LM3218SEX

LM3218 650 mA Miniature, Adjustable, Step-Down DC-DC Converter with Integrated Inductor for RF Power Amplifiers January 21, 2009 LM3218 650 mA Miniature, Adjustable, Step-Down DC-DC Converter for RF Power Amplifiers General Description The LM3218 is a DC-DC converter with inductor which is optimized for powering RF power amplifiers (PAs) from a single Lithium-Ion cell. It steps down an input voltage in the range from 2.7V to 5.5V to an adjustable output voltage of 0.8V to 3.6V. Output voltage is set by using a VCON analog input to control power levels and efficiency of the RF PA. The LM3218 offers superior electrical performance for mobile phones and similar RF PA applications with a reduced footprint (3mm x 2.5mm x 1.2mm). Fixed-frequency PWM operation minimizes RF interference. A shutdown function turns the device off and reduces battery consumption to 0.01 µA (typ.). The LM3218 is available in an integrated inductor 8–pin LTCC package. A high switching frequency (2 MHz typ.) allows use of tiny surface-mount components. Only two small external surface-mount components, two ceramic capacitors, are required. The overall board space is reduced up to 25% from the typical discrete inductor solution. Features ■ Includes 2.6 µH Inductor in very small form factor (3mm x ■ ■ ■ ■ ■ ■ ■ ■ ■ 2.5mm x 1.2mm) 2 MHz (typ.) PWM Switching Frequency Operates from a single Li-Ion cell (2.7V to 5.5V) Adjustable Output Voltage (0.8V to 3.6V) Fast Output Voltage Transient (0.8V to 3.4V in 25 µs typ.) 650 mA Maximum load capability High Efficiency (95% typ. at 3.9 VIN, 3.4 VOUT at 400 mA) 8-pin LTCC Package Current Overload Protection Thermal Overload Protection Applications ■ ■ ■ ■ Cellular Phones Hand-Held Radios RF PC Cards Battery-Powered RF Devices Typical Application 30050401 FIGURE 1. LM3218 Typical Application © 2009 National Semiconductor Corporation 300504 www.national.com LM3218 Connection Diagram 30050420 NS Package Number SE08A Order Information Order Number LM3218SE LM3218SEX Package Marking (Note) XVS SA XVS SA Supplied As 250 units, Tape-and-Reel 3000 units, Tape-and-Reel Note: The actual physical placement of the package marking will vary from part to part. The package marking “X” designates the date code. “V” is a NSC internal code for die traceability. Both will vary in production. “S” designates device type as switcher and “SA” identifies the device (part number). Pin Descriptions Pin # 1 2 3 4 5 6 7 8 Name EN VCON FB SGND VOUT PGND PVIN VDD Description Enable Input. Set this digital input high for normal operation. For shutdown, set low. Voltage Control Analog input. VCON controls VOUT in PWM mode. Feedback Analog Input. Connect to the VOUT pin. Analog and Control Ground. Output Voltage, connects to one terminal of 2.6 µH inductor. Connect output filter capacitor C2 to get DC voltage out. Power Ground Power Supply Voltage Input to the internal Buck PFET switch. Analog Supply Input. www.national.com 2 LM3218 Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VDD, PVIN to SGND PGND to SGND EN, FB, VCON −0.2V to +6.0V −0.2V to +0.2V (SGND −0.2V) to (VDD +0.2V) w/6.0V max (PGND −0.2V) to (PVIN +0.2V) w/6.0V max −0.2V to +0.2V Internally Limited +150°C −65°C to +150°C +260°C 2000V 200V Operating Ratings (Notes 1, 2) 2.7V to 5.5V 0 mA to 650 mA −30°C to +125°C −30°C to +85°C Input Voltage Range Recommended Load Current Junction Temperature (TJ) Range Ambient Temperature (TA) Range  (Note 5) Thermal Properties Junction-to-Ambient Thermal Resistance (θJA), TLP08 Package (Note 6) 120°C/W VOUT PVIN to VDD Continuous Power Dissipation  (Note 3) Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temperature (Soldering, 10 sec.) ESD Rating (Notes 4, 13) Human Body Model: Machine Model: Electrical Characteristics Symbol VFB, MIN VFB, MAX ISHDN IQ Parameter Feedback voltage at minimum setting Feedback voltage at maximum setting Shutdown supply current DC bias current into VDD (Notes 2, 7, 8) Limits in standard typeface are for TA = TJ = 25°C. Limits in boldface type apply over the full operating ambient temperature range (−30°C ≤ TA = TJ ≤ +85°C). Unless otherwise noted, all specifications apply to the LM3218 with: PVIN = VDD = EN = 3.6V. Conditions VCON = 0.32V VIN = 3.6V(Note 8) VCON = 1.44V, VIN = 4.2V(Note 8) EN = VOUT = VCON = 0V, (Note 9) VCON = 0V, FB = 0V, No Switching (Note 10) IOUT = 200mA, VCON = 0.5V VCON = 0.5V (Note 11) VCON = 0.32V (Note 11) Min 0.75 3.526 Typ 0.80 3.600 0.01 0.6 300 1100 800 2.0 1.2 0.5 EN = 3.6V 5 0.15 VCON = 1.0V 0.32V ≤ VCON ≤ 1.44V 2.5 ±1 10 Max 0.85 3.696 2 0.7 400 Units V V µA mA mΩ mA mA MHz V V µA V µA V/V RDROPOUT PinVout - PinVin resistance ILIM Large PFET (L) Switch peak (L_PFET) current limit ILIM Small PFET (S) Switch peak (S_PFET) current limit FOSC Internal oscillator frequency VIH,ENABLE Logic high input threshold VIL,ENABLE Logic low input threshold IPIN,ENABL E Pin pull down current VCON Threshold for turning on switches VCON pin leakage current VCON to VOUT Gain VCON,ON ICON Gain 3 www.national.com LM3218 The following spec table entries are guaranteed by design providing the component values in the typical application circuit are used (L = LTCC Inductor, 2.6 µH; DCR = 150 mΩ; CIN = 10 µF, 6.3V, 0603, TDK C1608X5R0J106K; COUT = 4.7 µF, 6.3V, 0603, C1608X5R0J475M). These parameters are not guaranteed by production testing. Min and Max values are specified over the ambient temperature range TA = −30°C to 85°C. Typical values are specified at PVIN = VDD = EN = 3.6V and TA = 25°C unless otherwise specified. Symbol Parameter Conditions Min Typ 25 35 5 5 0.39 0.37 650 400 −3 −50 +3 +50 0.42 0.40 Max 40 45 10 10 0.45 0.43 Units µs µs pF pF V V mA mA % mV TRESPONSE Time for VOUT to rise from 0.8V VIN = 4.2V (Rise Time) to 3.4V (to reach 3.35V) RLOAD = 5.5Ω TRESPONSE Time for VOUT to fall from 3.4V to VIN = 4.2V (Fall Time) 0.8V RLOAD = 15Ω CCON CEN VCON (S>L) VCON (L>S) IOUT, MAX VCON input capacitance EN input capacitance VCON = 1V, VIN=2.7V to 5.5V Test frequency = 100 KHz EN = 2V, VIN= 2.7V to 5.5V Test frequency = 100 KHz from 960 mΩ to 140 mΩ RDSON(P) management threshold Threshold for PFET RDSON(P) to change from 140 mΩ to 960 mΩ Maximum Output Current VIN = 2.7V to 5.5V VCON = 0.45V to 1.44V VIN = 2.7V to 5.5V VCON = 0.32V to 0.45V Linearity TON Linearity in control range 0.32V VIN = 3.9V (Note 14) to 1.44V Monotonic in nature Turn on time EN = Low to High (time for output to reach 97% of VIN = 4.2V, VOUT = 3.4V, final value after Enable low-to- I OUT ≤ 1mA high transition) Efficiency VIN = 3.6V, VOUT = 0.8V IOUT = 90mA VIN = 3.6V, VOUT = 1.5V IOUT = 150mA VIN = 3.9V, VOUT = 3.4V IOUT = 400 mA VO_ripple Ripple voltage at no pulse skip condition Ripple voltage at pulse skip condition Line_tr Line transient response VIN = 2.7V to 4.5V, VOUT = 0.8V to 3.4V, Differential voltage = VIN - VOUT > 1V, IOUT = 0 mA to 400 mA (Note 12) VIN = 5.5V to dropout, VOUT = 3.4V, IOUT = 650 mA (Note 12) VIN = 3.6V to 4.2V, TR = TF = 10 µs, VOUT = 0.8V, IOUT = 100 mA VIN = 3.1/3.6/4.5V, VOUT = 0.8V, IOUT = 50 mA to 150 mA 100 System Characteristics RDSON(P) management threshold Threshold for PFET RDSON(P) to change 40 60 µs η 81 89 95 % % % 10 mVp-p 60 mVp-p 50 mVpk Load_tr Max Duty cycle Load transient response Maximum duty cycle 50 mVpk % www.national.com 4 LM3218 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 pins. The LM3218 is designed for mobile phone applications where turn-on after power-up is controlled by the system controller and where requirements for a small package size overrule increased die size for internal Under Voltage Lock-Out (UVLO) circuitry. Thus, it should be kept in shutdown by holding the EN pin low until the input voltage exceeds 2.7V. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ = 150°C (typ.) and disengages at TJ = 125°C (typ.). Note 4: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. (MIL-STD-883 3015.7) The machine model is a 200 pF capacitor discharged directly into each pin. Note 5: In applications where high power dissipation and/or poor package thermal 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-OP = 125°C), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX). Note 6: Junction-to-ambient thermal resistance (θJA) is taken from thermal measurements, performed under the conditions and guidelines set forth in the JEDEC standard JESD51-7. A 4–layer, 4" x 4", 2/1/1/2 oz. Cu board as per JEDEC standards is used for the measurements. Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Due to the pulsed nature of the testing TA = TJ for the electrical characteristics table. Note 8: The parameters in the electrical characteristics table are tested under open loop conditions at PVIN = VDD = 3.6V unless otherwise specified. For performance over the input voltage range and closed-loop results, refer to the datasheet curves. Note 9: Shutdown current includes leakage current of PFET. Note 10: IQ specified here is when the part is not switching. For operating quiescent current at no load, refer to datasheet curves. Note 11: Current limit is built-in, fixed, and not adjustable. Electrical Characteristic table reflects open loop data (FB = 0V and current drawn from SW pin ramped up until cycle by cycle limit is activated). Refer to System Characteristics table for maximum output current. Note 12: Ripple voltage should be measured at COUT electrode on a well-designed PC board and using the suggested inductor and capacitors. Note 13: National Semiconductor recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper ESD handling procedures can result in damage. Note 14: Linearity limits are ±3% or ±50 mV whichever is larger. 5 www.national.com LM3218 Typical Performance Characteristics otherwise specified.). Quiescent Current vs Supply Voltage (VCON = 0V, FB = 0V, No Switching) (Circuit in Figure 3, PVIN = VDD = EN = 3.6V and TA = 25°C unless Shutdown Current vs Temperature (VCON = 0V, EN = 0V) 30050428 30050426 Switching Frequency vs Temperature (VOUT = 1.3V, IOUT = 200 mA) Output Voltage vs Supply Voltage (VOUT = 1.3V) 30050410 30050411 Output Voltage vs Temperature (VIN = 3.6V, VOUT = 0.8V) Output Voltage vs Temperature (VIN = 4.2V, VOUT = 3.4V) 30050447 30050427 www.national.com 6 LM3218 Current Limit vs Temperature (Large PFET) Current Limit vs Temperature (Small PFET) 30050430 30050448 VCON Voltage vs Output Voltage (RLOAD = 10Ω) VCON Voltage vs Output Voltage (RLOAD = 10Ω) 30050416 30050417 Efficiency vs Output Voltage (VIN = 3.9V) EN High Threshold vs Supply Voltage 30050479 30050413 7 www.national.com LM3218 Efficiency vs Output Current (VOUT = 0.8V) Efficiency vs Output Current (VOUT = 3.6V) 30050449 30050415 Efficiency vs Output Current (RDSON Management) Efficiency vs Output Current (RDSON Management, VIN=4.5V) 30050440 30050441 Dark curves are efficiency profiles of either large PFET or small PFET whichever is higher. VIN-VOUT vs Output Current (100% Duty Cycle) Load Transient Response (VOUT = 0.8V) 30050442 30050444 www.national.com 8 LM3218 Load Transient Response (VIN = 4.2V, VOUT = 3.4V) Startup (VIN = 3.6V, VOUT = 1.3V, RLOAD = 1 kΩ) 30050443 30050418 Startup (VIN = 4.2V, VOUT = 3.4V, RLOAD = 5 kΩ) Shutdown Response (VIN = 4.2V, VOUT = 3.4V, RLOAD = 10Ω) 30050433 30050439 Line Transient Reponse (VIN = 3.0V to 3.6V, IOUT = 100 mA) VCON Transient Response (VIN = 4.2V, VCON = 0.32V/1.44V, RLOAD = 10Ω) 30050419 30050446 9 www.national.com LM3218 Timed Current Limit Response (VIN = 3.6V) Output Voltage Ripple (VOUT = 1.3V) 30050438 30050434 Output Voltage Ripple (VOUT = 3.4V) Output Voltage Ripple in Pulse Skip (VIN = 3.96V, VOUT = 3.4V, RLOAD = 5Ω) 30050405 30050437 www.national.com 10 LM3218 Block Diagram 30050404 FIGURE 2. Functional Block Diagram Operation Description The LM3218 is a simple, step-down DC-DC converter with a 2.6 µH series inductor substrate optimized for powering RF power amplifiers (PAs) in mobile phones, portable communicators, and similar battery powered RF devices. It is designed to allow the RF PA to operate at maximum efficiency over a wide range of power levels from a single Li-Ion battery cell. It is based on current mode buck architecture, with synchronous rectification for high efficiency. It is designed for a maximum load capability of 650 mA when VOUT > 1.05V (typ.) and 400 mA when VOUT < 1.00V (typ.) in