LM2770SDX-12157

LM2770 High Efficiency Switched Capacitor Step-Down DC/DC Regulator with Sleep Mode December 2004 LM2770 High Efficiency Switched Capacitor Step-Down DC/DC Regulator with Sleep Mode General Description The LM2770 is a switched capacitor step-down regulator that is ideal for powering low-voltage applications in portable systems. The LM2770 can supply load currents up to 250mA and operates over an input voltage range of 2.7V to 5.5V. This makes the LM2770 a great choice for systems powered by 1-cell Li-Ion batteries and chargers. The output voltage of the LM2770 can be dynamically switched between two output levels with a logic input pin. Output voltage pairs currently available include 1.2V/1.5V and 1.2V/1.575V. Other pairs of voltage options can be developed upon request. LM2770 efficiency is superior to both fixed-gain switched capacitor buck regulators and low-dropout linear regulators (LDO’s). Multiple fractional gains maximize power efficiency over the entire input voltage and output current ranges. The LM2770 can also be switched into a low-power sleep mode when load currents are light (≤ 20mA). In sleep mode, the charge pump is off, and the output is driven with a low-noise, low-power linear regulator. Soft-start, short-circuit protection, current-limit protection, and thermal-shutdown protection are also included. The LM2770 is available in National’s small 10-pin Leadless Leadframe Package (LLP-10). Features n High Efficiency Multi-Gain Architecture: Peak Power Efficiency > 85% n Output Voltage Pairs: 1.2V/1.5V and 1.2V/1.575V n Dynamic Output Voltage Selection n ± 3% Output Voltage Accuracy n Output Currents up to 250mA n 2.7V to 5.5V Input Range n Low-Supply-Current Sleep Mode n 55µA Quiescent Supply Current in Full-Power Mode n Soft-Start n Short-Circuit Protection in Full-Power Mode n Current-Limit Protection in Sleep Mode n LLP-10 Package (3mm x 3mm x 0.8mm) Applications n n n n DSP Power Supplies Baseband Power Supplies Mobile Phones and Pagers Portable Electronic Equipment Typical Application Circuit LM2770 Efficiency vs. Low-Dropout Linear Regulator (LDO) Efficiency 20126011 20126001 © 2004 National Semiconductor Corporation DS201260 www.national.com LM2770 Connection Diagram and Package Mark Information 10-Pin Non-Pullback Leadless Frame Package (LLP-10) National Semiconductor Package Number SDA10A 20126002 Pin Description Pin # 1 Name VSEL Description Output Voltage Select Logic Input. If VSEL is high, VOUT = high voltage. If VSEL is low, VOUT = low voltage. (See Order Information for available voltage options) Enable Pin Logic Input. If high, part is enabled. If low, part is in shutdown. Flying Capacitor 2: Positive Termial Ground Flying Capacitor 2: Negative terminal Sleep Mode Logic Input. If high, the part operates in sleep mode, and the output is driven by a low power linear regulator. If low, the part operates in full-power mode, and the output is driven by the switched capacitor regulator Input Voltage. Recommended VIN operating range: 2.7V to 5.5V Flying Capacitor 1: Positive Terminal Flying Capacitor 1: Negative Terminal Output Voltage 2 3 4 5 10 EN C2+ GND C2SLEEP 9 8 7 6 VIN C1+ C1VOUT Order Information Output Voltages 1.2V / 1.5V 1.2V / 1.5V 1.2V / 1.575V 1.2V / 1.575V Order Number LM2770SD-1215 LM2770SDX-1215 LM2770SD-12157 LM2770SDX-12157 Package Mark ID L162B L162B L166B L166B SDA10A Non-Pullback LLP Package Supplied as: 1000 Units, Tape and Reel 4500 Units, Tape and Reel 1000 Units, Tape and Reel 4500 Units, Tape and Reel www.national.com 2 LM2770 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. VIN Pin Voltage EN, SLEEP, and VSEL Pin Voltages Continuous Power Dissipation (Note 3) VOUT Short to GND Duration (Note 4) Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temperature (Note 5) ESD Rating (Note 6) Human Body Model: Machine Model -0.3V to 6.0V -0.3V to (VIN+0.3V) w/ 6.0V max Internally Limited Infinite 150oC -65oC to +150o C 265oC Operating Ratings Input Voltage Range (Notes 1, 2) 2.7V to 5.5V 0mA to 250mA -30˚C to +105˚C -30˚C to +85˚C Recommended Load Current Range Junction Temperature (TJ) Range Ambient Temperature (TA) Range (Note 