LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits General Description Features The LM3670 step-down DC-DC converter is optimized for powering ultra-low voltage circuits from a single Li-Ion cell or 3 cell NiMH/NiCd batteries. It provides up to 350 mA load current, over an input voltage range from 2.5V to 5.5V. There are several different fixed voltage output options available as well as an adjustable output voltage version (see ordering information). n VOUT = Adj (0.7V min), 1.2, 1.5, 1.6, 1.8, 1.875, 2.5, 3.3V n 2.5V ≤ VIN ≤ 5.5V n 15 µA typical quiescent current n 350 mA maximum load capability n 1 MHz PWM fixed switching frequency (typ.) n Automatic PFM/PWM mode switching n Available in fixed output voltages as well as an adjustable version n SOT23-5 package n Low drop out operation - 100% duty cycle mode n Internal synchronous rectification for high efficiency n Internal soft start n 0.1 µA typical shutdown current n Operates from a single Li-Ion cell or 3 cell NiMH/NiCd batteries n Only three tiny surface-mount external components required (one inductor, two ceramic capacitors) n Current overload protection 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 full-power operation, a fixed-frequency 1 MHz (typ). PWM mode drives loads from ∼70 mA to 350 mA max, with up to 95% efficiency. Hysteretic PFM mode extends the battery life through reduction of the quiescent current to 15 µA (typ) during light current loads and system standby. Internal synchronous rectification provides high efficiency (90 to 95% typ. at loads between 1 mA and 100 mA). In shutdown mode (Enable pin pulled low) the device turns off and reduces battery consumption to 0.1 µA (typ.). The LM3670 is available in a SOT23-5 package. A high switching frequency - 1 MHz (typ) - allows use of tiny surface-mount components. Only three external surfacemount components, an inductor and two ceramic capacitors, are required. Applications n n n n n n Mobile phones HandHeld PDAs Palm-top PCs Portable Instruments Battery Powered Devices Typical Application 20075801 FIGURE 1. Fixed Output Voltage - Typical Application Circuit © 2006 National Semiconductor Corporation DS200758 www.national.com LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits January 2006 LM3670 Typical Application (Continued) 20075830 FIGURE 2. Adjustable Output Voltage - Typical Application Circuit Connection Diagram and Package Mark Information SOT23-5 Package NS Package Number MF05A 20075802 Note: The actual physical placement of the package marking will vary from part to part. FIGURE 3. Top View Pin Descriptions Pin # Name 1 VIN 2 GND 3 EN 4 FB Feedback analog input. Connect to the output filter capacitor (Figure 1). 5 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier. Connect to an inductor with a saturation current rating that exceeds the 750 mA max. Switch Peak Current Limit specification. www.national.com Description Power supply input. Connect to the input filter capacitor (Figure 1). Ground pin. Enable input. 2 Voltage Option (V) 3.3 Order Number (Level 95) SPEC LM3670MF-3.3 NOPB LM3670MFX-3.3 NOPB Package Marking Supplied As (#/reel) SDEB 1000 3000 LM3670MF-3.3 1000 LM3670MFX-3.3 2.5 3000 LM3670MF-2.5 NOPB LM3670MFX-2.5 NOPB SDDB 3000 LM3670MF-2.5 1000 LM3670MFX-2.5 1.875 3000 LM3670MF-1.875 NOPB LM3670MFX-1.875 NOPB SEFB LM3670MF-1.875 1000 3000 LM3670MF-1.8 NOPB LM3670MFX-1.8 NOPB SDCB 1000 LM3670MFX-1.8 1.5 3000 LM3670MF-1.6 NOPB LM3670MFX-1.6 NOPB SDBB 3000 1000 LM3670MFX-1.6 3000 LM3670MF-1.5 NOPB LM3670MFX-1.5 NOPB S82B 1000 3000 1000 LM3670MFX-1.5 3000 LM3670MF-1.2 NOPB LM3670MFX-1.2 NOPB SCZB 1000 3000 LM3670MF-1.2 1000 LM3670MFX-1.2 Adjustable 1000 LM3670MF-1.6 LM3670MF-1.5 1.2 1000 3000 LM3670MF-1.8 1.6 1000 3000 LM3670MFX-1.875 1.8 1000 3000 LM3670MF-ADJ NOPB LM3670MFX-ADJ NOPB SDFB 1000 3000 LM3670MF-ADJ 1000 LM3670MFX-ADJ 3000 3 www.national.com LM3670 Ordering Information LM3670 Absolute Maximum Ratings (Note 1) ESD Rating (Note 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Human Body Model: VIN Pin: Voltage to GND EN Pin: Voltage to GND FB, SW Pin: VIN, SW, FB, EN, GND 2.0kV Machine Model: 200V −0.2V to 6.0V −0.2V to 6.0V Operating Ratings (Notes 1, 2) (GND−0.2V) to (VIN + 0.2V) Input Voltage Range 2.5V to 5.5V Junction Temperature (TJ-MAX) −45˚C to +125˚C Recommended Load Current 0A to 350 mA Storage Temperature Range −45˚C to +150˚C Junction Temperature (TJ) Range −40˚C to +125˚C Maximum Lead Temperature (Soldering, 10 sec.) 260˚C Ambient Temperature (TA) Range −40˚C to +85˚C Thermal Properties Junction-to-Ambient Thermal Resistance (θJA) (SOT23-5) 250˚C/W Electrical Characteristics Limits in standard typeface are for TJ = 25˚C. Limits in boldface type apply over the full operating junction temperature range (−40˚C ≤ TJ ≤ +125˚C). Unless otherwise noted VIN = 3.6V, VOUT = 1.8V, IO = 150mA, EN = VIN Max Units VIN Symbol Input Voltage Range Parameter (Note 5) 2.5 5.5 V VOUT Fixed Output Voltage: 1.2V 2.5V ≤ VIN ≤ 5.5V IO = 10 mA -2.0 +4.0 % 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 150 mA -4.5 +4.0 2.5V ≤ VIN ≤ 5.5V IO = 10 mA -2.5 +4.0 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA -5.0 +4.0 2.5V ≤ VIN ≤ 5.5V IO = 10 mA -2.5 +4.0 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA -5.5 +4.0 2.5V ≤ VIN ≤ 5.5V IO = 10 mA -1.5 +3.0 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA −4.5 +3.0 3.6V ≤ VIN ≤ 5.5V IO = 10 mA -2.0 +4.0 3.6V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA -6.0 +4.0 2.5V ≤ VIN ≤ 5.5V IO = 10 mA -2.5 +4.5 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 150 mA -4.0 +4.5 Fixed Output Voltage: 1.5V Fixed Output Voltage: 1.6V, 1.875V Fixed Output Voltage: 1.8V Fixed Output Voltage: 2.5V, 3.3V Adjustable Output Voltage (Note 4) Condition Line_reg Line Regulation 2.5V ≤ VIN ≤ 5.5V IO = 10 mA Load_reg Load Regulation 150 mA ≤ IO ≤ 350 mA VREF Internal Reference Voltage IQ_SHDN Shutdown Supply Current www.national.com Min Typ % % % %/V 0.0014 %/mA 0.1 4 % 0.26 0.5 TA=85oC % V 1 µA Symbol Parameter IQ DC Bias Current into VIN VUVLO Minimum VIN below which VOUT will be disabled Condition Min No load, device is not switching (VOUT forced higher than programmed output voltage) Typ Max Units 15 30 µA V 2.4 RDSON (P) Pin-Pin Resistance for PFET VIN=VGS=3.6V 360 690 mΩ RDSON (N) Pin-Pin Resistance for NFET VIN=VGS=3.6V 250 660 mΩ ILKG (P) P Channel Leakage Current VDS=5.5V 0.1 1 µA ILKG (N) N Channel Leakage Current VDS=5.5V ILIM Switch Peak Current Limit η Efficiency (VIN = 3.6V, VOUT = 1.8V) 400 Logic High Input VIL Logic Low Input IEN Enable (EN) Input Current FOSC Internal Oscillator Frequency 1.5 µA 750 mA ILOAD = 1 mA 91 ILOAD = 10 mA 94 ILOAD = 100 mA 94 ILOAD = 200 mA 94 ILOAD = 300 mA 92 ILOAD = 350 mA VIH 0.1 620 % 90 1.3 V 0.4 PWM Mode 550 V 0.01 1 µA 1000 1300 kHz Note 1: 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 2: All voltages are with respect to the potential at the GND pin. Note 3: 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 4: Output voltage specification for the adjustable version includes tolerance of the external resistor divider. Note 5: The input voltage range recommended for the specified output voltages are given below: VIN = 2.5V to 5.5V for 0.7V ≤ VOUT < 1.875V VIN = ( VOUT + VDROP OUT) to 5.5V for 1.875 ≤ VOUT≤ 3.3V Where VDROP OUT = ILOAD * (RDSON (P) + RINDUCTOR) 5 www.national.com LM3670 Electrical Characteristics Limits in standard typeface are for TJ = 25˚C. Limits in boldface type apply over the full operating junction temperature range (−40˚C ≤ TJ ≤ +125˚C). Unless otherwise noted VIN = 3.6V, VOUT = 1.8V, IO = 150mA, EN = VIN (Continued) LM3670 20075832 FIGURE 4. Simplified Functional Diagram www.national.com 6 LM3670 Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V) IQ (Non-switching) vs. VIN IQ vs. Temp 20075805 20075804 VOUT vs. VIN VOUT vs. IOUT 20075806 20075807 Efficiency vs. IOUT Efficiency vs. VIN 20075808 20075809 7 www.national.com LM3670 Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V) (Continued) RDSON vs. VIN P & N Channel Frequency vs. Temperature 20075810 20075811 Line Transient (VIN = 3.6V to 4.6V , ILOAD = 100 mA) Line Transient (VIN = 2.6V to 3.6V, ILOAD = 100 mA) 20075812 20075813 Load Transient ILOAD = 0mA to 70mA Load Transient ILOAD = 3mA to 280mA 20075817 20075816 www.national.com 8 Load Transient ILOAD = 0mA to 280mA (Continued) Load Transient ILOAD = 0mA to 350mA 20075818 20075819 Load Transient ILOAD = 100mA to 300mA Load Transient ILOAD = 50mA to 350mA 20075820 20075821 PWM Mode