LM3677 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits General Description Features The LM3677 step-down DC-DC converter is optimized for powering ultra-low voltage circuits from a single Li-Ion cell battery and input voltage rails from 2.7V to 5.5V. It provides up to 600 mA load current over the entire input voltage range. The LM3677 is configured to different fixed voltage output options as well as an adjustable output voltage version range from 1.2V to 3.3V. The device offers superior features and performance for mobile phones and similar portable applications with complex power management systems. Automatic intelligent switching between PWM low-noise and PFM low-current mode offers improved system control. During PWM mode operation, the device operates at a fixed frequency of 3 MHz (typ). PWM mode drives loads from ~ 80 mA to 600 mA max. Hysteretic PFM mode extends the battery life by reducing the quiescent current to 16 µA (typ.) during light load and standby operation. Internal synchronous rectification provides high efficiency. In shutdown mode (Enable pin pulled down), the device turns off and reduces battery consumption to 0.01 µA (typ.). The LM3677 is available in a lead-free (NOPB) 5-bump micro SMD package and 6-pin LLP package. A switching frequency of 3 MHz (typ.) allows use of tiny surface-mount components. Only three external surface-mount components, an inductor and two ceramic capacitors, are required. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 16 µA typical quiescent current 600 mA maximum load capability 3 MHz PWM fixed switching frequency (typ.) Automatic PFM/PWM mode switching Available in 5-bump micro SMD package and 6-pin LLP package Internal synchronous rectification for high efficiency Internal soft start 0.01 µA typical shutdown current Operates from a single Li-Ion cell battery Only three tiny surface-mount external components required (solution size less than 20 mm2) Current overload and thermal shutdown protection Applications ■ ■ ■ ■ ■ ■ ■ Mobile Phones PDAs MP3 Players W-LAN Portable Instruments Digital Still Cameras Portable Hard Disk Drives Typical Application Circuits Efficiency vs. Output Current (VOUT = 1.8V) 30008401 FIGURE 1. Typical Application Circuit 30008407 30008487 FIGURE 2. Typical Application Circuit - ADJ. Version © 2008 National Semiconductor Corporation 300084 www.national.com LM3677 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits October 28, 2008 LM3677 Connection Diagram and Package Mark Information 5-Bump micro SMD Package NS Package Number TLA05FEA 30008444 FIGURE 3. 5 Bump Micro SMD Package 30008400 FIGURE 4. 6 Pin LLP Package Pin Descriptions Pin # Name Description A1 1 VIN A3 6 GND Power supply input. Connect to the input filter capacitor (Figure 1). C1 3 EN Enable pin. The device is in shutdown mode when voltage to this pin is < 0.4V and enabled when > 1.0V. Do not leave this pin floating. Ground pin. C3 4 FB Feedback analog input. Connect directly to the output filter capacitor ( FIGURE 1). B2 2, 5 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier. Ordering Information Order Number Spec LM3677TL-1.2 (Note 1) NOPB LM3677TLX-1.2 (Note 1) NOPB LM3677TL-1.3 NOPB LM3677TLX-1.3 NOPB LM3677TL-1.5 NOPB LM3677TLX-1.5 NOPB www.national.com Package Marking Supplied As 250 units, Tape-and-Reel 3 3000 units, Tape-and-Reel 250 units, Tape-and-Reel V 3000 units, Tape-and-Reel 250 units, Tape-and-Reel X 3000 units, Tape-and-Reel 2 Spec LM3677TL-1.8 NOPB LM3677TLX-1.8 NOPB LM3677TL-1.875 NOPB LM3677TLX-1.875 NOPB LM3677TL-2.5 NOPB LM3677TLX-2.5 NOPB LM3677TL-ADJ NOPB LM3677TLX-ADJ NOPB LM3677LEE-1.2 NOPB LM3677LE-1.2 NOPB LM3677LEX-1.2 NOPB LM3677LEE-1.5 NOPB LM3677LE-1.5 NOPB LM3677LEX-1.5 NOPB LM3677LEE-1.8 NOPB LM3677LE-1.8 NOPB LM3677LEX-1.8 NOPB LM3677LEE-1.82 NOPB LM3677LE-1.82 NOPB LM3677LEX-1.82 NOPB Package Marking LM3677 Order Number Supplied As 250 units, Tape-and-Reel Y 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 9 3000 units, Tape-and-Reel 250 units, Tape-and-Reel Z 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 4 3000 units, Tape-and-Reel 250 units, Tape-and-Reel 1000 units, Tape-and-Reel K 4500 units, Tape-and-Reel 250 units, Tape-and-Reel L 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel 250 units, Tape-and-Reel N 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel 250 units, Tape-and-Reel 5 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel Note 1: For output voltage 1.2V or lower, input voltage needs to be derated to the range of 2.7V to 5.0V in order to perform within specification. 