frequency to control and minimize harmonics. Idle Mode™ (PWM/PFM) extends battery life by switching to a PFM pulse-skipping mode during light loads. Shutdown mode places the device in standby, reducing quiescent supply current to under 0.1µA. The MAX1692 can deliver over 600mA. The output voltage can be adjusted from 1.25V to VIN with the input range of +2.7V to +5.5V. Other features of the MAX1692 include high efficiency, low dropout voltage, and a 1.2%-accurate 1.25V reference. It is available in a space-saving 10-pin µMAX package with a height of only 1.11mm. ♦ +2.7V to +5.5V Input Range ♦ Adjustable Output from 1.25V to VIN ♦ 600mA Guaranteed Output Current ♦ 95% Efficiency ♦ No Schottky Diode Required ♦ 85µA Quiescent Current ♦ 100% Duty Cycle in Dropout ♦ 750kHz Fixed-Frequency PWM Operation ♦ Synchronizable Switching Frequency ♦ Accurate Reference: 1.25V (±1.2%) ♦ Small 10-Pin µMAX Package Ordering Information PART MAX1692EUB CPU I/O Supplies Cordless Phones Notebook Chipset Supplies PIN-PACKAGE -40°C to +85°C 10 µMAX Pin Configuration TOP VIEW IN 1 Applications Cellular Phones TEMP. RANGE PDAs and Handy-Terminals Battery-Operated Devices (1 Li-Ion or 3 NiMH/NiCd) 10 PGND BP 2 GND 3 REF 4 7 SYNC/PWM FB 5 6 LIM MAX1692 9 LX 8 SHDN µMAX Typical Operating Circuit L IN VIN = 2.7V TO 5.5V VOUT = 1.25V TO VIN LX SHDN C1 R1 LIM MAX1692 BP C2 FB SYNC/PWM R2 PGND C3 AGND REF C4 Idle Mode is a trademark of Maxim Integrated Products. ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX1692 General Description MAX1692 Low-Noise, 5.5V-Input, PWM Step-Down Regulator ABSOLUTE MAXIMUM RATINGS IN, BP, SHDN, SYNC/PWM, LIM to GND ................ -0.3V to +6V BP to IN .................................................................-0.3V to +0.3V PGND to GND ...................................................... -0.3V to +0.3V LX to PGND................................................. -0.3V to (VIN + 0.3V) FB, REF to GND ......................................... -0.3V to (VBP + 0.3V) Reference Current ............................................................. ±1mA LX Peak Current (internally limited)...................................... 1.6A Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) ............444mW Operating Temperature Range .......................... -40°C to +85°C Maximum Junction Temperature .................................... +150°C Storage Temperature Range ............................ -65°C to +160°C Lead Temperature (soldering, 10sec) .............................+300°C 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 = +3.6V, SYNC/PWM = GND, VLIM = 3.6V, SHDN = IN, circuit of Figure 2; TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Input Voltage Range SYMBOL CONDITIONS VIN MIN TYP 2.7 MAX UNITS 5.5 V FB = OUT, VIN = VLIM = 2.7V to 5.5V, IOUT = 0 1.223 1.249 1.275 FB = OUT, VIN = 2.7V to 5.5V, IOUT = 0 to 600mA, LIM = IN or IOUT = 0 to 250mA, LIM = GND 1.190 1.232 1.275 Output Adjustment Range (Note 1) VREF Feedback Voltage FB = OUT, VIN = VLIM = 5.5V, IOUT = 0 (duty cycle = 23%) (Note 2) 1.223 Output Voltage VOUT VFB V 1.249 VIN V 1.275 V Line Regulation Duty cycle = 100% to 23% +1 % Load Regulation IOUT = 0 to 600mA, LIM = IN or IOUT = 0 to 250mA, LIM = GND -1.3 % FB Input Current IFB VFB = 1.4V P-Channel On-Resistance PRDS(ON) ILX = 180mA N-Channel On-Resistance NRDS(ON) ILX = 180mA -50 LIM = GND LIM = IN VFB = 1.4V SYNC/PWM = IN, FB = REF P-Channel Current-Limit Threshold N-Channel Current-Limit Threshold 0.35 0.75 -450 0 0.01 0.3 0.4 0.4 0.5 0.6 1.2 -850 50 0.85 1.55 -1600 100 80 120 160 mA VIN = 3.6V VIN = 2.7V VIN = 3.6V VIN = 2.7V Pulse-Skipping Current-Limit Threshold 50 0.65 nA 0.8 Ω Ω A mA Quiescent Current SYNC/PWM = GND, VFB = 1.4V, LX unconnected 85 140 µA Shutdown Supply Current SHDN = LX = GND, includes LX leakage current 0.1 10 µA LX Leakage Current VIN = 5.5V, VLX = 0 or 5.5V -20 0.1 20 µA 650 750 830 kHz 1000 kHz Oscillator Frequency fOSC SYNC Capture Range 500 Maximum Duty Cycle dutyMAX Minimum Duty Cycle dutyMIN Reference Output Voltage 2 VREF 100 IREF = 0 1.235 % 1.250 _______________________________________________________________________________________ 22 % 1.265 V Low-Noise, 5.5V-Input, PWM Step-Down Regulator (VIN = +3.6V, SYNC/PWM = GND, VLIM = 3.6V, SHDN = IN, circuit of Figure 2; TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL MIN TYP MAX UNITS 2.3 3 15 mV 2.4 2.5 V 0 ≤ IREF ≤ 50µA Reference Load Regulation Undervoltage Lockout Threshold CONDITIONS UVLO VIN rising, typical hysteresis is 85mV Logic Input High VIH SHDN, SYNC/PWM, LIM Logic Input Low VIL SHDN, SYNC/PWM, LIM Logic Input Current SHDN, SYNC/PWM, LIM SYNC/PWM Minimum Pulse Width High or low 2 -1 V 0.1 0.4 V 1 µA 500 ns ELECTRICAL CHARACTERISTICS (VIN = +3.6V, SYNC/PWM = GND, VLIM = 3.6V, SHDN = IN, circuit of Figure 2, TA = -40°C to +85°C, unless otherwise noted.) (Note 3) PARAMETER MIN MAX UNITS 2.7 5.5 V FB = OUT, VIN = VLIM = 2.7V to 5.5V, IOUT = 0 1.213 1.285 FB = OUT, VIN = 2.7V to 5.5V, IOUT = 0 to 600mA, LIM = IN or IOUT = 0 to 250mA, LIM = GND 1.185 1.285 REF VIN V 1.213 1.285 V VFB =1.4V LIM = GND LIM = IN -50 0.3 0.7 50 0.9 1.6 nA N-Channel Current-Limit Threshold SYNC/PWM = IN, FB = REF -15 110 mA Quiescent Current SYNC/PWM = GND, LX = unconnected, VFB = 1.4V 140 µA Shutdown Supply Current SHDN = LX = GND, includes LX leakage current 10 µA Input Voltage Range Output Voltage SYMBOL CONDITIONS VIN VOUT Output Adjustment Range (Note 1) Feedback Voltage VFB FB = OUT, VIN = VLIM = 5.5V, IOUT = 0 (duty cycle = 23%) (Note 2) FB Input Current IFB P-Channel Current-Limit Threshold Oscillator Frequency fOSC Reference Output Voltage VREF IREF = 0 Undervoltage Lockout Threshold UVLO VIN rising, typical hysteresis is 85mV Logic Input High VIH SHDN, SYNC/PWM, LIM Logic Input Low VIL SHDN, SYNC/PWM, LIM Logic Input Current SHDN, SYNC/PWM, LIM V A 630 840 kHz 1.230 1.268 V 2.3 2.5 V 2 -1 V 0.4 V 1 µA Note 1: Guaranteed by minimum and maximum duty-factor tests. Note 2: The following equation can be used to calculate FB accuracy for output voltages other than 1.232V: (see Feedback Voltage vs. Load Current) VFB = VFB (NOMINAL) - (Line Reg) (VOUT / VIN - 0.23) / 0.77 - (Load Reg)(IOUT + 0.5 · IRIPPLE) / IMAX where: Line Reg = the line regulation Load Reg = the load regulation IRIPPLE = (1- VOUT / VIN) · VOUT / (fOSC · L) where L is the inductor value IMAX = 250mA (LIM = GND) or 600mA (LIM = IN) Note 3: Specifications to -40°C are guaranteed by design, not production tested. _______________________________________________________________________________________ 3 MAX1692 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (SYNC/PWM = GND, circuit of Figure 2, L = Sumida CD43-100, TA = +25°C, unless otherwise noted.) DROPOUT VOLTAGE vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) 300 VOUT = 3.3V 200 80 75 VIN = 5.0V 70 65 150 300 450 600 750 10 100 1000 MAX1692-03 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (VOUT = 1.8V) FEEDBACK VOLTAGE vs. LOAD CURRENT BATTERY INPUT CURRENT vs. INPUT VOLTAGE 1.24 FB VOLTAGE (V) 85 VIN = 3.6V 80 75 VIN = 5.0V 70 65 1.235 1.23 LIM = IN 1.225 1.22 LIM = GND 1.215 60 LIM = IN R1 = 138kΩ R2 = 301kΩ 55 1.21 95 50 1.2 1 10 100 1000 TA = +85°C 90 85 80 TA = +25°C 75 TA = -40°C 70 VOUT = 1.8V SYNC/PWM = GND 65 1.205 MAX1692-06 100 BATTERY INPUT CURRENT (µA) VIN = 5.0V R1 = 309kΩ R2 = 301kΩ SYNC/PWM = GND 1.245 90 MAX1692-05 1.25 MAX1692-04 VIN = 2.7V 60 0 100 200 300 400 500 600 700 800 900 1000 2.7 3.1 3.5 3.9 4.3 4.7 LOAD CURRENT (mA) LOAD CURRENT (mA) INPUT VOLTAGE (V) BATTERY INPUT CURRENT vs. INPUT VOLTAGE BATTERY INPUT CURRENT vs. INPUT VOLTAGE AND TEMPERATURE OUTPUT VOLTAGE vs. LOAD CURRENT VOUT = 3.3V 3.5 VOUT = 2.5V 3.0 2.5 2.0 