PWM mode. Maximum load range may vary from this depending on input voltage, output voltage and the inductor chosen. Efficiency is typically around 95% for a 400 mA load with 3.4V output, 3.9V input. The LM3218 has an RDSON management scheme to increase efficiency when VOUT ≤ 1V. The output voltage is dynamically programmable from 0.8V to 3.6V by adjusting the voltage on the control pin without the need for external feedback resistors. This prolongs battery life by changing the PA supply voltage dynamically depending on its transmitting power. Additional features include current overload protection and thermal overload shutdown. The LM3218 is constructed using a chip-scale 8-pin micro SMD package and a LTCC inductor substrate. This package offers the smallest possible integrated solution footprint for space-critical applications such as cell phones, where board area is an important design consideration. Use of a high switching frequency (2 MHz) reduces the size of external components. As shown in Figure 1, only two external capacitors are required for implementation. Use of this module requires special design considerations for implementation. (See LTCC Module Package Assembly and Use in the Applications Information section). The board mounting requires careful board design and precision assembly equipment. Use of this package is best suited for opaque-case applications, where its edges are not subject to high-intensity ambient red or infrared light. Also, the system controller should set EN low during power-up and other low supply voltage conditions. (See Shutdown Mode in the Device Information section.) 11 www.national.com LM3218 30050436 FIGURE 3. Typical Operating System Circuit Circuit Operation Referring to Figure 1 and Figure 2, the LM3218 operates as follows: During the first part of each switching cycle, the control block in the LM3218 turns on the internal PFET (Pchannel MOSFET) 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 around (VIN - VOUT) / L, by storing energy in a magnetic field. During the second part of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET (N-channel MOSFET) synchronous rectifier on. In response, the inductor’s magnetic field collapses, generating a voltage that forces current from ground through the synchronous rectifier to the output filter capacitor and load. As the stored energy is transferred back into the circuit and depleted, the inductor current ramps down with a slope around VOUT / L. The output filter capacitor stores charge when the inductor current is high, and releases it when 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 power MOSFET switch and synchronous rectifier to a low-pass filter formed by the inductor and output filter capacitor. The output voltage is equal to the average voltage at the terminal of the power MOSFET inverter. While in operation, the output voltage is regulated by switching at a constant frequency and then modulating the energy per cycle to control power to the load. Energy per cycle is set by modulating the PFET switch on-time pulse width to control the peak inductor current. This is done by comparing the signal from the current-sense amplifier with a slope compensated error signal from the voltage-feedback error amplifier. At the beginning of each cycle, the clock turns on the PFET switch, causing the inductor current to ramp up. When the current sense signal ramps past the error amplifier signal, the PWM comparator turns off the PFET switch and turns on the NFET synchronous rectifier, ending the first part of the cycle. If an increase in load pulls the output down, the error amplifier output increases, which allows the inductor current to ramp higher before the comparator turns off the PFET. This increases the average current sent to the output and adjusts for the increase in the load. Before appearing at the PWM comwww.national.com 12 parator, a slope compensation ramp from the oscillator is subtracted from the error signal for stability of the current feedback loop. The minimum on time of PFET is 55 ns (typ.) Shutdown Mode Setting the EN digital pin low (1.2V) enables normal operation. EN should be set low to turn off the LM3218 during power-up and under-voltage conditions when the power supply is less than the 2.7V minimum operating voltage. The LM3218 is designed for compact portable applications, such as mobile phones. In such applications, the system controller determines power supply sequencing and requirements for small package size outweigh the additional size required for inclusion of UVLO (Under Voltage Lock-Out) circuitry. Internal Synchronous Rectification While in PWM mode, the LM3218 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. The internal NFET synchronous rectifier is turned on during the inductor current down slope in the second part of each cycle. The synchronous rectifier is turned off prior to the next cycle. The NFET is designed to conduct through its intrinsic body diode during transient intervals before it turns on, eliminating the need for an external diode. RDSON(P) Management The LM3218 has a unique RDSON(P) management function to improve efficiency in the low output current region up to 100 mA. When the VCON voltage is less than 0.40V (typ.), the device uses only a small part of the PFET to minimize drive loss of the PFET. When VCON is greater than 0.42V (typ.), the entire PFET is used to minimize RDSON(P) loss. This threshold has about 20 mV (typ.) of hysteresis. LM3218 VCON,ON The output is disabled when VCON is below 125 mV (typ.). It is enabled when VCON is above 150 mV (typ.). The threshold has about 25 mV (typ.) of hysteresis. Current Limiting A current limit feature allows the LM3218 to protect itself and external components during overload conditions. In PWM mode, an 1100 mA (typ.) cycle-by-cycle current limit is normally used when VCON is above 0.42V (typ.), and an 800 mA (typ.) is used when VCON is below 0.40V (typ.). If an excessive load pulls the output voltage down to approximately 0.375V, then the device switches to a timed current limit mode when VCON is above 0.42V (typ.). In timed current limit mode the internal PFET switch is turned off after the current comparator trips and the beginning of the next cycle is inhibited for 3.5us to force the instantaneous inductor current to ramp down to a safe value. The synchronous rectifier is off in timed current limit mode. Timed current limit prevents the loss of current control seen in some products when the output voltage is pulled low in serious overload conditions. can go lower than 0.8V providing a limited VIN range is used. Refer to datasheet curve (VCON Voltage vs Output Voltage) for details. This curve is for a typical part and there could be part-to-part variation for output voltages less than 0.8V over the limited VIN range. When the control pin voltage is more than 0.15V (typ.), the switches are turned on. When it is less than 0.125V (typ.), the switches are turned off. This on/off function has 25 mV (typ.) hysteresis. The quiescent current when (VCON = 0V and VEN = Hi) is around 600 µA. ESTIMATION OF MAXIMUM OUTPUT CURRENT CAPABILITY Referring to Figure 3, the Inductor peak-to-peak ripple current can be estimated by: IIND_PP = (VIN - VOUT ) × VOUT / (L1 × FSW × VIN) Where, Fsw is switching frequency. Therefore, maximum output current can be calculated by: IOUT_MAX = ILIM - 0.5 × IIND_PP For the worst case calculation, the following parameters should be used: FSW (Lowest switching frequency): 1.8 MHz ILIM (Lowest current limit value): 985 mA L1 (Lowest inductor value): refer to inductor datasheet. Note that inductance will drop with DC bias current and temperature. The worst case is typically at 85°C. For example, VIN = 4.2V, VOUT = 3.2V, L1 = 2.0 µH (Inductance value at 985 mA DC-bias current and 85°C), FSW = 1.8 MHz , ILIM = 985 mA. IIND_PP = 212 mA IOUT_MAX = 985 – 106 = 876 mA The effects of switch, inductor resistance and dead time are ignored. In real application, the ripple current would be 10% to 15% higher than ideal case. This should be taken into account when calculating maximum output current. Special attention needs to be paid that a delta between maximum output current capability