3 www.national.com LM3677 ESD Rating (Note 5) Human Body Model: All Pins Machine Model: All Pins Absolute Maximum Ratings (Note 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Operating Ratings If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributor for availability and specifications. VIN Pin: Voltage to GND FB, SW, EN Pin: Continuous Power Dissipation (Note 4) Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temperature (Soldering, 10 sec.) 2.0 kV 200V (Note 2), (Note 3) Input Voltage Range 2.7V to 5.5V Recommended Load Current 0 mA to 600 mA Junction Temperature (TJ) Range −30°C to +125°C Ambient Temperature (TA) Range (Note −30°C to +85°C 6) −0.2V to 6.0V (GND−0.2V) to (VIN + 0.2V) Internally Limited Thermal Properties +125°C −65°C to +150°C 260°C Junction-to-Ambient Thermal Resistance (θJA) (Note 7) 85°C/W Electrical Characteristics (Note 3), (Note 9), (Note 10) Limits in standard typeface are for TJ = TA = 25°C. Limits in boldface type apply over the operating ambient temperature range (−30°C ≤ TA ≤ +85°C). Unless otherwise noted, specifications apply to the LM3677 with VIN = EN = 3.6V. Symbol VIN VFB Parameter Condition Input Voltage (Note 11) Feedback Voltage (TL) PWM mode Feedback Voltage (LE) Min Max Units 2.7 Typ 5.5 V -2.5 +2.5 -4.0 +4.0 % VREF Internal Reference Voltage 0.5 ISHDN Shutdown Supply Current EN = 0V 0.01 1 µA IQ DC Bias Current into VIN No load, device is not switching 16 35 µA RDSON (P) Pin-Pin Resistance for PFET VIN= VGS= 3.6V, ISW= 100mA 350 450 mΩ RDSON (N) Pin-Pin Resistance for NFET VIN= VGS= 3.6V, ISW= -100mA 150 250 mΩ ILIM Switch Peak Current Limit Open Loop(Note 8) 1220 1375 mA VIH Logic High Input VIL Logic Low Input IEN Enable (EN) Input Current FOSC Internal Oscillator Frequency 1085 V V 1.0 PWM Mode 2.5 0.4 V 0.01 1 µA 3 3.5 MHz Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics tables. Note 3: All voltages are with respect to the potential at the GND pin. Note 4: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ= 150°C (typ.) and disengages at TJ= 130°C (typ.). Note 5: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged directly into each pin. MIL-STD-883 3015.7 Note 6: In Applications where high power dissipation and/or poor package resistance is present, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX), the maximum power dissipation of the device in the application (PD-MAX) and the junction to ambient thermal resistance of the package (θJA) in the application, as given by the following equation: TA-MAX= TJ-MAX − (θJAx PD-MAX). Refer to Dissipation rating table for PD-MAX values at different ambient temperatures. Note 7: Junction to ambient thermal resistance is highly application and board layout dependent. In applications where high power dissipation exists, special care must be given to thermal dissipation issues in board design. Value specified here 85 °C/W is based on measurement results using a 4 layer board as per JEDEC standards. Note 8: Refer to datasheet curves for closed loop data and its variation with regards to supply voltage and temperature. Electrical Characteristic table reflects open loop data (FB=0V and current drawn from SW pin ramped up until cycle by cycle current limit is activated). Closed loop current limit is the peak inductor current measured in the application circuit by increasing output current until output voltage drops by 10%. Note 9: Min and Max limits are guaranteed by design, test or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Note 10: The parameters in the electrical characteristic table are tested under open loop conditions at VIN= 3.6V unless otherwise specified. For performance over the input voltage range and closed loop condition, refer to the datasheet curves. Note 11: For output voltage 1.2V or lower, input voltage needs to be derated to the range of 2.7V to 5.0V in order to perform within specification. www.national.com 4 LM3677 Dissipation Rating Table θJA TA≤ 25°C Power Rating TA= 60°C Power Rating TA= 85°C Power Rating 85°C/W (4-layer board) micro SMD 1178 mW 785 mW 470 mW 117°C/W (4-layer board) LLP 855 mW 556 mW 342 mW 5 www.national.com LM3677 Block Diagram 30008418 FIGURE 5. Simplified Functional Diagram