VSW, VOUT, IINDUCTOR vs. Time PFM Mode VSW, VOUT, IINDUCTOR vs. Time 20075822 20075823 9 www.national.com LM3670 Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V) LM3670 Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V) (Continued) Soft Start VIN, VOUT, IINDUCTOR vs. Time (ILOAD = 350mA) 20075824 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 Operation Description DEVICE INFORMATION The LM3670, a high efficiency step down DC-DC switching buck converter, delivers a constant voltage from either a single Li-Ion or three cell NiMH/NiCd battery to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, the LM3670 has the ability to deliver up to 350 mA depending on the input voltage and output voltage (voltage head room), and the inductor chosen (maximum current capability). There are three modes of operation depending on the current required - PWM (Pulse Width Modulation), PFM (Pulse Frequency Modulation), and shutdown. PWM mode handles current loads of approximately 70 mA or higher. Lighter output current loads cause the device to automatically switch into PFM for reduced current consumption (IQ = 15 µA typ) and a longer battery life. Shutdown mode turns off the device, offering the lowest current consumption (ISHUTDOWN = 0.1 µA typ). The LM3670 can operate up to a 100% duty cycle (PMOS switch always on) for low drop out control of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage. Additional features include soft-start, under voltage lock out, current overload protection, and thermal overload protection. As shown in Figure 1, only three external power components are required for implementation. The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage across the load. PWM OPERATION During PWM operation the converter operates as a voltagemode controller with input voltage feed forward. This allows the converter to achieve excellent 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. Internal Synchronous Rectification While in PWM mode, the LM3670 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 LM3670 operates as follows. During the first portion of each switching cycle, the control block in the LM3670 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 Current Limiting A current limit feature allows the LM3670 to protect itself and external components during overload conditions PWM mode implements cycle-by-cycle current limiting using an internal comparator that trips at 620 mA (typ). PFM OPERATION At very light load, the converter enters PFM mode and operates with reduced switching frequency and supply current to maintain high efficiency. www.national.com 10 LM3670 Operation Description (Continued) The part automatically transition into PFM mode when either of two conditions occurs for a duration of 32 or more clock cycles: A. The inductor current becomes discontinuous 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 5), 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 less than 30 µA, which allows the part to achieve high efficiencies under extremely light load conditions. When the output drops below the ‘low’ PFM threshold, the cycle repeats to restore the output voltage to ∼1.6% above the nominal PWM output voltage. If the load current should increase during PFM mode (see Figure 5) causing the output voltage to fall below the ‘low2’ PFM threshold, the part automatically transitions into fixedfrequency PWM mode. B. The peak PMOS switch current drops below the IMODE level: During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage in PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy load. The PFM comparator senses the output voltage via the feedback pin and control the switching of the output FETs such that the output voltage ramps between 0.8% and 1.6% (typ) above the nominal PWM output voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on. It remains on until the output voltage exceeds the ‘high’ PFM threshold or the peak current exceeds the IPFM level set for PFM mode. The peak current in PFM mode is: 20075803 FIGURE 5. Operation in PFM Mode and Transition to PWM Mode Soft-Start The LM3670 has a soft-start circuit that limits in-rush current during start-up. Typical start-up times with a 10µF output capacitor and 350mA load is 400µs: Inrush Current (mA) Duration (µSec) 0 32 70 224 140 256 Inrush Current (mA) Duration (µSec) 280 256 620 until soft start ends Note 6: The first 32µS are to allow the bias currents to stabilize 11 www.national.com LM3670 Operation Description (Continued) LDO - Low Drop Out Operation The LM3670 can operate at 100% duty cycle (no switching, PMOS switch is completely on) for low drop out support of the output voltage. In this way the output voltage is controlled down to the lowest possible input voltage. A pole can also be used at higher output voltages. For example, in the table Table 3, there is an entry for 1.24V with both a pole and zero at approximately 10kHz for noise rejection. The minimum input voltage needed to support the output voltage is INDUCTOR SELECTION • ILOAD • RDSON, PFET There are two main considerations when choosing an inductor; the inductor current should not saturate, and the inductor current ripple is small enough to achieve the desired output voltage ripple. Load current Drain to source resistance of PFET switch in the triode region There are two methods to choose the inductor current rating. Method 1: • RINDUCTOR Inductor resistance The total current is the sum of the load and the inductor ripple current. This can be written as Application Information OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE LM3670 The output voltage of the adjustable parts can be programmed through the resistor network connected from VOUT to VFB then to GND. VOUT is adjusted to make VFB equal to 0.5V. The resistor from VFB to GND (R2) should be at least 100KΩ to keep the current sunk through this network well below the 15µA quiescent current level (PFM mode with no switching) but large enough that it is not susceptible to noise. If R2 is 200KΩ, and VFB is 0.5V, then the current through the resistor feedback network is 2.5µA ( IFB =0.5V/R2). The output voltage formula is: • ILOAD load current • VIN input voltage • L inductor • f switching frequency • IRIPPLE peak-to-peak Method 2: A more conservative approach is to choose an inductor that can handle the current limit of 700 mA. Given a peak-to-peak current ripple (IPP) the inductor needs to be at least • VOUT Output Voltage (V) • VFB Feedback Voltage (0.5V typ) • R1 Resistor from VOUT to VFB (Ω) • R2 Resistor from VOUT to GND (Ω) For any output voltage greater than or equal to 0.7V a frequency zero must be added at 10kHz for stability. The formula is: A 10 µH inductor with a saturation current rating of at least 800 mA is recommended for most applications. The inductor’s resistance should be less than around 0.3Ω for good efficiency. Table 1 lists suggested inductors and suppliers. For low-cost applications, an unshielded bobbin inductor is suggested. 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 toroidal inductor, in the event that noise from low-cost bobbin models is unacceptable. For any output voltages below 0.7 and above or equal to 2.5V, a pole must also be placed at 10kHz as well. The lowest output voltage possible is 0.7V. At low output voltages the duty cycle is very small and, as the input voltage increases, the duty cycle decreases even further. Since the duty cycle is so low any change due to noise is an appreciable percentage. In other words, it is susceptible to noise. Capacitors C1 and C2 act as noise filters rather than frequency poles and zeros. If the pole and zero are at the same frequency the formula is: www.national.com INPUT CAPACITOR SELECTION A ceramic input capacitor of 4.7 µF is sufficient for most applications. A larger value may be used for improved input voltage filtering. The input filter capacitor supplies current to the PFET switch of the LM3670 in the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capcitor’s low ESR provides the best 12 LM3670 Application Information (Continued) noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with a surge current rating sufficient for the power-up surge from the input power source. The power-up surge current is approximately the capacitor’s value (µF) times the voltage rise rate (V/µs). The input current ripple can be calculated as: TABLE 1. Suggested Inductors and Their Suppliers Model Vendor Phone FAX IDC2512NB100M Vishay 408-727-2500 408-330-4098 DO1608C-103 Coilcraft 847-639-6400 