LM2771 Low-Ripple 250mA Switched Capacitor Step-Down DC/DC Converter General Description Features The LM2771 is a switched capacitor step-down regulator that produces a 1.5V output without the use of an inductor. It is capable of supplying loads up to 250mA. The LM2771 operates with an input voltage from 2.7V to 5.5V, and requires only 3 low-cost ceramic capacitors. The LM2771 uses a regulated 0.5x charge pump to give power conversion efficiencies nearly twice as high as an LDO. Pre-regulated 1.1MHz fixed-frequency switching results in very low ripple and noise on both the input and the output. When output currents are low ( < 30mA typ.), the part automatically switches to a low-ripple PFM regulation mode to maintain high efficiency over the entire load range. At input voltages below 3.5V (Typ), the charge pump goes into pass mode, with efficiencies similar to an LDO. The LM2771 is available in National’s 10-pad Leadless Leadframe No-Pullback Package (LLP-10). n n n n n n n n n n n n Low-Noise Fixed Frequency Operation 1.5V Output Voltage Li-Ion (3.6V) to 1.5V with 81% Efficiency 1.7% Output Voltage Accuracy Very Low Output Ripple: 8mV @ 250mA Output Currents up to 250mA 2.7V to 5.5V Input Range Shutdown Disconnects Load from VIN 1.1MHz Switching Frequency No Inductors…Small Solution Size Current Limit and Thermal Protection LLP-10 Package (3mm x 3mm x 0.8mm) Applications n DSP, Memory, and Microprocessor Power Supplies n Mobile Phones and Pagers n Portable Electronic Devices Typical Application Circuit LM2771 Efficiency vs. Low-Dropout Linear Regulator (LDO) Efficiency 20183801 20183813 © 2006 National Semiconductor Corporation DS201838 www.national.com LM2771 Low-Ripple 250mA Switched Capacitor Step-Down DC/DC Converter March 2006 LM2771 Connection Diagram and Package Mark Information 10-Pin Non-Pullback Leadless Frame Package (LLP-10) National Semiconductor Package Number SDA10A 20183802 Pin Descriptions Pin # Name 1 VIN 2 GND Description Input Voltage: Recommended VIN operating range 3.0V to 5.5V. Ground 3 VOUT 4 NC No Connect 5 NC No Connect 6 C1- Flying Capacitor 1: Negative Terminal 7 C1+ Flying Capacitor 1: Positive Terminal 8 NC No Connect 9 GND 10 EN Output Voltage Ground Enable Pin Logic Input. Applying a logic HIGH voltage signal enables the part. A logic LOW voltage signal places the the device in shutdown. Order Information Output Voltages Order Number Package Mark ID 1.5V LM2771SD XXXXX = CZC2CX YYYYY = L2771 1.5V LM2771SDX XXXXX = CZC2CX YYYYY = L2771 www.national.com 2 Package Supplied as: 1000 Units, Tape and Reel SDA10A Non-Pullback LLP 4500 Units, Tape and Reel Operating Ratings If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN Pin Voltage -0.3V to 6.0V EN Pin Voltage -0.3V to (VIN+0.3V) w/ 6.0V max Continuous Power Dissipation (Note 3) Input Voltage Range 150 C -65oC to +150o C Maximum Lead Temperature (Note 4) 265oC ESD Rating (Note 5) Human Body Model: Machine Model 0mA to 250mA Junction Temperature (TJ) Range -30˚C to +110˚C Ambient Temperature (TA) Range (Note 6) -30˚C to +85˚C Thermal Properties o Storage Temperature Range 2.7V to 5.5V Recommended Load Current Range Internally Limited Junction Temperature (TJ-MAX) (Notes 1, 2) 55˚C/W Junction-to-Ambient Thermal Resistance (θJA), LLP10 Package (Note 7) 2.0kV 200V Electrical Characteristics (Notes 2, 8) o Limits in standard typeface are for TJ = 25 C. Limits in boldface type apply over the full operating junction temperature range (-30˚C ≤ TJ ≤ +110˚C) . Unless otherwise noted, specifications apply to the LM2771 Typical Application Circuit (pg. 1) with: VIN = 3.6V; V(EN) = 1.8V, CIN = C1 = 1.0µF, COUT = 4.7µF. (Note 9) Symbol VOUT Parameter 1.5V Output Voltage Regulation VOUT/IOUT Output Load Regulation VOUT/VIN Output Line Regulation Condition Min Typ Max Units 1.455 (−3%) 1.5 1.545 (+3%) V 3.0V ≤ VIN ≤ 5.5V IOUT = 150mA 1.475 (−1.7%) 1.5 1.525 (+1.7%) 3.0V < VIN ≤ 5.5V, 0mA ≤ IOUT ≤ 250mA 1.445 (−3.7%) 1.5 1.545 (+3%) 3.0V ≤ VIN ≤ 5.5V 0mA ≤ IOUT ≤ 200mA VOUT = 1.5V 0mA ≤ IOUT ≤ 250mA 0.17 mV/mA 0.1 %/V E Power Efficiency IOUT = 200mA 81 IQ Quiescent Supply Current IOUT = 0mA (Note 10) 45 VR Fixed Frequency Output Ripple 40mA ≤ IOUT ≤ 250mA 8 mV VR–PFM PFM–Mode Output Ripple IOUT < 40mA 12 mV ISD Shutdown Current V(EN) = 0V FSW Switching Frequency 3.2V ≤ VIN ≤ 5.5V ROL Open–Loop Output Resistance IOUT = 200mA (Note 11) 1.0 Ω ICL Output Current Limit VIN = 5.5V 0V ≤ VOUT ≤ 0.2V (Note 12) 500 mA tON Turn-on Time VIL Logic-low Input Voltage 3.0V ≤ VIN ≤ 5.5V 0 0.5 V VIH Logic-high Input Voltage 3.0V ≤ VIN ≤ 5.5V 0.95 VIN V IIH Logic-high Input Current V(EN) = 1.8V (Note 13) IIL Logic-low Input Current Logic Input = 0V 0.80 % 50 µA 0.1 0.5 µA 1.1 1.40 MHz 150 µs 5 µA 0.1 µA 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. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=150oC (typ.) and disengages at TJ=140oC (typ.). 3 www.national.com LM2771 Absolute Maximum Ratings (Notes 1, 2) LM2771 Electrical Characteristics (Notes 2, 8) (Continued) Note 4: For detailed information on soldering requirements and recommendations, please refer to National Semiconductor’s Application Note 1187 (AN-1187): Leadless Leadframe Package (LLP). Note 5: The Human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. MIL-STD-883 3015.7 Note 6: Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 110oC), 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 x PD-MAX). Note 7: Junction-to-ambient thermal resistance is highly application and board-layout dependent. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues. Note 8: 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 9: CIN, COUT, C1: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics. Note 10: VOUT is set to 1.6V during this test. Note 11: Open loop output resistance can be used to predict output voltage when, under low VIN and high IOUT conditions, VOUT falls out of regulation. VOUT = VIN/2 − (ROL x IOUT) Note 12: Maximum input current is equal to half the maximum output current for buck-mode switched capacitor converters. Note 13: There is a 350kΩ pull-down resistor connected internally between the EN pin and GND. Block Diagram 20183803 www.national.com 4 Output Voltage vs. Input Voltage Output Voltage vs. Output Current 20183812 20183814 Efficiency vs. Input Voltage Efficiency vs. Output Current 20183813 20183826 Input Voltage Ripple, Load=6Ω(250mA) Output Voltage Ripple, Load=6Ω(250mA) 20183815 20183821 CH1: VIN, CIN = 1µF; Scale: 50mV/Div, AC Coupled CH2: VIN, CIN = 10µF; Scale: 50mV/Div, AC Coupled CH2: VOUT, COUT = 4.7µF; Scale: 20mV/Div, AC Coupled Time scale: 200ns/Div Time scale: 200ns/Div 5 www.national.com LM2771 Typical Performance Characteristics Unless otherwise specified: VIN = 3.6V, CIN = C1 = 1.0µF, COUT = 4.7µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). LM2771 Typical Performance Characteristics Unless otherwise specified: VIN = 3.6V, CIN = C1 = 1.0µF, COUT = 4.7µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued) Load Step, 15mA to 200mA Load Step, 200mA to 15mA 20183823 20183824 CH2: VOUT; Scale: 20mV/Div, AC Coupled CH2: VOUT; Scale: 20mV/Div, AC Coupled CH4: IOUT; Scale: 200mA/Div CH4: IOUT; Scale: 200mA/Div Time scale: 20µs/Div Time scale: 10µs/Div Line Step, 3.6V to 4.5V with Load=7.5Ω(200mA) Oscillator Frequency vs. Input Voltage 20183822 CH1: VIN; Scale: 1V/Div, AC Coupled CH2: VOUT; Scale: 20mV/Div, AC Coupled Time scale: 40µs/Div 20183825 Startup and Shutdown Behavior, Load=6Ω(250mA) 20183816 CH1: VEN; Scale: 2V/Div, DC Coupled CH2: VOUT; Scale: 500mV/Div, DC Coupled Time scale: 100µs/Div www.national.com 6 OVERVIEW The LM2771 is a switched capacitor converter that produces a regulated, low voltage output. The core of the part is a highly efficient charge pump that utilizes fixed frequency pre-regulation and Pulse Frequency Modulation to minimize ripple and power losses over wide input voltage and output current ranges. A description