www.national.com 6 LM3677 Typical Performance Characteristics LM3677, Circuit of Figure 1, VIN = 3.6V, VOUT = 1.8V, TA = 25°C, unless otherwise noted. Quiescent Supply Current vs. Supply Voltage (Switching) Shutdown Current vs. Temp 30008482 30008481 Switching Frequency vs. Temperature RDS(ON) vs. Temperature 30008483 30008451 Open/Closed Loop Current Limit vs. Temperature Output Voltage vs. Supply Voltage (VOUT = 1.8V) 30008449 30008484 7 www.national.com LM3677 Output Voltage vs. Supply Voltage (VOUT = 2.5V) Output Voltage vs. Temperature (VOUT = 1.3V) 30008438 30008468 Output Voltage vs. Temperature (VOUT = 1.8V) Output Voltage vs. Temperature (VOUT = 2.5V) 30008485 30008469 Output Voltage vs. Output Current (VOUT = 1.8V) Output Voltage vs. Output Current (VOUT = 2.5V) 30008486 www.national.com 30008437 8 LM3677 Efficiency vs. Output Current (VOUT = 1.3V) Efficiency vs. Output Current (VOUT = 1.8V) 30008441 30008487 Efficiency vs. Output Current (VOUT = 2.5V) Output Current vs. Input Voltage at Mode Change Point (VOUT = 1.3V) 30008432 30008435 Output Current vs. Input Voltage at Mode Change Point (VOUT = 1.8V) Output Current vs. Input Voltage at Mode Change Point (VOUT = 2.5V) 30008488 30008436 9 www.national.com LM3677 Line Transient Response VOUT = 1.3V (PWM Mode) Line Transient Response VOUT = 1.8V (PWM Mode) 30008477 30008433 Line Transient Response VOUT = 1.8V (PWM Mode) Line Transient Response VOUT = 2.5V (PWM Mode) 30008478 30008439 Load Transient Response (VOUT = 1.3V) (PFM Mode 1mA to 50mA) Load Transient Response (VOUT = 1.3V) (PFM Mode 50mA to 1mA) 30008493 www.national.com 30008494 10 Load Transient Response (VOUT = 1.8V) (PFM Mode 50mA to 1mA) 30008473 30008474 Load Transient Response (VOUT = 2.5V) (PFM Mode 1mA to 50mA) Load Transient Response (VOUT = 2.5V) (PFM Mode 50mA to 1mA) 30008498 30008430 Mode Change by Load Transients VOUT = 1.3V (PFM to PWM) Mode Change by Load Transients VOUT = 1.3V (PWM to PFM) 30008495 30008496 11 www.national.com LM3677 Load Transient Response (VOUT = 1.8V) (PFM Mode 1mA to 50mA) LM3677 Mode Change by Load Transients VOUT = 1.8V (PFM to PWM) Mode Change by Load Transients VOUT = 1.8V (PWM to PFM) 30008475 30008476 Load Transient Response VOUT = 1.3V (PWM Mode) Load Transient Response VOUT = 1.8V (PWM Mode) 30008472 30008497 Load Transient Response VOUT = 2.5V (PWM Mode) Start Up into PWM Mode VOUT = 1.3V (Output Current= 300mA) 30008431 www.national.com 30008491 12 LM3677 Start Up into PFM Mode VOUT = 1.3V (Output Current= 1mA) Start Up into PWM Mode VOUT = 1.8V (Output Current= 300mA) 30008470 30008492 Start Up into PFM Mode VOUT = 1.8V (Output Current= 1mA) Start Up into PWM Mode VOUT = 2.5V (Output Current= 300mA) 30008471 30008489 Start Up into PFM Mode VOUT = 2.5V (Output Current= 1mA) 30008490 13 www.national.com LM3677 control logic turns off the switch. The current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock turning off the NFET and turning on the PFET. Operation Description DEVICE INFORMATION The LM3677, a high-efficiency step-down DC-DC switching buck converter, delivers a constant voltage from a single LiIon battery and input voltage rails from 2.7V to 5.5V to devices such as cell phones and PDAs. Using a voltage-mode architecture with synchronous rectification, the LM3677 has the ability to deliver up to 600 mA depending on the input voltage and output voltage, ambient temperature, and the inductor chosen. There are three modes of operation depending on the current required: PWM (Pulse Width Modulation), PFM (Pulse Frequency Modulation), and shutdown. The device operates in PWM mode at load current of approximately 80 mA or higher, having a voltage precision of ±2.5% with 90% efficiency or better. Lighter load current causes the device to automatically switch into PFM mode for reduced current consumption (IQ = 16 µA typ.) and a longer battery life. Shutdown mode turns off the device, offering the lowest current consumption (ISHUTDOWN = 0.01 µA (typ.). Additional features include soft-start, under voltage protection, current overload protection, and thermal shutdown protection. As shown in Figure 1, only three external power components are required for implementation. The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the input voltage exceeds 2.7V. 30008480 FIGURE 6. Typical PWM Operation Internal Synchronous Rectification While in PWM mode, the LM3677 