6) Thermal Properties Juntion-to-Ambient Thermal Resistance (θJA), LLP10 Package (Note 7) 55˚C/W 2.0kV 200V (Notes 2, 9) Electrical Characteristics Limits in standard typeface are for TJ = 25oC. Limits in boldface type apply over the full operating junction temperature range (-30˚C ≤ TJ ≤ +105˚C) . Unless otherwise noted, specifications apply to the LM2770 Typical Application Circuit (pg. 1) with: VIN = 3.6V; V(EN) = VSEL = 1.8V, V(SLEEP) = 0V, CIN = COUT = 10µF, C1 = C2 = 1.0µF. (Note 10) Symbol VOUT-1215 Parameter LM2770-1215: 1.5V Output Voltage Regulation Condition VIN = 3.5V, IOUT = 150mA, VSEL = 1.8V 3.0V ≤ VIN ≤ 4.5V IOUT = 150mA, VSEL = 1.8V 4.5V < VIN ≤ 5.5V, IOUT = 150mA, VSEL = 1.8V LM2770-1215: 1.2V Output Voltage Regulation VIN = 3.5V, IOUT = 150mA, VSEL = 0V 3.0V ≤ VIN ≤ 4.5V IOUT - 150mA, VSEL =0V 4.5V < VIN ≤ 5.5V, IOUT = 150mA, VSEL = 0V VOUT-12157 LM2770-12157: 1.575V Output Voltage Regulation VIN = 3.5V, IOUT = 150mA, VSEL = 1.8V 3.1V ≤ VIN ≤ 4.5V IOUT = 150mA, VSEL = 1.8V 4.5V < VIN ≤ 5.5V, IOUT = 150mA, VSEL = 1.8V LM2770-12157: 1.2V Output Voltage Regulation VIN = 3.5V, IOUT = 150mA, VSEL = 0V 3.0V ≤ VIN ≤ 4.5V IOUT - 150mA, VSEL =0V 4.5V < VIN ≤ 5.5V, IOUT = 150mA, VSEL = 0V VOUT/IOUT Load Regulation IOUT = 1mA to 250mA Min 1.443 1.420 1.428 1.157 1.140 1.135 1.528 1.500 1.504 1.162 1.145 1.145 Typ 1.495 1.495 1.495 1.205 1.205 1.205 1.575 1.575 1.575 1.210 1.210 1.210 0.18 Max 1.547 1.570 1.562 1.253 1.270 1.275 1.622 1.650 1.646 1.258 1.275 1.275 mV/mA V Units V Output Voltage Specifications: Specific to Individual LM2770 Options 3 www.national.com LM2770 Electrical Characteristics (Notes 2, 9) (Continued) Limits in standard typeface are for TJ = 25oC. Limits in boldface type apply over the full operating junction temperature range (-30˚C ≤ TJ ≤ +105˚C) . Unless otherwise noted, specifications apply to the LM2770 Typical Application Circuit (pg. 1) with: VIN = 3.6V; V(EN) = VSEL = 1.8V, V(SLEEP) = 0V, CIN = COUT = 10µF, C1 = C2 = 1.0µF. (Note 10) Symbol VLDO-1215 Parameter LM2770-1215: 1.5V Output Voltage Regulation - SLEEP Mode LM2770-1215: 1.2V Output Voltage Regulation - SLEEP Mode VLDO-12157 LM2770-12157: 1.575V Output Voltage Regulation - SLEEP Mode LM2770-12157: 1.2V Output Voltage Regulation - SLEEP Mode E EAVG IQ Power Efficiency Average Eficiency over Li-Ion Input Voltage Range (Note 11) Quiescent Supply Current: Full-power Mode Quiescent Supply Current: Sleep Mode Shutdown Current Current Limit - Sleep Mode Turn-on Time Switching Frequency Logic-low Input Voltage Logic-high Input Voltage Logic-high Input Current: SLEEP and VSEL pins Condition 3.0V ≤ VIN ≤ 5.5V, 0mA ≤ IOUT ≤ 20mA, VSEL= 0V, V(SLEEP) = 1.8V 3.0V ≤ VIN ≤ 5.5V, 0mA ≤ IOUT ≤ 20mA, VSEL = 0V, V(SLEEP) = 1.8V 3.0V ≤ VIN ≤ 5.5V, 0mA ≤ IOUT ≤ 20mA, VSEL= 0V, V(SLEEP) = 1.8V 3.0V ≤ VIN ≤ 5.5V, 0mA ≤ IOUT ≤ 20mA, VSEL = 0V, V(SLEEP) = 1.8V VIN = 3.6V, IOUT = 150mA VOUT =1.5V 3.0V ≤ VIN ≤ 4.2V IOUT = 200mA, VOUT = 1.5V 2.7V ≤ VIN ≤ 5.5V IOUT = 0mA V(SLEEP) = 0V 2.7V ≤ VIN ≤ 5.5V IOUT = 0mA V(SLEEP) = 1.8V 2.7V ≤ VIN ≤ 5.5V V(EN) = 0V 0V ≤ VOUT ≤ 0.2V V(SLEEP) = 1.8V VIN = 3.6V, COUT = 10µF 2.7V ≤ VIN ≤ 5.5V 2.7V ≤ VIN ≤ 5.5V 2.7V ≤ VIN ≤ 5.5V Logic Input = 3.0V 475 0 1.0 0.1 6 0 Min 1.435 Typ 1.495 Max 1.555 Units V 1.145 1.205 1.265 1.520 1.575 1.630 V 1.150 1.210 1.270 Specifications Below Apply to All LM2770 Options 82 73 55 75 % % µA ISLEEP 50 65 µA ISD ICL tON FSW VIL VIH IIH IIH-EN IIL 0.1 60 200 700 0.5 µA mA µs 925 0.4 VIN kHz V V µA µA µA Logic Pin Specifications: EN, ENA, ENB Logic-high Input Current: EN pin V(EN) = 1.8V (Note 12) Logic-low Input Current: All Logic Pins Logic Input = 0V 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: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=150oC (typ.) and disengages at TJ=140oC (typ.). Note 4: Short circuit protection circuitry protects the part from immediate destructive failure when VOUT is shorted to GND. Applying a continuous GND short to the output may shorten the lifetime of the device. Note 5: For detailed information on soldering requirements and recommendations, please refer to National Semiconductor’s Application Note 1187 (AN-1187): Leadless