847-639-1469 ELL6RH100M Panasonic 714-373-7366 714-373-7323 CDRH5D18-100 Sumida 847-956-0666 847-956-0702 OUTPUT CAPACITOR SELECTION The output filter capacitor smoothes out current flow from the inductor to the load, maintaining 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. Voltage peak-to-peak ripple, root mean squared = The output ripple current can be calculated as: Voltage peak-to-peak ripple due to capacitance = Note that the output ripple is dependent on the current ripple and the equivalent series resistance of the output capacitor (RESR). Because these two components are out of phase the rms value is used. The RESR is frequency dependent (as well as temperature dependent); make sure the frequency of the RESR given is the same order of magnitude as the switching frequency. Voltage peak-to-peak ripple due to ESR = TABLE 2. Suggested Capacitors and Their Suppliers Model Type Vendor Phone FAX 10 µF for COUT VJ1812V106MXJAT Ceramic Vishay 408-727-2500 408-330-4098 LMK432BJ106MM Ceramic Taiyo-Yuden 847-925-0888 847-925-0899 JMK325BJ106MM Ceramic Taiyo-Yuden 847-925-0888 847-925-0899 4.7 µF for CIN VJ1812V475MXJAT Ceramic Vishay 408-727-2500 408-330-4098 EMK325BJ475MN Ceramic Taiyo-Yuden 847-925-0888 847-925-0899 C3216X5R0J475M Ceramic TDK 847-803-6100 847-803-6296 TABLE 3. Adjustable LM3670 Configurations for Various VOUT VOUT (V) R1 (KΩ) R2 (KΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF) 0.7 80.6 200 200 150 4.7 4.7 10 0.8 120 200 130 none 4.7 4.7 10 0.9 160 200 100 none 4.7 4.7 10 1.0 200 200 82 none 4.7 4.7 10 13 www.national.com LM3670 Application Information (Continued) TABLE 3. Adjustable LM3670 Configurations for Various VOUT (Continued) VOUT (V) R1 (KΩ) R2 (KΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF) 1.1 240 200 68 none 4.7 4.7 10 1.2 280 200 56 none 4.7 4.7 10 1.24 300 200 56 none 4.7 4.7 10 1.24 221 150 75 120 4.7 4.7 10 1.5 402 200 39 none 10 4.7 10 1.6 442 200 39 none 10 4.7 10 1.7 487 200 33 none 10 4.7 10 1.875 549 200 30 none 10 4.7 14.7 Note: (10 || 4.7) 2.5 www.national.com 806 200 22 82 14 10 4.7 22 DC-DC 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. (Continued) BOARD LAYOUT CONSIDERATIONS PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a 20075831 FIGURE 6. Board Layout Design Rules for the LM3670 Good layout for the LM3670 can be implemented by following a few simple design rules, as illustrated in . 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 LM3670 by giving it a low-impedance ground connection. 4. Use wide traces between the power components and for power connections to the DC-DC converter circuit. This reduces voltage errors caused by resistive losses across the traces. 5. 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 LM3670 circuit and should be direct but should be routed opposite to noisy components. This reduces EMI radiated onto the DC-DC converter’s own voltage feedback trace. 6. 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, 1. Place the LM3670, 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 radiated noise. Place the capacitors and inductor within 0.2 in. (5 mm) of the LM3670. 2. 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 LM3670 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 LM3670 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. 3. Connect the ground pins of the LM3670, and filter capacitors together using generous component-side copper fill as a pseudo-ground plane. Then, connect this to 15 www.national.com LM3670 Application Information LM3670 Application Information IF stages on the diagonally opposing corner. Often, the sensitive circuitry is shielded with a metal pan and power to it is post-regulated to reduce conducted noise, using lowdropout linear regulators. (Continued) arrange the CMOS digital circuitry around it (since this also generates noise), and then place sensitive preamplifiers and www.national.com 16 inches (millimeters) unless otherwise noted 5-Lead SOT23-5 Package NS Package Number MF05A National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. BANNED SUBSTANCE COMPLIANCE National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. Leadfree products are RoHS compliant. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits Physical Dimensions