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier diode. CIRCUIT OPERATION The LM3677 operates as follows. During the first portion of each switching cycle, the control block in the LM3677 turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of (VIN–VOUT)/L, by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of - VOUT/L. The output filter stores charge when the inductor current is high, and releases it when inductor current is low, smoothing the voltage across the load. The output voltage is regulated by modulating the PFET switch-on time to control the average current sent to the load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The output voltage is equal to the average voltage at the SW pin. Current Limiting A current limit feature allows the LM3677 to protect itself and external components during overload conditions. PWM mode implements current limiting using an internal comparator that trips at 1220 mA (typ.). If the output is shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration until the inductor current falls below a low threshold, ensuring inductor current has more time to decay, thereby preventing runaway. PFM OPERATION At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply current to maintain high efficiency. The part will automatically transition into PFM mode when either of the following conditions occurs for a duration of 32 or more clock cycles: A. The NFET current reaches zero. B. The peak PMOS switch current drops below the IMODE level, (Typically IMODE < 75 mA + VIN/55Ω ). PWM OPERATION During PWM operation, the converter operates as a voltagemode controller with input voltage feed forward. This allows the converter to achieve good load and line regulation. The DC gain of the power stage is proportional to the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is introduced. While in PWM mode, the output voltage is regulated by switching at a constant frequency and then modulating the energy per cycle to control power to the load. At the beginning of each clock cycle the PFET switch is turned on and the inductor current ramps up until the comparator trips and the www.national.com 14 30008479 FIGURE 7. Typical PFM Operation During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage during PWM operation allowing additional headroom for voltage drop during a load transient from light to heavy load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output FETs such that the output voltage ramps between ~0.2% and ~1.8% above the nominal PWM output voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch 30008403 FIGURE 8. Operation in PFM Mode and Transfer to PWM Mode SHUTDOWN MODE Setting the EN input pin low (<0.4V) places the LM3677 in shutdown mode. During shutdown the PFET switch, NFET switch, reference, control and bias circuitry of the LM3677 are turned off. Setting EN high (>1.0V) enables normal operation. It is recommended to set EN pin low to turn off the LM3677 during system power up and undervoltage conditions when the supply is less than 2.7V. Do not leave the EN pin floating. SOFT START The LM3677 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current limit is increased in steps. Soft start is activated only if EN goes from logic low to logic high after VIN reaches 2.7V. Soft start is implemented by increasing switch current limit in steps of 200 mA, 400 mA, 600 mA and 1220 mA (typical switch current limit). The start-up time thereby depends on the output capacitor and load current demanded at start-up. Typical start15 www.national.com LM3677 is turned on. It remains on until the output voltage reaches the ‘high’ PFM threshold or the peak current exceeds the IPFM level set for PFM mode. The typical peak current in PFM mode is: IPFM = 112 mA + VIN/20Ω . Once the PMOS power switch is turned off, the NMOS power switch is turned on until the inductor current ramps to zero. When the NMOS zero-current condition is detected, the NMOS power switch is turned off. If the output voltage is below the ‘high’ PFM comparator threshold (see Figure 8), the PMOS switch is again turned on and the cycle is repeated until the output reaches the desired level. Once the output reaches the ‘high’ PFM threshold, the NMOS switch is turned on briefly to ramp the inductor current to zero, and then both output switches are turned off and the part enters an extremely low-power mode. Quiescent