Leadframe Package (LLP). Note 6: 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 www.national.com 4 LM2770 Electrical Characteristics (Notes 2, 9) (Continued) Note 7: Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 105oC), 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 8: 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 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: CIN, COUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics. Note 11: Efficiency is measured versus VIN, with VIN being swept in small increments from 3.0V to 4.2V. The average is calculated from these measurement results. Weighting to account for battery voltage discharge characteristics (VBAT vs. Time) is not done in computing the average. Note 12: There is a 300kΩ pull-down resistor connected internally between the EN pin and GND. 5 www.national.com LM2770 Block Diagram 20126003 www.national.com 6 LM2770 Typical Performance Characteristics Unless otherwise specified: CIN = 10µF, C1 = 1.0µF, C2 = 1.0µF, COUT = 10µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). Output Voltage vs. Input Voltage: VOUT = 1.2V Efficiency vs. Input Voltage: VOUT = 1.2V 20126006 20126007 Output Voltage vs. Input Voltage: VOUT = 1.5V Efficiency vs. Input Voltage: VOUT = 1.5V 20126004 20126005 Output Voltage vs. Input Voltage: VOUT = 1.575V Efficiency vs. Input Voltage: VOUT = 1.575V 20126018 20126019 7 www.national.com LM2770 Typical Performance Characteristics Unless otherwise specified: CIN = 10µF, C1 = 1.0µF, C2 = 1.0µF, COUT = 10µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued) Load Regulation: VOUT = 1.2V Load Regulation: VOUT = 1.5V 20126009 20126008 Load Regulation: VOUT = 1.575V Output Voltage Ripple 20126013 VIN = 3.6V, VOUT = 1.5V, IOUT = 200mA CH1: CIN = COUT = 2x10µF; C1 = C2 = 1µF; Scale: 50mV/Div 20126020 CH2: CIN = COUT = 10µF; C1 = C2 = 1µF; Scale: 50mV/Div Time scale: 4µs/Div Input Voltage RIpple Start-up Behavior 20126014 20126012 VIN = 3.6V, VOUT = 1.5V, IOUT = 200mA CH1: CIN = COUT = 2x10µF; C1 = C2 = 1µF; Scale: 50mV/Div CH2: CIN = COUT = 10µF; C1 = C2 = 1µF; Scale: 50mV/Div Time scale: 4µs/Div VIN = 3.6V, VOUT = 1.5V, Load = 7.5Ω (200mA) CH1: EN pin; Scale: 1V/Div CH2: VOUT; Scale: 500mV/Div Time scale: 40µs/Div www.national.com 8 LM2770 Typical Performance Characteristics Unless otherwise specified: CIN = 10µF, C1 = 1.0µF, C2 = 1.0µF, COUT = 10µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued) Load Step Active-to-Sleep Mode Transitions 20126010 20126017 VIN = 3.6V, VOUT = 1.5V, Load = 10mA - 150mA step CH1 (top): Output Current; Scale: 100mA/Div CH2: VOUT; Scale: 100mV/Div Time scale: 40µs/Div VIN = 3.6V, VOUT = 1.5V, Load = 20mA CH1: SLEEP pin; Scale: 2V/Div CH2: VOUT; Scale: 200mV/Div Time scale: 200µs/Div Dynamic Output Voltage Switching: 1.5V to 1.2V 20126016 VIN = 3.6V, VOUT = 1.5V, Load = 10mA - 150mA step CH1: VSEL pin; Scale: 2V/Div CH2: VOUT; Scale: 500mV/Div Time scale: 40µs/Div 9 www.national.com LM2770 Operation Description OVERVIEW The LM2770 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 multiple fractional gains and pulse-frequency-modulated (PFM) switching to minimize power losses over wide input voltage and output current ranges. A description of the principal operational characteristics of the LM2770 is broken up into the following sections: PFM Regulation, Fractional Multi-Gain Charge Pump, and Multi-Gain Efficiency Performance. Each of these sections refers to the Block Diagram. PFM REGULATION The LM2770 achieves tightly regulated output voltages with pulse-frequency-modulated (PFM) regulation. PFM simply means the part only pumps when charge needs to be delivered to the output in order to keep the output voltage in regulation. When the output voltage is above the target regulation voltage, the part idles and consumes minimal supply-current. In this state, the load current is supplied solely by the charge stored on the output capacitor. As this capacitor discharges and the output voltage falls below the target regulation voltage, the charge pump activates, and charge is delivered to the output. This charge supplies the load current and boosts the