and the current limit is necessary to satisfy transient response requirements. In practice, transient response requirements may not be met for output current greater than 650 mA. INDUCTOR SELECTION The inductor is an integrated LTCC 2.6 µH substrate within the LM3218 module and has a saturation current rating over 1200 mA. The integrated inductor’s low 1.2 mm maximum height provides ease of use into small design constraints. Integrating the inductor can eliminate layout issues associated with DC/DC converters and reduce potential EMI problems. CAPACITOR SELECTION The LM3218 is designed for use with ceramic capacitors for its input and output filters. Use a 10 µF ceramic capacitor for input and a 4.7 µF ceramic capacitor for output. They should maintain at least 50% capacitance at DC bias and temperature conditions. Ceramic capacitor types such as X5R, X7R and B are recommended for both filters. Table 1 lists some suggested part numbers and suppliers. DC bias characteristics of the capacitors must be considered when selecting the voltage rating and case size of the capacitor. If it is necessary to choose a 0603-size capacitor for CIN and COUT, the operation of the LM3218 should be carefully evaluated on the system board. Use of multiple 2.2 µF or 1 µF capacitors in parallel may also be considered. Dynamically Adjustable Output Voltage The LM3218 features dynamically adjustable output voltage to eliminate the need for external feedback resistors. The output can be set from 0.8V to 3.6V by changing the voltage on the analog VCON pin. This feature is useful in PA applications where peak power is needed only when the handset is far away from the base station or when data is being transmitted. In other instances, the transmitting power can be reduced. Hence the supply voltage to the PA can be reduced, promoting longer battery life. See Setting the Output Voltage in the Application Information section for further details. The LM3218 moves into Pulse Skipping mode when duty cycle is over 92% and the output voltage ripple increases slightly. Thermal Overload Protection The LM3218 has a thermal overload protection function that operates to protect itself from short-term misuse and overload conditions. When the junction temperature exceeds around 150°C, the device inhibits operation. Both the PFET and the NFET are turned off in PWM mode. When the temperature drops below 125°C, normal operation resumes. Prolonged operation in thermal overload conditions may damage the device and is considered bad practice. Application Information SETTING THE OUTPUT VOLTAGE The LM3218 features a pin-controlled variable output voltage to eliminate the need for external feedback resistors. It can be programmed for an output voltage from 0.8V to 3.6V by setting the voltage on the VCON pin, as in the following formula: VOUT = 2.5 x VCON When VCON is between 0.32V and 1.44V, the output voltage will follow proportionally by 2.5 times of VCON. If VCON is over 1.44V (VOUT = 3.6V), sub-harmonic oscillation may occur because of insufficient slope compensation. If VCON voltage is less than 0.32V (VOUT = 0.8V), the output voltage may not be regulated due to the required on-time being less than the minimum on-time (55 ns). The output voltage 13 www.national.com LM3218 TABLE 1. Suggested Capacitors And Their Suppliers Model C1608X5R0J106K, 10 µF, 6.3V C1608X5R0J475M, 4.7 µF, 6.3V 0805ZD475KA 4.7 µF, 10V Vendor TDK TDK AVX BOARD LAYOUT CONSIDERATIONS The input filter capacitor supplies AC current drawn by the PFET switch of the LM3218 in the first part of each cycle and reduces the voltage ripple imposed on the input power source. The output filter capacitor absorbs the AC inductor current, 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 (Equivalent Series Resistance) to perform these functions. The ESR of the filter capacitors is generally a major factor in voltage ripple. EN PIN CONTROL Drive the EN pin using the system controller to turn the LM3218 ON and OFF. Use a comparator, Schmidt trigger or logic gate to drive the EN pin. Set EN high (>1.2V) for normal operation and low (