supply current during this ‘sleep’ mode is 16 µA (typ.), which allows the part to achieve high efficiencies under extremely light load conditions. If the load current should increase during PFM mode (Figure 8) causing the output voltage to fall below the ‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode. When VIN =2.7V the part transitions from PWM to PFM mode at ~ 35 mA output current and from PFM to PWM mode at ~ 95 mA , when VIN=3.6V, PWM to PFM transition occurs at ~ 42 mA and PFM to PWM transition occurs at ~ 115 mA, when VIN =4.5V, PWM to PFM transition occurs at ~ 60 mA and PFM to PWM transition occurs at ~ 135 mA. LM3677 A 1.0 µH inductor with a saturation current rating of at least 1375 mA is recommended for most applications. The inductor’s resistance should be less than 0.15Ω for good efficiency. Table 1 lists suggested inductors and suppliers. For low-cost applications, an unshielded bobbin inductor could be considered. For noise critical applications, a toroidal or shielded-bobbin inductor should be used. A good practice is to lay out the board with overlapping footprints of both types for design flexibility. This allows substitution of a low-noise shielded inductor in the event that noise from low-cost bobbin models is unacceptable. up times with a 10 µF output capacitor and 300 mA load is 300 µs and with 1 mA load is 200 µs. Application Information INDUCTOR SELECTION There are two main considerations when choosing an inductor: the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested form the manufacturer. The minimum value of inductance to guarantee good performance is 0.7 µH at ILIM (typ.) DC current over the ambient temperature range. Shielded inductors radiate less noise and should be preferred. There are two methods to choose the inductor saturation current rating. INPUT CAPACITOR SELECTION A ceramic input capacitor of 4.7 µF, 6.3V is sufficient for most applications. Place the input capacitor as close as possible to the VIN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0603 and 0805. The minimum input capacitance to guarantee good performance is 2.2 µF at 3V DC bias; 1.5 µF at 5V DC bias including tolerances and over ambient temperature range. The input filter capacitor supplies current to the PFET switch of the LM3677 in the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor’s low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a capacitor with sufficient ripple current rating. The input current ripple can be calculated as: Method 1: The saturation current is greater than the sum of the maximum load current and the worst case average to peak inductor current. This can be written as • • • • IRIPPLE: average to peak inductor current IOUTMAX: maximum load current (600 mA) VIN: maximum input voltage in application L : min inductor value including worst case tolerances (30% drop can be considered for method 1) • f : minimum switching frequency (2.5 MHz) • VOUT: output voltage Method 2: A more conservative and recommended approach is to choose an inductor that has saturation current rating greater than the max current limit of 1375 mA. www.national.com 16 Model Vendor Dimensions LxWxH(mm) D.C.R (max) MIPSA2520D 1R0 FDK 2.5 x 2.0 x 1.2 100 mΩ LQM2HP 1R0 Murata 2.5 x 2.0 x 0.95 100 mΩ BRL2518T1R0M Taiyo Yuden 2.5x 1.8 x 1.2 80 mΩ Voltage peak-to-peak ripple due to ESR can be expressed as follows VPP-ESR = (2 * IRIPPLE) * RESR Because these two components are out of phase the rms (root mean squared) value can be used to get an approximate value of peak-to-peak ripple. Voltage peak-to-peak ripple, rms can be expressed as follow: OUTPUT CAPACITOR SELECTION A ceramic output capacitor of 10 µF, 6.3V is sufficient for most applications. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0603 and 0805. DC bias characteristics vary from manufacturer to manufacturer and dc bias curves should be requested from them as part of the capacitor selection process. The minimum output capacitance to guarantee good performance is 5.75 µF at 2.5V DC bias including tolerances and over ambient temperature range. The output filter capacitor smooths out current flow from the inductor to the load, helps maintain a steady output voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR to perform these functions. The output voltage ripple is caused by the charging and discharging of the output capacitor