1.5 VOUT = 1.8V 1.0 0.5 2.0 TA = +25°C 1.5 TA = -40°C VOUT = 1.8V SYNC/PWM = IN SYNC/PWM = IN 0 3.5 3.9 4.3 4.7 INPUT VOLTAGE (V) 5.1 5.5 5.5 1.80 1.78 1.76 1.74 1.0 3.1 VIN = 2.7V VOUT = 1.8V R1 = 138kΩ R2 = 301kΩ 1.82 TA = +85°C 5.1 1.84 OUTPUT VOLTAGE (V) 4.0 2.5 MAX1692-09 4.5 BATTERY INPUT CURRENT (mA) MAX1692-07 5.0 2.7 LIM = IN R1 = 309kΩ R2 = 301kΩ 55 LOAD CURRENT (mA) 100 95 VIN = 5.0V 70 50 1 900 VIN = 3.6V 75 60 LIM = IN R1 = 505kΩ R2 = 301kΩ 50 0 VIN = 2.7V 65 60 0 85 80 MAX1692-10 400 90 VIN = 3.6V 85 EFFICIENCY (%) VOUT = 2.5V 55 EFFICIENCY (%) 95 90 100 4 100 MAX1692-02 95 EFFICIENCY (%) DROPOUT VOLTAGE (mV) 500 EFFICIENCY vs. LOAD CURRENT (VOUT = 2.5V) 100 MAX1692-01 600 BATTERY INPUT CURRENT (mA) MAX1692 Low-Noise, 5.5V-Input, PWM Step-Down Regulator 2.7 3.1 3.5 3.9 4.3 4.7 INPUT VOLTAGE (V) 5.1 5.5 0 100 200 300 400 500 600 700 800 900 LOAD CURRENT (mA) _______________________________________________________________________________________ Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE TA = +25°C 700 TA = -40°C 650 1.1 VOUT 1V/div 0.8 IIN 0.5A/div LIM = GND IOUT = 200mA 600 VOUT = 1.8V 0.5 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 2.7 3.1 3.5 3.9 4.3 4.7 5.1 HEAVY LOAD SWITCHING WAVEFORMS LOAD-TRANSIENT RESPONSE 2ms/div 5.5 LINE-TRANSIENT RESPONSE MAX1692-17 INPUT VOLTAGE (V) MAX1692-15 SUPPLY VOLTAGE (V) VLX 5V/div VSHDN 2V/div LIM = IN VIN ACCOUPLED 2V/div VLX 5V/div ILX 0.5A/div VOUT ACCOUPLED 50mV/div IOUT 2.5A/div VOUT ACCOUPLED 100mV/div MAX1692-18 750 MAX1692-12 OSCILLATOR FREQUENCY (kHz) TA = +85°C START-UP FROM SHUTDOWN 1.4 MAXIMUM OUTPUT CURRENT (A) MAX1692-11 800 MAX1692-14 OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE VOUT AC-COUPLED 100mV/div 500µs/div 2ms/div 2ms/div ILOAD = 30mA to 700mA VIN = 5V, VOUT = 3.3V, IOUT = 700mA VIN = 3V to 5V, IOUT = 300mA SWITCHING HARMONICS AND NOISE MAX1692-19 MAX1692-22 RECOVERY FROM 100% DUTY CYCLE VIN 2V/div VLX 5V/div 1mV/div VOUT ACCOUPLED 500mV/div 2ms/div VIN = 3.3V to 4.5V , VOUT = 3.3V, IOUT = 500mA 100kHz IOUT = 500mA 1MHz 10MHz 1ms/div _______________________________________________________________________________________ 5 MAX1692 Typical Operating Characteristics (continued) (SYNC/PWM = GND, TA = +25°C, unless otherwise noted.) Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAX1692 Pin Description PIN NAME FUNCTION 1 IN Supply Voltage Input. Input range from +2.7V to +5.5V. Bypass with a 10µF capacitor. 2 BP Supply Bypass Pin. Internally connected to IN. Bypass with a 0.1µF capacitor. Do not connect to an external power source other than IN. 3 GND Ground 4 REF 1.25V, 1.2% Reference Output. Capable of delivering 50µA to external loads. Bypass with a 0.22µF capacitor to GND. 5 FB Feedback Input 6 LIM Current-Limit Select Input. Connect LIM to GND for 0.6A current limit or LIM to IN for 1.2A current limit. 7 SYNC/ PWM Oscillator Sync and Low-Noise, Mode-Control Input. SYNC/PWM = IN (Forced PWM Mode) SYNC/PWM = GND (PWM/PFM Mode) An external clock signal connected to this pin allows for LX switching synchronization. 8 SHDN Active-Low, Shutdown-Control Input. Reduces quiescent current to 0.1µA. In shutdown, output becomes high impedance. 9 LX 10 PGND Inductor Connection to the Drains of the Internal Power MOSFETs Power Ground BP CHIP SUPPLY PFM CURRENT COMPARATOR 10Ω MAX1692 SHDN IN REF 1Ω 12mV REF P 120mV GND LX 0.1X LIM COMPARATOR SENSE FET PWM COMPARATOR RAMP GEN SYNC/ PWM SYNC CELL CONTROL AND DRIVER LOGIC SLOPE COMPENSATION PWM SENSE FET 0.1X FB REF N PWM ON SIGNAL 40mV FB FB 1Ω ON ON REF NEGLIM COMPARATOR PFM COMPARATOR REF OVERVOLTAGE COMPARATOR PGND 5mV IN PFM ADJ. IN PWM Figure 1. Simplified Functional Diagram 6 _______________________________________________________________________________________ Low-Noise, 5.5V-Input, PWM Step-Down Regulator The MAX1692 step-down, pulse-width-modulated (PWM), DC-DC converter has an adjustable output range from 1.25V to the input voltage. An internal synchronous rectifier improves efficiency and eliminates an external Schottky diode. Fixed-frequency operation enables easy post-filtering, thereby providing excellent noise characteristics. As a result, the MAX1692 is an ideal choice for many small wireless systems. The MAX1692 accepts inputs as low as +2.7V while still delivering 600mA. The MAX1692 can operate in four modes to optimize performance. A forced (PWM) mode switches at a fixed frequency, regardless of load, for easy post-filtering. A synchronizable PWM mode uses an external clock to minimize harmonics. A PWM/PFM mode extends battery life by operating in PWM mode under heavy loads and PFM mode under light loads for reduced power consumption. Shutdown mode reduces quiescent current to 0.1µA. PWM Control Scheme The MAX1692 uses a slope-compensated, currentmode PWM controller capable of achieving 100% duty cycle. The device uses an oscillator-triggered, minimum on-time, current-mode control scheme. The minimum on-time is approximately 150ns unless in dropout. The maximum on-time is approximately 2/fOSC, allowing operation to 100% duty cycle. Current-mode feedback provides cycle-by-cycle current limiting for superior load- and line-response and protection of the internal MOSFET and rectifier. At each falling edge of the internal oscillator, the SYNC cell sends a PWM ON signal to the control and drive logic, turning on the internal P-channel MOSFET (main switch) (Figure 1). This allows current to ramp up through the inductor (Figure 2) to the load, and stores energy in a magnetic field. The switch remains on until either the current-limit (LIM) comparator is tripped or the PWM comparator signals that the output is in regulation. When the switch turns off during the second half of each cycle, the inductor’s magnetic field collapses, releasing the stored energy and forcing current through the N-channel synchronous rectifier to the output-filter capacitor and load. The output-filter capacitor stores charge when the inductor current is high and releases it when the inductor current is low, thus smoothing the voltage across the load. During normal operation, the MAX1692 regulates output voltage by switching at a constant frequency and then modulating the power transferred to the load each cycle using the PWM comparator. A multi-input comparator sums three weighted differential signals: the L1 10µH VIN +2.7V TO +5.5V IN C1 10µF MAX1692 Detailed Description VOUT = 1.8V @ 600mA LX C2 47µF LIM MAX1692 ON/OFF C4 0.22µF SHDN C5 47pF REF FB BP C3 0.1µF SYNC/ PWM R1 138k R2 300k GND PGND Figure 2. Standard Application Circuit output voltage with respect to the reference, the main switch current sense, and the slope-compensation ramp. It modulates output power by adjusting the inductor-peak current during the first half of each cycle, based on the output-error voltage. The MAX1692’s loop gain is relatively low to enable the use of a small, lowvalued output-filter capacitor. The resulting load regulation is 1.3% (typ) at 0 to 600mA. 100% Duty-Cycle Operation The maximum on-time can exceed one internal oscillator cycle, which permits operation up to 100% duty cycle. As the input voltage drops, the duty cycle increases until the P-channel MOSFET is held on continuously. Dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal switch and inductor, around 280mV (I OUT = 600mA). In PWM mode, subharmonic oscillation can occur near dropout but subharmonic voltage ripple is small, since the ripple current is low. Synchronous Rectification An N-channel, synchronous-rectifier improves efficiency during the second half of each cycle (off time). When the inductor current ramps below the threshold set by the NEGLIM comparator (Figure 1) or when the PWM reaches the end of