LM3674 2MHz, 600mA Step-Down DC-DC Converter in SOT 23-5 General Description Features The LM3674 step-down DC-DC converter is optimized for powering low voltage circuits from a single Li-Ion cell battery and input voltage rails from 2.7V to 5.5V. It provides up to 600mA load current, over the entire input voltage range. There are several fixed output voltages and adjustable output voltage versions. n 600mA max load current n Input voltage range from 2.7V to 5.5V n Available in fixed and adjustable output voltages ranging from 1.0V to 3.3V n Operates from a single Li-Ion cell Battery n Internal synchronous rectification for high efficiency n Internal soft start n 0.01 µA typical shutdown current n 2 MHz PWM fixed switching frequency (typ) n SOT23-5 package n Current overload protection and Thermal shutdown protection The device offers superior features and performance for mobile phones and similar portable systems. During PWM mode, the device operates at a fixed-frequency of 2 MHz (typ). Internal synchronous rectification provides high efficiency during PWM mode operation. In shutdown mode, the device turns off and reduces battery consumption to 0.01 µA (typ). The LM3674 is available in SOT23-5 in leaded (PB) and lead-free (NO PB) versions. A high switching frequency of 2 MHz (typ) allows use of only three tiny external surfacemount components, an inductor and two ceramic capacitors. Applications n n n n n n n Mobile phones PDAs MP3 players Portable instruments W-LAN Digital still cameras Portable Hard disk drives Typical Application 20167201 FIGURE 1. Typical Application Circuit © 2006 National Semiconductor Corporation DS201672 www.national.com LM3674 2MHz, 600mA Step-Down DC-DC Converter in SOT 23-5 September 2006 LM3674 Typical Application (Continued) 20167230 FIGURE 2. Typical Application Circuit Connection Diagram and Package Mark Information SOT23-5 Package NS Package Number MF05A 20167202 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 Enable input. The device is in shutdown mode when voltage to this pin is < 0.4V and enable when > 1.0V. Do not leave this pin floating. 4 FB Feedback analog input. Connect to the output filter capacitor for fixed voltage versions. For adjustable version external resistor dividers are required ( Figure 2). The internal resistor dividers are disabled for the adjustable version. 5 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier. www.national.com Description Power supply input. Connect to the input filter capacitor ( Figure 1). Ground pin. 2 Voltage Option (V) 1.2 Order Number (Level 95) SPEC LM3674MF-1.2 NO PB LM3674MFX-1.2 NO PB Package Marking Supplied As (#/reel) SLRB 1000 3000 LM3674MF-1.2 1000 LM3674MFX-1.2 1.5 3000 LM3674MF-1.5 NO PB LM3674MFX-1.5 NO PB SLSB 3000 LM3674MF-1.5 1000 LM3674MFX-1.5 1.8 3000 LM3674MF-1.8 NO PB LM3674MFX-1.8 NO PB SLHB LM3674MF-1.8 1000 3000 LM3674MF-1.875 NO PB LM3674MF-1.875 NO PB SNNB 1000 LM3674MF-1.875 ADJ 1000 3000 LM3674MF-1.875 2.8 1000 3000 LM3674MFX-1.8 1.875 1000 3000 LM3674MF-2.8 NO PB LM3674MFX-2.8 NO PB SLZB 1000 3000 LM3674MF-2.8 1000 LM3674MFX-2.8 3000 LM3674MF-ADJ NO PB LM3674MFX-ADJ NO PB SLTB 1000 3000 LM3674MF-ADJ 1000 LM3674MFX-ADJ 3000 3 www.national.com LM3674 Ordering Information LM3674 Absolute Maximum Ratings (Note 1) Operating Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN Pin: Voltage to GND Input Voltage Range (Note 11) Recommended Load Current −0.2V to 6.0V EN, FB, SW Pin: (GND−0.2V) to (VIN + 0.2V) Continuous Power Dissipation Junction Temperature (TJ-MAX) +125˚C −65˚C to +150˚C Maximum Lead Temperature (Soldering, 10 sec.) 260˚C ESD Rating (Note 3) Human Body model: All Pins 2 kV Machine Model: All Pins 200V Junction Temperature (TJ) Range −30˚C to +125˚C Ambient Temperature (TA) Range −30˚C to +85˚C Thermal Properties Internally Limited Storage Temperature Range 2.7V to 5.5V 0A to 600 mA Junction-to-Ambient Thermal Resistance (θJA) (SOT23-5) for a 2 layer board (Note 6) Junction-to-Ambient Thermal Resistance (θJA) (SOT23-5) for a 4 layer board (Note 6) 250˚C/W 130˚C/W Electrical Characteristics (Notes 2, 9, 10) Limits in standard typeface are for TJ = 25˚C. Limits in boldface type apply over the full operating junction temperature range (−30˚C ≤ TJ ≤ 125˚C). Unless otherwise noted, specifications apply to the LM3674 with