and by the RESR and can be calculated as: Voltage peak-to-peak ripple due to capacitance can be expressed as follows Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series resistance of the output capacitor (RESR). The RESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations is at the switching frequency of the part. TABLE 2. Suggested Capacitors and Their Suppliers Type Vendor Voltage Rating Case Size Inch (mm) C1608X5R0J475 Ceramic, X5R TDK 6.3V 0603 (1608) Model 4.7 µF for CIN C2012X5R0J475 Ceramic, X5R TDK 6.3V 0805 (2012) GRM21BR60J475 Ceramic, X5R muRata 6.3V 0805 (2012) JMK212BJ475 Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012) C1608X5R0J106 Ceramic, X5R TDK 6.3V 0603 (1608) C2012X5R0J106 Ceramic, X5R TDK 6.3V 0805 (2012) GRM21BR60J106 Ceramic, X5R muRata 6.3V 0805 (2012) JMK212BJ106 Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012) 10 µF for COUT 17 www.national.com LM3677 TABLE 1. Suggested Inductors and Their Suppliers LM3677 R1: feedback resistor from VOUT to FB (Ω) R2: feedback resistor from to FB to GND (Ω) For the fixed output voltage parts the feedback resistors are internal and R1 is 0Ω. The bypass capacitors C1 and C2 (labeled C3 and C4 on Evaluation Board) in parallel with the feedback resistors are chosen for increased stability. Below are the formulas for C1 and C2: OUTPUT VOLTAGE SELECTION FOR LM3677-ADJ The output voltage of the adjustable parts can be programmed through the resistor network connected from VOUT to FB to GND. The resistor and FB to GND (R2) should be 200 kΩ to keep the current drawn through this network well below 16 µA quiescent current level (PFM mode) but large enough that it is not susceptible to noise. If R2 is 200 kΩ, and given the VFB is 0.5V, then the current through the resistor feedback network will be 2.5 µA. The output voltage of the adjustable parts ranges from 1.2V and 3.3V. The output voltage formula is: 30008404 30008406 VOUT: output voltage (V) VFB: feedback voltage (0.5V typical) 30008405 TABLE 3. LM3677–ADJ Configurations for Various VOUT (Circuit of Figure 2) VOUT (V) R1(kΩ ) R2(kΩ ) C1(pF) C2(pF) L (µH) CIN (µF) COUT (µF) 1.2 280 200 8.2 none 1.0 4.7 10 1.3 320 200 8.2 none 1.0 4.7 10 1.5 357 178 6.8 none 1.0 4.7 10 1.6 442 200 5.6 none 1.0 4.7 10 1.8 464 178 5.6 none 1.0 4.7 10 2.5 402 100 6.0 none 1.0 4.7 10 2.8 464 100 5.6 24 1.0 4.7 10 3.3 562 100 5.6 24 1.0 4.7 10 www.national.com 18 Use of the micro SMD package requires specialized board layout, precision mounting and careful re-flow techniques, as detailed in National Semiconductor Application Note 1112. Refer to the section "Surface Mount Technology (SMD) Assembly Considerations". For best results in assembly, alignment ordinals on the PC board should be used to facilitate placement of the device. The pad style used with micro SMD package must be the NSMD (non-solder mask defined typ.). This means that the solder-mask opening is larger than the pad size. This prevents a lip that otherwise forms if the soldermask and pad overlap, from holding the device off the surface of the board and interfering with mounting. See Application Note 1112 for specific instructions how to do this. The 5-bump package used for LM3677 has 300–micron solder balls and requires 10.82 mils pads for mounting on the circuit board. The trace to each pad should enter the pad with a 90° entry angle to prevent debris from being caught in deep corners. Initially, the trace to each pad should be 7 mil wide, for a section approximately 7 mil long or longer, as a thermal relief. Then each trace should neck up or down to its optimal width. The important criteria is symmetry. This ensures the solder bumps on the LM3677 re-flow evenly and that the device solders level to the board. In particular, special attention must be BOARD LAYOUT CONSIDERATIONS PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a DCDC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss in the traces. These can send erroneous signals to the DC-DC converter IC, resulting in poor regulation or instability. Poor layout can also result in re-flow problems leading to poor solder joints between the micro SMD package and board pads. Poor solder joints can result in erratic or degraded performance. 30008454 FIGURE 9. Board Layout Design Rules for the LM3677 Good layout for the LM3677 can be implemented by following a few simple design rules, as illustrated in . 