the oscillator period, the synchronous rectifier turns off. This keeps excess current from flowing backward through the inductor, from the output-filter capacitor to GND, or through the switch and synchronous rectifier to GND. During PWM operation, the NEGLIM threshold adjusts to permit small _______________________________________________________________________________________ 7 MAX1692 Low-Noise, 5.5V-Input, PWM Step-Down Regulator amounts of reverse current to flow from the output during light loads. This allows regulation with a constantswitching frequency and eliminates minimum load requirements. The NEGLIM comparator threshold is 50mA if VFB < 1.25V, and decreases as VFB exceeds 1.25V to prevent the output from rising. The NEGLIM threshold in PFM mode is fixed at 50mA. (See Forced PWM and PWM/PFM Operation section.) Forced PWM and PWM/PFM Operation Connect SYNC/PWM to IN for normal forced PWM operation. Forced PWM operation is desirable in sensitive RF and data-acquisition applications, to ensure that switching-noise harmonics do not interfere with sensitive IF and data-sampling frequencies. A minimum load is not required during forced PWM operation, since the synchronous rectifier passes reverse-inductor current as needed to allow constant-frequency operation with no load. Forced PWM operation uses higher supply current with no load (2mA typ). Connecting SYNC/PWM to GND enables PWM/PFM operation. This proprietary control scheme overrides PWM mode and places the MAX1692 in PFM mode at light loads to improve efficiency and reduce quiescent current to 85µA. With PWM/PFM enabled, the MAX1692 initiates pulse-skipping PFM operation when the peak inductor current drops below 120mA. During PFM operation, the MAX1692 switches only as needed to service the load, reducing the switching frequency and associated losses in the internal switch, the synchronous rectifier, and the external inductor. During PFM mode, a switching cycle initiates when the PFM comparator senses that the output voltage has dropped too low. The P-channel MOSFET switch turns on and conducts current to the output-filter capacitor and load until the inductor current reaches the PFM peak current limit (120mA). Then the switch turns off and the magnetic field in the inductor collapses, forcing current through the synchronous rectifier to the output filter capacitor and load. Then the MAX1692 waits until the PFM comparator senses a low output voltage again. The PFM current comparator controls both entry into PWM mode and the peak switching current during PFM mode. Consequently, some jitter is normal during transition from PFM to PWM modes with loads around 100mA, and it has no adverse impact on regulation. Output ripple is higher during PFM operation. A larger output-filter capacitor can be used to minimize ripple. 8 SYNC Input and Frequency Control The MAX1692’s internal oscillator is set for a fixedswitching frequency of 750kHz or can be synchronized to an external clock. Connect SYNC to IN for forcedPWM operation. Do not leave SYNC/PWM unconnected. Connecting SYNC/PWM to GND enables PWM/PFM operation to reduce supply current at light loads. SYNC/PWM is a negative-edge triggered input that allows synchronization to an external frequency ranging between 500kHz and 1000kHz. When SYNC/PWM is clocked by an external signal, the converter operates in forced PWM mode. If SYNC is low or high for more than 100µs, the oscillator defaults to 750kHz. Shutdown Mode Connecting SHDN to GND places the MAX1692 in shutdown mode. In shutdown, the reference, control circuitry, internal switching MOSFET, and the synchronous rectifier turn off and the output falls to 0V. Connect SHDN to IN for normal operation. Current-Sense Comparators The MAX1692 uses several internal current-sense comparators. In PWM operation, the PWM comparator sets the cycle-by-cycle