VIN = EN = 3.6V Symbol Parameter Condition Min Typ Units +4 % IO = 10mA Line Regulation 2.7V ≤ VIN ≤ 5.5V IO = 100 mA 0.083 %/V Load Regulation 100 mA ≤ IO ≤ 600 mA VIN = 3.6V 0.0010 %/mA VREF Internal Reference Voltage (Note 7) 0.5 V VFB -4 Max Feedback Voltage (Note 12, 13) ISHDN Shutdown Supply Current EN = 0V 0.01 1 µA IQ DC Bias Current into VIN No load, device is not switching (FB=0V) 300 600 µA RDSON (P) Pin-Pin Resistance for PFET ISW = 200mA 380 500 mΩ RDSON (N) Pin-Pin Resistance for NFET ISW = 200mA 250 400 mΩ ILIM Switch Peak Current Limit Open Loop (Note 8) 1020 1200 mA VIH Logic High Input VIL Logic Low Input 0.4 V IEN Enable (EN) Input Current FOSC Internal Oscillator Frequency 830 1.0 PWM Mode 1.6 V 0.01 1 µA 2 2.6 MHz Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device 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). National Semiconductor recommends that all intergrated circuits be handled with appropriate precautions. Failure to observe proper ESD handling techniques can result in damage. 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 Note 5: 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(θJA x PD-MAX). Refer to Dissipation ration table for PD-MAX values at different ambient temperatures. Note 6: 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 250˚C/W is based on measurement results using a 2 layer, 4" X 3", 2 oz. Cu board as per JEDEC standards. The (θJA) is 130˚C/W if a 4 layer, 4" X 3", 2/1/1/2 oz. Cu board as per JEDEC standards is used. Note 7: For the ADJ version the resistor dividers should be selected such that at the desired output voltage, the voltage at the FB pin is 0.5V. 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 at VIN = 3.6V unless otherwise specified. For performance over the input voltage range refer to datasheet curves. www.national.com 4 Note 11: Input voltage range recommended for ideal applications performance for the specified output voltages are given below VIN = 2.7V to 5.5V for 1.0V ≤ VOUT < 1.8V VIN = ( VOUT + VDROP OUT) to 5.5V for 1.8 ≤ VOUT≤ 3.3V Where VDROP OUT = ILOAD * (RDSON (P) + RINDUCTOR) Note 12: ADJ configured to 1.5V output. Note 13: For VOUT less than 2.5V, VIN = 3.6V, for VOUT greater than or equal to 2.5V, VIN = VOUT +1. Dissipation Rating Table θJA TA ≤ 25˚C (Power Rating) TA = 60˚C (Power Rating) TA = 85˚C (Power Rating) 250˚C/W (2 layer board) 400mW 260mW 160mW 130˚C/W (4 layer board) 770mW 500mW 310mW 5 www.national.com LM3674 Electrical Characteristics (Notes 2, 9, 10) Limits in standard typeface are for TJ = 25˚C. Limits in boldface type apply over the full operating junction temperature range (−30˚C ≤ TJ ≤ 125˚C). Unless otherwise noted, specifications apply to the LM3674 with VIN = EN = 3.6V (Continued) LM3674 Block Diagram 20167232 FIGURE 4. Simplified Functional Diagram www.national.com 6 (unless otherwise stated: VIN = 3.6V, VOUT = 1.5V, TA = 25˚C) Quiescent Current vs. Supply Voltage (FB = 0V, No Switching) IQ Shutdown vs. Temp 20167244 20167205 Feedback Bias Current vs. Temp Output Voltage vs. Supply Voltage 20167265 20167206 Output Voltage vs. Temperature Output Voltage vs. Output Current 20167298 20167266 7 www.national.com LM3674 Typical Performance Characteristics LM3674 Typical Performance Characteristics (unless otherwise stated: VIN = 3.6V, VOUT = 1.5V, TA = 25˚C) (Continued) Efficiency vs. Output Current (VOUT = 1.2V, L = 2.2uH, DCR = 200mΩ) RDSON vs. Temperature 20167210 20167267 Efficiency vs. Output Current (VOUT = 1.8V, L = 2.2uH, DCR = 200mΩ) Efficiency vs. Output Current (VOUT = 1.5V, L = 2.2uH, DCR = 200mΩ) 20167268 20167269 Efficiency vs. Output Current (VOUT = 3.3V, L = 2.2uH, DCR = 200mΩ) Switching Frequency vs. Temperature 20167216 20167299 www.national.com 8 Open/Closed Loop Current Limit vs. Temperature Line Transient Response 20167218 20167297 Start Up (Output Current = 300mA) Load Transient 20167223 20167247 Start Up (Output Current = 10mA) 20167224 9 www.national.com LM3674 Typical Performance Characteristics (unless otherwise stated: VIN = 3.6V, VOUT = 1.5V, TA = 25˚C) (Continued) LM3674 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 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 LM3674, a high efficiency step down DC-DC switching buck converter, delivers a constant voltage from a single Li-Ion battery and input voltage rails from 2.7V to 5.5V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, the LM3674 has the ability to deliver up to 600 mA depending on the input voltage, output voltage, ambient temperature and the inductor chosen. There are two modes of operation depending on the current required - PWM (Pulse Width Modulation), and shutdown. The device operates in PWM throughout the IOUT range. 