1. Place the LM3677 on 10.82 mil pads. As a thermal relief, connect to each pad with a 7 mil wide, approximately 7 mil long trace, and then incrementally increase each trace to its optimal width. The important criterion is 2. 19 symmetry to ensure the solder bumps on the re-flow evenly (see Micro SMD Package Assembly and Use). Place the LM3677, inductor and filter capacitors close together and make the traces short. The traces between these components carry relatively high switching currents and act as antennas. Following this rule reduces www.national.com LM3677 paid to the pads for bumps A1 and A3, because GND and VIN are typically connected to large copper planes, inadequate thermal relief can result in late or inadequate re-flow of these bumps. The micro SMD package is optimized for the smallest possible size in applications with red or infrared opaque cases. Because the micro SMD package lacks the plastic encapsulation characteristic of larger devices, it is vulnerable to light. Backside metallization and/or epoxy coating, along with frontside shading by the printed circuit board, reduce this sensitivity. However, the package has exposed die edges. In particular, micro SMD devices are sensitive to light, in the red and infrared range, shining on the package’s exposed die edges. Micro SMD Package Assembly And Use LM3677 3. 4. 5. radiated noise. Special care must be given to place the input filter capacitor very close to the VIN and GND pin. Arrange the components so that the switching current loops curl in the same direction. During the first half of each cycle, current flows from the input filter capacitor, through the LM3677 and inductor to the output filter capacitor and back through ground, forming a current loop. In the second half of each cycle, current is pulled up from ground, through the LM3677 by the inductor, to the output filter capacitor and then back through ground, forming a second current loop. Routing these loops so the current curls in the same direction prevents magnetic field reversal between the two half-cycles and reduces radiated noise. Connect the ground pins of the LM3677, and filter capacitors together using generous component-side copper fill as a pseudo-ground plane. Then connect this to the ground-plane (if one is used) with several vias. This reduces ground-plane noise by preventing the switching currents from circulating through the ground plane. It also reduces ground bounce at the LM3677 by giving it a lowimpedance ground connection. Use wide traces between the power components and for power connections to the DC-DC converter circuit. This www.national.com reduces voltage errors caused by resistive losses across the traces 6. Route noise sensitive traces such as the voltage feedback path away from noisy traces between the power components. The voltage feedback trace must remain close to the LM3677 circuit and should be routed directly from FB to VOUT at the output capacitor and should be routed opposite to noise components. This reduces EMI radiated onto the DC-DC converter’s own voltage feedback trace. 7. Place noise sensitive circuitry, such as radio IF blocks, away from the DC-DC converter, CMOS digital blocks and other noisy circuitry. Interference with noisesensitive circuitry in the system can be reduced through distance. In mobile phones, for example, a common practice is to place the DC-DC converter on one corner of the board, arrange the CMOS digital circuitry around it (since this also generates noise), and then place sensitive preamplifiers and IF stages on the diagonally opposing corner. Often, the sensitive circuitry is shielded with a metal pan and power to it is postregulated to reduce conducted noise, using low-dropout linear regulators. 20 LM3677 Physical Dimensions inches (millimeters) unless otherwise noted 5-Bump (Large) Micro SMD Package, 0.5 mmPitch NS Package Number TLA05FEA The dimensions for X1, X2, and X3 are as given: X1 = 1.107 mm +/- 0.030 mm X2 = 1.488 mm +/- 0.030 mm X3 = 0.600 mm +/- 0.075 mm 21 www.national.com LM3677 6-pin LLP Package, 0.5 mm Pitch NS Package Number LEB06A The dimensions for A, B, and C are as given: A = 2.0 mm +/- 0.1 mm B = 1.5 mm +/- 0.1 mm C = 0.60 mm +/- 0.06 mm www.national.com 22 LM3677 Notes 23 www.national.com LM3677 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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