of the principal operational characteristics of the LM2771 is detailed in the Circuit Description, and Efficiency Performance sections. These sections refer to details in the Block Diagram. SOFT START The LM2771 employs soft start circuitry to prevent excessive input inrush currents during startup. At startup, the output voltage gradually rises from 0V to the nominal output voltage. This occurs in 150µs (typ.). Soft-start is engaged when the part is enabled, including situations where voltage is established simultaneously on the VIN and EN pins. CIRCUIT DESCRIPTION The core of the LM2771 is a two-phase charge pump controlled by an internally generated non-overlapping clock. The charge pump operates by using an external flying capacitor, C1, to transfer charge from the input to the output. At input voltages below 3.5V (typ.) the LM2771 operates in a "pass mode", with the input current being equal to the load current. At input voltages above 3.5V (typ.) the part utilizes a gain of 1⁄2, resulting in an input current equal to half the load current. The two phases of the switched capacitor switching cycle will be referred to as the "charge phase" and the "discharge phase". During the charge phase, the flying capacitor is charged by the input supply. After half of the switching cycle [ t = 1/(2xFSW) ], the LM2771 switches to the discharge phase. In this configuration, the charge that was stored on the flying capacitor in the charge phase is transferred to the output. The LM2771 uses fixed frequency pre-regulation to regulate the output voltage to 1.5V during moderate to high load currents. The input and output connections of the flying capacitor is made with internal MOS switches. Preregulation limits the gate drive of the MOS switch connected between the voltage input and the flying capacitor. Controlling the on resistance of this switch limits the amount of charge transferred into and out of the flying capacitor during the charge and discharge phases, and in turn helps to keep the output ripple very low. When output currents are low ( < 30mA typ.), the LM2771 automatically switches to a low-ripple Pulse Frequency Modulation (PFM) form of regulation. In PFM mode, the flying capacitor stays in the discharge phase until the output voltage drops below a predetermined trip point. When this occurs, the flying capacitor switches back to the charge phase. After being charged, the flying capacitor repeats the process of staying in the discharge phase and switching to the charge phase when necessary. THERMAL SHUTDOWN Protection from damage related to overheating is achieved with a thermal shutdown feature. When the junction temperature rises to 150oC (typ.), the part switches into shutdown mode. The LM2771 disengages thermal shutdown when the junction temperature of the part is reduced to 140oC (typ.). Due to the high efficiency of the LM2771, thermal shutdown and/or thermal cycling should not be encountered when the part is operated within specified input voltage, output current, and ambient temperature operating ratings. If thermal cycling is seen under these conditions, the most likely cause is an inadequate PCB layout that does not allow heat to be sufficiently dissipated out of the LLP package. CURRENT LIMIT PROTECTION The LM2771 charge pump contains current limit protection circuitry that protects the device during VOUT fault conditions where excessive current is drawn. Output current is limited to 500mA (typ). Application Information RECOMMENDED CAPACITOR TYPES The LM2771 requires 3 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2771 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use with the LM2771. These capacitors have tight capacitance tolerance (as good as ± 10%) and hold their value over temperature (X7R: ± 15% over -55oC to 125oC; X5R: ± 15% over -55oC to 85oC). Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2771. These types of capacitors typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30oC to +85oC range; Z5U: +22%, -56% over +10oC to +85oC range). Under some conditions, a 1µF-rated Y5V or Z5U capacitor could have a capacitance as low as 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2771. EFFICIENCY PERFORMANCE Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are neglected for simplicity): IIN = G x IOUT E = (VOUT x IOUT) ÷ (VIN x IIN) = VOUT ÷ (G x VIN) In the equations, G represents the charge pump gain. Efficiency is at its highest as GxVIN approaches VOUT. Refer to the efficiency graph in the Typical Performance Characteristics section for detailed efficiency data. The transition between Pass mode and the gain of 1⁄2 is clearly distinguished by the sharp discontinuity in the efficiency curve. 7 www.national.com LM2771 SHUTDOWN The LM2771 is in shutdown mode when the voltage on the enable pin (EN) is logic-low. In shutdown, the LM2771 draws virtually no supply current. When in shutdown, the output of the LM2771 is completely disconnected from the input. The internal feedback resistors will pull the output voltage down to 0V. Operation Description LM2771 Application Information pacitor in parallel with the primary output capacitor. The low ESR of the ceramic capacitor will be in parallel with the higher ESR, resulting in a low net ESR based on the principles of parallel resistance reduction. (Continued) Net capacitance of a ceramic capacitor decreases with increased DC bias. This degradation can result in lower capacitance than expected on the input and/or output, resulting in higher ripple voltages and currents. Using capacitors at DC bias voltages significantly below the capacitor voltage rating will usually minimize DC bias effects. Consult capacitor manufacturers for information on capacitor DC bias characteristics. Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and capacitor manufacturers. It is strongly recommended that the LM2771 circuit be thoroughly evaluated early in the design-in process with the mass-production capacitors of choice. This will help ensure that any such variability in capacitance does not negatively impact circuit performance. INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE The input capacitor (CIN) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying capacitor during the charge phase of operation. The input capacitor helps to keep the input voltage from drooping at the start of the charge phase when the flying capacitor is connected to the input. It also filters noise on the input pin, keeping this noise out of sensitive internal analog circuitry that is biased off the input line. Much like the relationship between the output capacitance and output voltage ripple, input capacitance has a dominant and first-order effect on input ripple magnitude. Increasing (decreasing) the input capacitance will result in a proportional decrease (increase) in input voltage ripple. Input voltage, output current, and flying capacitance also will affect input ripple levels to some degree. In typical high-current applications, a 1µF low-ESR ceramic capacitor is recommended on the input. Different input capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the solution. But changing the input capacitor may also require changing the flying capacitor and/or output capacitor to maintain good overall circuit performance. Performance of the LM2771 with different capacitor setups is discussed below in Recommended Capacitor Configurations. The table below lists some leading ceramic capacitor manufacturers. Manufacturer Contact Information AVX www.avx.com Murata www.murata.com Taiyo-Yuden www.t-yuden.com TDK www.component.tdk.com Vishay-Vitramon www.vishay.com OUTPUT CAPACITOR AND OUTPUT VOLTAGE RIPPLE The output capacitor in the LM2771 circuit (COUT) directly impacts the magnitude of output voltage ripple. Other prominent factors also affecting output voltage ripple include input voltage, output current and flying capacitance. Due to the complexity of