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 overload 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 is 2.7V or higher. CIRCUIT OPERATION During the first portion of each switching cycle, the control block in the LM3674 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 20167275 Internal Synchronous Rectification While in PWM mode, the LM3674 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. 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 Current Limiting A current limit feature allows the LM3674 to protect itself and external components during overload conditions. PWM mode implements current limiting using an internal comparator that trips at 1020 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. The output filter stores charge when the inductor current is high, and releases it when the 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. SOFT-START The LM3674 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 70mA, 140mA, 280mA, and 1020mA (typ. switch current limit). The start-up time thereby depends on the output capacitor and load current demanded at start-up. Typical start-up times with 10µF output capacitor and 300mA load current is 350µs and with 10mA load current is 240µs. PWM OPERATION During PWM ( Pulse Width Modulation) operation the converter operates as a voltage-mode controller with input voltage feed forward. This allows the converter to achieve excellent load and line regulation. The DC gain of the power www.national.com LDO - LOW DROP OUT OPERATION The LM3674-ADJ can operate at 100% duty cycle (no switching, PMOS switch completely on) for low drop out support of the output voltage. In this way the output voltage 10 • VFB = Feedback Voltage (0.5V typ) • R1 = Resistor from VOUT to FB (Ω) • R2 = Resistor from FB to GND (Ω) For any output voltage greater than or equal to 1.0V a frequency zero must be added at 45KHz for stability. The formula is: (Continued) will be controlled down to the lowest possible input voltage. When the device operates near 100% duty cycle, the output voltage supply ripple is slightly higher, approximately 25mV. The minimum input voltage needed to support the output voltage is Load current • ILOAD • RDSON,PFET Drain to source resistance of PFET switch in the triode region • RINDUCTOR Inductor resistance For output voltages greater than or equal to 2.5V, a pole must also be placed at 45KHz as well. If the pole and zero are at the same frequency the formula for calculation of C2 is: Application Information OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE (LM3674-ADJ) The formula for location of zero and pole frequency created by adding C1,C2 are given below. It can be seen that by adding C1, a zero as well as a higher frequency pole is introduced. The output voltage of the adjustable parts can be programmed through the resistor network connected from VOUT to FB the to GND. VOUT will be adjusted to make FB equal to 0.5V. The resistor from FB to GND (R2) should be 200 kΩ to keep the current drawn through this network small but large enough that it is not susceptible to noise. If R2 is 200KΩ, and given the VFB is 0.5V, then the current through the resistor feedback network will be 2.5µA. The output voltage formula is: See the " LM3674-ADJ Configurations for " Various VOUT" table. • VOUT = Output Voltage (V) TABLE 1. Adjustable LM3674 Configurations for Various VOUT VOUT (V) R1 (KΩ) R2 (KΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF) 1.0 200 200 18 None 2.2 4.7 10 1.1 191 158 18 None 2.2 4.7 10 1.2 280 200 12 None 2.2 4.7 10 1.5 357 178 10 None 2.2 4.7 10 1.6 442 200 8.2 None 2.2 4.7 10 1.7 432 178 8.2 None 2.2 4.7 10 1.8 464 178 8.2 None 2.2 4.7 10 1.875 523 191 6.8 None 2.2 4.7 10 2.5 402 100 8.2 None 2.2 4.7 10 2.8 464 100 8.2 33 2.2 4.7 10 3.3 562 100 6.8 33 2.2 4.7 10 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 from the manufacturer. The minimum value of inductance to guarantee good performance is 1.76µH 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. 