current limit (Figure 1) and provides improved load and line response, allowing tighter specification of the inductor-saturation current limit to reduce inductor cost. A second 120mA current-sense comparator used across the P-channel switch controls entry into PFM mode. A third current-sense comparator monitors current through the internal N-channel MOSFET to set the NEGLIM threshold and determine when to turn off the synchronous rectifier. A fourth comparator (LIM) used at the P-channel MOSFET switch detects overcurrent. This protects the system, external components, and internal MOSFETs under overload conditions. Applications Information Output Voltage Selection Select an output voltage between 1.25V and V IN by connecting FB to a resistor-divider between the output and GND (Figure 2). Select feedback resistor R2 in the 5kΩ to 500kΩ range. R1 is then given by: R1 = R2 [(VOUT / VFB) - 1] where V FB = 1.232V (See Note 2 of the Electrical Characteristics). Add a small ceramic capacitor (C5) around 47pF to 100pF in parallel with R1 to compensate for stray capacitance at the FB pin and output capacitor equivalent series resistance (ESR). _______________________________________________________________________________________ Low-Noise, 5.5V-Input, PWM Step-Down Regulator IRMS = IOUT[VOUT (VIN - VOUT)]1/2 · VIN When selecting an output capacitor, consider the output-ripple voltage and approximate it as the product of the ripple current and the ESR of the output capacitor. VRIPPLE = [VOUT (VIN - VOUT)] / [2 · fOSC(L) (VIN)] · ESRC2 where ESRC2 is the equivalent-series resistance of the output capacitor. The MAX1692’s loop gain is relatively low, enabling the use of small, low-value output filter capacitors. Higher values provide improved output ripple and transient response. Lower oscillator frequencies require a largervalue output capacitor. When PWM/PFM is used, verify capacitor selection with light loads during PFM operation, since output ripple is higher under these conditions. Low-ESR capacitors are recommended. Capacitor ESR is a major contributor to output ripple (usually more than 60%). Ordinary aluminum-electrolytic capacitors have high ESR and should be avoided. Low-ESR aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors are better and provide a compact solution for space-constrained surface-mount designs. Do not exceed the ripple-current ratings of tantalum capacitors. Ceramic capacitors have the lowest ESR overall, and OS-CON™ capacitors have the lowest ESR of the high-value electrolytic types. It is generally not necessary to use ceramic or OS-CON capacitors for the MAX1692; consider them only in very compact, high-reliability, or wide-temperature applications where the expense is justified. When using verylow-ESR capacitors, such as ceramic or OS-CON, check for stability while examining load-transient response. The output capacitor is determined by ensuring that the minimum capacitance value and maximum OS-CON is a trademark of Sanyo Corp. ESR values are met: C2 > 2VREF(1 + VOUT/VIN(MIN)) / (VOUT · RSENSE · fOSC) RESR < (RSENSE)(VOUT) / (VREF) where C2 is the output filter capacitor, VREF is the internal reference voltage of 1.25V, VIN(min) is the minimum input voltage (2.7V), RSENSE is the internal sense resistance of 0.1Ω, and fOSC is the internal oscillator frequency (typically 750kHz). These equations provide the minimum requirements. The value of C2 may need to be increased for operation at duty-cycle extremes. Tables 1 and 2 provide recommended inductor and capacitor sizes at various external sync frequencies. Table 3 lists suppliers for the various components used with the MAX1692. Standard Application Circuits Figures 2 and 3 are standard application circuits optimized for power and board space respectively. The circuit of Figure 2 is the most general of the two, and generates 