LM3670

LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits January 2006 LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits General Description 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). 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. Features 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 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 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 # 1 2 3 4 5 Name VIN GND EN FB SW Description Power supply input. Connect to the input filter capacitor (Figure 1). Ground pin. Enable input. Feedback analog input. Connect to the output filter capacitor (Figure 1). 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 2 LM3670 Ordering Information Voltage Option (V) 3.3 Order Number (Level 95) LM3670MF-3.3 LM3670MFX-3.3 LM3670MF-3.3 LM3670MFX-3.3 2.5 LM3670MF-2.5 LM3670MFX-2.5 LM3670MF-2.5 LM3670MFX-2.5 1.875 LM3670MF-1.875 LM3670MFX-1.875 LM3670MF-1.875 LM3670MFX-1.875 1.8 LM3670MF-1.8 LM3670MFX-1.8 LM3670MF-1.8 LM3670MFX-1.8 1.6 LM3670MF-1.6 LM3670MFX-1.6 LM3670MF-1.6 LM3670MFX-1.6 1.5 LM3670MF-1.5 LM3670MFX-1.5 LM3670MF-1.5 LM3670MFX-1.5 1.2 LM3670MF-1.2 LM3670MFX-1.2 LM3670MF-1.2 LM3670MFX-1.2 Adjustable LM3670MF-ADJ LM3670MFX-ADJ LM3670MF-ADJ LM3670MFX-ADJ NOPB NOPB SDFB NOPB NOPB SCZB NOPB NOPB S82B NOPB NOPB SDBB NOPB NOPB SDCB NOPB NOPB SEFB NOPB NOPB SDDB SPEC NOPB NOPB Package Marking SDEB Supplied As (#/reel) 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 1000 3000 3 www.national.com LM3670 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN Pin: Voltage to GND EN Pin: Voltage to GND FB, SW Pin: Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temperature (Soldering, 10 sec.) −0.2V to 6.0V −0.2V to 6.0V (GND−0.2V) to (VIN + 0.2V) −45˚C to +125˚C −45˚C to +150˚C 260˚C ESD Rating (Note 3) Human Body Model: VIN, SW, FB, EN, GND Machine Model: 2.0kV 200V Operating Ratings (Notes 1, 2) Input Voltage Range Recommended Load Current Junction Temperature (TJ) Range Ambient Temperature (TA) Range 2.5V to 5.5V 0A to 350 mA −40˚C to +125˚C −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 Symbol VIN VOUT Parameter Input Voltage Range Fixed Output Voltage: 1.2V (Note 5) 2.5V ≤ VIN ≤ 5.5V IO = 10 mA 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 150 mA Fixed Output Voltage: 1.5V 2.5V ≤ VIN ≤ 5.5V IO = 10 mA 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA Fixed Output Voltage: 1.6V, 1.875V 2.5V ≤ VIN ≤ 5.5V IO = 10 mA 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA Fixed Output Voltage: 1.8V 2.5V ≤ VIN ≤ 5.5V IO = 10 mA 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA Fixed Output Voltage: 2.5V, 3.3V 3.6V ≤ VIN ≤ 5.5V IO = 10 mA 3.6V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 350 mA Adjustable Output Voltage (Note 4) 2.5V ≤ VIN ≤ 5.5V IO = 10 mA 2.5V ≤ VIN ≤ 5.5V 0 mA ≤ IO ≤ 150 mA Line_reg Load_reg VREF IQ_SHDN Line Regulation Load Regulation Internal Reference Voltage Shutdown Supply Current TA=85oC 2.5V ≤ VIN ≤ 5.5V IO = 10 mA 150 mA ≤ IO ≤ 350 mA Condition Min 2.5 -2.0 -4.5 -2.5 -5.0 -2.5 -5.5 -1.5 −4.5 -2.0 -6.0 -2.5 -4.0 0.26 0.0014 0.5 0.1 1 Typ Max 5.5 +4.0 +4.0 +4.0 +4.0 +4.0 +4.0 +3.0 +3.0 +4.0 +4.0 +4.5 +4.5 %/V %/mA V µA % % % % % Units V % www.national.com 4 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) Symbol IQ Parameter DC Bias Current into VIN Condition No load, device is not switching (VOUT forced higher than programmed output voltage) Min Typ 15 Max 30 Units µA VUVLO RDSON (P) RDSON (N) ILKG (P) ILKG (N) ILIM η Minimum VIN below which VOUT will be disabled Pin-Pin Resistance for PFET Pin-Pin Resistance for NFET P Channel Leakage Current N Channel Leakage Current Switch Peak Current Limit Efficiency (VIN = 3.6V, VOUT = 1.8V) ILOAD = 1 mA ILOAD = 10 mA ILOAD = 100 mA ILOAD = 200 mA ILOAD = 300 mA ILOAD = 350 mA VIN=VGS=3.6V VIN=VGS=3.6V VDS=5.5V VDS=5.5V 400 2.4 360 250 0.1 0.1 620 91 94 94 94 92 90 1.3 0.4 0.01 PWM Mode 550 1000 1 1300 690 660 1 1.5 750 V mΩ mΩ µA µA mA % VIH VIL IEN FOSC Logic High Input Logic Low Input Enable (EN) Input Current Internal Oscillator Frequency V V µA 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 20075832 FIGURE 4. Simplified Functional Diagram www.national.com 6 LM3670 Typical Performance Characteristics IQ (Non-switching) vs. VIN (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V) 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) Frequency vs. Temperature RDSON vs. VIN P & N Channel (Continued) 20075810 20075811 Line Transient (VIN = 2.6V to 3.6V, ILOAD = 100 mA) Line Transient (VIN = 3.6V to 4.6V , ILOAD = 100 mA) 20075812 20075813 Load Transient ILOAD = 3mA to 280mA Load Transient ILOAD = 0mA to 70mA 20075816 20075817 www.national.com 8 LM3670 Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V) Load Transient ILOAD = 0mA to 280mA Load Transient ILOAD = 0mA to 350mA (Continued) 20075818 20075819 Load Transient ILOAD = 50mA to 350mA Load Transient ILOAD = 100mA to 300mA 20075820 20075821 PFM Mode VSW, VOUT, IINDUCTOR vs. Time PWM 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) Soft Start VIN, VOUT, IINDUCTOR vs. Time (ILOAD = 350mA) (Continued) 20075824 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. 