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: 11 www.national.com LM3674 Operation Description LM3674 Application Information for design flexibility. This allows substitution of a low-noise toroidal inductor, in the event that noise from low-cost bobbin models is unacceptable. (Continued) 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 0805 and 0603. The minimum input capacitance to guarantee good performance is 2.2µF at 3V dc bias; 1.5µF at 5V dc bias including tolerances and over ambient temperature range. The input filter capacitor supplies current to the PFET switch of the LM3674 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: IRipple : average to peak inductor current Ioutmax: maximum load current (600mA) 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 (1.6 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 1200 mA. A 2.2 µH inductor with a saturation current rating of at least 1200 mA is recommended for most applications. The inductor’s resistance should be less than around 0.3Ω for good efficiency. Table 2 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 TABLE 2. Suggested Inductors and Their Suppliers Model Vendor Dimensions LxWxH(mm) D.C.R (max) DO3314-222MX Coilcraft 3.3 x 3.3 x 1.4 200 mΩ LPO3310-222MX Coilcraft 3.3 x 3.3 x 1.0 150 mΩ ELL5GM2R2N Panasonic 5.2 x 5.2 x 1.5 53 mΩ CDRH2D14NP-2R2NC Sumida 3.2 x 3.2 x 1.55 94 mΩ www.national.com 12 LM3674 Application Information (Continued) 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 0805 and 0603. DC bias characteristics vary from manufacturer to manufacturer and dc bias curves should be requested from them as part of the capacitor selection process. Because these two components are out of phase the rms value can be used to get an approximate value of peak-topeak ripple. Voltage peak-to-peak ripple, root mean squared = The minimum output capacitance to guarantee good performance is 5.75µF at 1.8V dc bias including tolerances and over ambient temperature range. The output filter capacitor smoothes 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: Note that the output ripple is dependent on the 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. Voltage peak-to-peak ripple due to capacitance can be expressed as follow: Voltage peak-to-peak ripple due to ESR = TABLE 3. Suggested Capacitors and Their Suppliers Model Type Vendor Voltage Rating Case size inch (mm) GRM21BR60J106K Ceramic, X5R Murata 6.3V 0805 (2012) C2012X5R0J106K Ceramic, X5R TDK 6.3V 0805 (2012) JMK212BJ106K Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012) 10 µF for COUT 4.7 µF for CIN GRM21BR60J475K Ceramic, X5R Murata 6.3V 0805 (2012) JMK212BJ475K Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012) C2012X5R0J475K Ceramic, X5R TDK 6.3V 0805 (2012) BOARD LAYOUT CONSIDERATIONS 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. 13 www.national.com LM3674 Application Information (Continued) 20167231 FIGURE 5. Board Layout Design Rules for the LM3674 Good layout for the LM3674 can be implemented by following a few simple design rules, as illustrated in . 1. Place the LM3674, 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. Special care must by given to place the input filter capacitor very close to the VIN and GND pin. 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 LM3674 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 LM3674 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 LM3674, 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 LM3674 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 www.national.com 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 LM3674 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. A good approach is to route the feedback trace on another layer and to have a ground plane between the top layer and layer on which the feedback trace is routed. In the same manner for the adjustable part it is desired to have the feedback dividers on the bottom layer. 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, 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. 14 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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