the regulation topology, providing equations or models to approximate the magnitude of the ripple can not be easily accomplished. But one important generalization can be made: increasing (decreasing) the output capacitance will result in a proportional decrease (increase) in output voltage ripple. In typical high-current applications, a 4.7µF low-ESR ceramic output capacitor is recommended. Different output capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the solution. But changing the output capacitor may also require changing the flying capacitor and/or input capacitor to maintain good overall circuit performance. Performance of the LM2771 with different capacitor setups in discussed in the section Recommended Capacitor Configurations. High ESR in the output capacitor increases output voltage ripple. If a ceramic capacitor is used at the output, this is usually not a concern because the ESR of a ceramic capacitor is typically very low and has only a minimal impact on ripple magnitudes. If a different capacitor type with higher ESR is used (tantalum, for example), the ESR could result in high ripple. To eliminate this effect, the net output ESR can be significantly reduced by placing a low-ESR ceramic ca- www.national.com FLYING CAPACITOR The flying capacitor (C1) transfers charge from the input to the output. Flying capacitance can impact both output current capability and ripple magnitudes. If flying capacitance is too small, the LM2771 may not be able to regulate the output voltage when load currents are high. On the other hand, if the flying capacitance is too large, the flying capacitor might overwhelm the input and output capacitors, resulting in increased input and output ripple. Polarized capacitors (tantalum, aluminum electrolytic, etc.) must not be used for the flying capacitor, as they could become reverse-biased during LM2771 operation. RECOMMENDED CAPACITOR CONFIGURATIONS The data in Table 1 can be used to assist in the selection of a capacitor configuration that best balances solution size and cost with the electrical requirements of the application. As previously discussed, input and output ripple voltages will vary with output current and input voltage. The numbers provided show expected ripple voltage when VIN = 3.6V and load currents are between 10mA and 200mA. The table offers first look at approximate ripple levels and provides a comparison for the different capacitor configurations presented, but is not intended to be a guarantee of performance. 8 Layout Guidelines (Continued) Proper board layout will help to ensure optimal performance of the LM2771 circuit. The following guidelines are recommended: • Place capacitors as close to the LM2771 as possible, and preferably on the same side of the board as the IC. • Use short, wide traces to connect the external capacitors to the LM2771 to minimize trace resistance and inductance. • Use a low resistance connection between ground and the GND pin of the LM2771. Using wide traces and/or multiple vias to connect GND to a ground plane on the board is most advantageous. TABLE 1. LM2771 Performance with Different Capacitor Configurations (Note 14) CAPACITOR CONFIGURATION (VIN = 3.6V) TYPICAL INPUT RIPPLE TYPICAL OUTPUT RIPPLE CIN = 1µF, COUT = 4.7µF, C1 = 1µF 45mV 8mV CIN = 1µF, COUT = 2.2µF, C1 = 1µF 94mV 19mV CIN = 0.47µF, COUT = 4.7µF, C1 = 1µF 105mV 11mV CIN = 0.47µF, COUT = 3.3µF, C1 = 1µF 102mV 16mV CIN = 0.47µF, COUT = 3.3µF, C1 = 0.33µF 120mV 15mV Note 14: Refer to the text in the Recommended Capacitor Configurations section for detailed information on the data in this table 9 www.national.com LM2771 Application Information LM2771 Low-Ripple 250mA Switched Capacitor Step-Down DC/DC Converter Physical Dimensions inches (millimeters) unless otherwise noted SDA10A: 10-Pin Non-Pullback Leadless Leadframe Package 3.0mm x 3.0mm x 0.8mm 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. 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