1.8V at 600mA. The circuit of Figure 3 is optimized for smallest overall size. Cellular phones are using low voltage for baseband logic and have critical area and height restrictions. This circuit operates from a single Li-ion battery (2.9V to 4.5V) and delivers up to 200mA at 1.8V. It uses small ceramic capacitors at the input and output and a tiny chip inductor such as the NLC322522T series from TDK. With the MAX1692 in a 10-pin µMAX package, the entire circuit can fit in only 60mm2 and have less than 2.4mm height. L1 10µH VIN +2.9V TO +4.5V IN C5 4.7µF VOUT = 1.8V @ 200mA LX C2 10µF BP 10µF MAX1692 ON/OFF C4 0.1µF SHDN C5 47pF REF R1 138k FB LIM SYNC/ PWM R2 301k GND PGND Figure 3. Miniaturized 200mA Output Circuit Fits in 60mm2 _______________________________________________________________________________________ 9 MAX1692 Capacitor Selection Choose input- and output-filter capacitors to service inductor currents with acceptable voltage ripple. The input-filter capacitor also reduces peak currents and noise at the voltage source. In addition, connect a lowESR bulk capacitor (>10µF suggested) to the input. Select this bulk capacitor to meet the input ripple requirements and voltage rating, rather than capacitor size. Use the following equation to calculate the maximum RMS input current: MAX1692 Low-Noise, 5.5V-Input, PWM Step-Down Regulator Bypass Considerations Bypass IN and OUT to PGND with 10µF and 47µF, respectively. Bypass BP and REF to GND with 0.1µF and 0.22µF, respectively. Locate the bypass capacitors as close as possible to their respective pins to minimize noise coupling. For optimum performance, place input and output capacitors as close to the device as feasible (see Capacitor Selection section). PC Board Layout and Routing High switching frequencies and large peak currents make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both Table 1. Suggested Inductors OUTPUT VOLTAGE RANGE (V) 1.25 to 2.5 2.5 to 4.0 4.0 to 5.5 INDUCTOR L VALUE (µH) 10 22 33 Table 3. Component Suppliers COMPANY SUGGESTED INDUCTORS Sumida CD43-100 Coilcraft D01608C-103 Sumida CD54-100 TDK NLC322522-100T Sumida CD43-220 Sumida CD54-220 Sumida CD43-330 Sumida CD54-330 Table 2. Suggested Capacitors MANUFACTURER PART NUMBER TYPE ESR (mΩ) Tantalum 150 Poscap 100 Sprague 594D686X9010C2T Tantalum 95 Taiyo Yuden JMK325BJ106MN Ceramic 50 AVX TPSD476M016R0150 Sanyo 6TPA47M 10 of which can result in instability or regulation errors. Connect the inductor, input filter capacitor, and output filter capacitor as close together as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star-ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces, such as from the LX pin, away from the voltage-feedback network; also keep them separate, using grounded copper. Connect GND and PGND at the highest quality ground. The MAX1692 evaluation kit manual illustrates an example PC board layout and routing scheme. PHONE FAX AVX 843-946-0238 843-626-3123 Coilcraft 847-639-6400 847-639-1469 Coiltronics 561-241-7876 561-241-9339 Kemet 408-986-0424 408-986-1442 Nihon USA 805- 867-2555 Japan 81-3-3494-7411 805- 867-2698 81-3-3494-7414 Sanyo USA 619-661-6835 Japan 81-7-2070-6306 619-661-1055 81-7-2070-1174 Sprague 603-224-1961 603- 224-1430 Sumida USA 847-956-0666 Japan 81-3-3607-5111 847- 956-0702 81-3-3607-5144 Taiyo Yuden 408-573-4150 408-573-4159 TDK 847-390-4373 847-390-4428 ______________________________________________________________________________________ Low-Noise, 5.5V-Input, PWM Step-Down Regulator TRANSISTOR COUNT: 1462 10LUMAXB.EPS Package Information ______________________________________________________________________________________ 11 MAX1692 Chip Information MAX1692 Low-Noise, 5.5V-Input, PWM Step-Down Regulator NOTES 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 © 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.