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 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 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. 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 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: 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. 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) 0 70 140 Duration (µSec) 32 224 256 11 www.national.com Inrush Current (mA) 280 620 Duration (µSec) 256 until soft start ends Note 6: The first 32µS are to allow the bias currents to stabilize LM3670 Operation Description LDO - Low Drop Out Operation (Continued) 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. The minimum input voltage needed to support the output voltage is 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. INDUCTOR SELECTION 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. There are two methods to choose the inductor current rating. Method 1: The total current is the sum of the load and the inductor ripple current. This can be written as • ILOAD • RDSON, PFET Load current Drain to source resistance of PFET switch in the triode region • RINDUCTOR Inductor resistance 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: • 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: • 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 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: 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. 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 www.national.com 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 IDC2512NB100M DO1608C-103 ELL6RH100M CDRH5D18-100 Vendor Vishay Coilcraft Panasonic Sumida Phone 408-727-2500 847-639-6400 714-373-7366 847-956-0666 FAX 408-330-4098 847-639-1469 714-373-7323 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. The output ripple current can be calculated as: Voltage peak-to-peak ripple due to capacitance = Voltage peak-to-peak ripple, root mean squared = Voltage peak-to-peak ripple due to ESR = 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. TABLE 2. Suggested Capacitors and Their Suppliers Model 10 µF for COUT VJ1812V106MXJAT LMK432BJ106MM JMK325BJ106MM 4.7 µF for CIN VJ1812V475MXJAT EMK325BJ475MN C3216X5R0J475M Ceramic Ceramic Ceramic Vishay Taiyo-Yuden TDK 408-727-2500 847-925-0888 847-803-6100 408-330-4098 847-925-0899 847-803-6296 Ceramic Ceramic Ceramic Vishay Taiyo-Yuden Taiyo-Yuden 408-727-2500 847-925-0888 847-925-0888 408-330-4098 847-925-0899 847-925-0899 Type Vendor Phone FAX TABLE 3. Adjustable LM3670 Configurations for Various VOUT VOUT (V) 0.7 0.8 0.9 1.0 R1 (KΩ) 80.6 120 160 200 R2 (KΩ) 200 200 200 200 C1 (pF) 200 130 100 82 C2 (pF) 150 none none none L (µH) 4.7 4.7 4.7 4.7 CIN (µF) 4.7 4.7 4.7 4.7 COUT (µF) 10 10 10 10 13 www.national.com LM3670 Application Information VOUT (V) 1.1 1.2 1.24 1.24 1.5 1.6 1.7 1.875 R1 (KΩ) 240 280 300 221 402 442 487 549 (Continued) TABLE 3. Adjustable LM3670 Configurations for Various VOUT (Continued) R2 (KΩ) 200 200 200 150 200 200 200 200 C1 (pF) 68 56 56 75 39 39 33 30 C2 (pF) none none none 120 none none none none L (µH) 4.7 4.7 4.7 4.7 10 10 10 10 CIN (µF) 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 COUT (µF) 10 10 10 10 10 10 10 14.7 Note: (10 || 4.7) 2.5 806 200 22 82 10 4.7 22 www.national.com 14 LM3670 Application Information BOARD LAYOUT CONSIDERATIONS (Continued) PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a 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. 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 . 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 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, www.national.com LM3670 Application Information (Continued) 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 post-regulated to reduce conducted noise, using lowdropout linear regulators. www.national.com 16 LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits Physical Dimensions 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. 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