voltage on the output capacitor. The primary benefit of PFM regulation is when output currents are light and the part is predominantly in the lowsupply-current idle state. Net supply current is minimal because the part only occasionally needs to recharge the output capacitor by activating the charge pump. With PFM regulation, input and output ripple frequencies vary significantly, and are dependent on output current, input voltage, and, to a lesser degree, other factors such as temperature and internal switch characteristics. FRACTIONAL MULTI-GAIN CHARGE PUMP The core of the LM2770 is a two-phase charge pump controlled by an internally generated non-overlapping clock. The charge pump operates by using the external flying capacitors, C1 and C2, to transfer charge from the input to the output. The two phases of the switching cycle will be referred to as the "charge phase" and the "hold/rest phase". During the charge phase, the flying capacitors are charged by the input supply. After charging the flying capacitors for half of a switching cycle [ t = 1/(2xFSW) ], the LM2770 switches to the hold/rest phase. In this configuration, the charge that was stored on the flying capacitors in the charge phase is transferred to the output. If the voltage on the output is below the target regulation voltage at completion of the switching cycle, the charge pump will switch back to the charge phase. But if the output voltage is above the target regulation voltage at the end of the switching cycle, the charge pump will remain in the hold/rest state. It will idle in this mode until the output voltage drops below the target regulation voltage. When this finally occurs, the LM2770 will switch back to the charge phase. Input, output, and intermediary connections of the flying capacitors are made with internal MOS switches. The LM2770 utilizes two flying capacitors and a versatile switch network to achieve three distinct fractional voltage gains: 1⁄3, 1⁄2, and 2⁄3. With this gain-switching ability, it is as if the LM2770 is three-charge-pumps-in-one. The "active" charge pump at any given time is the one that yields the highest efficiency based on the input and output conditions present. MULTI-GAIN EFFICIENCY PERFORMANCE The ability to switch gains based on input and output conditions results in optimal efficiency throughout the operating ranges of the LM2770. 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 graphs in the Typical Perfromance Characteristics section for detailed efficiency data. The gain regions are clearly distinguished by the sharp discontinuities in the efficiency curves and are identified at the bottom of each graph (G = 2⁄3, G = 1⁄2, and G = 1⁄3). DYNAMIC OUTPUT VOLTAGE SELECTION The output voltage of the LM2770 can be dynamically adjusted for the purpose of improving system efficiency. Each LM2770 version contains two built-in output voltage options: a high level and a low level (1.5V and 1.2V, for example). With the simple VSEL logic input pin, the output voltage can be switched between these two voltages. Dynamic voltage selection can be used to improve overall system efficiency. When comparing system efficiency between two different output voltages, evaluating power consumption often lends more insight than actually comparing converter efficiencies. An application powered with a Li-Ion battery is a good example to illustrate this. Referring to the LM2770 efficiency curves (see Typical Performance Charactersitics), all LM2770 output voltage options operate with G = 1⁄2 over the core Li-Ion battery voltage range (3.5V 3.9V). Thus, the LM2770 circuit will draw an input current that is approximately half the output current in the core Li-Ion voltage range, regardless of the output voltage (IIN = G x IOUT) . While varying the LM2770 output voltage does not directly improve system efficiency, it can have a secondary effect. Different output voltages often will result in different LM2770 load currents. This is where system efficiency can benefit from dynamic output voltage selection: the LM2770 load circuit can run at lower currents. This reduces LM2770 input current and improves overall system efficiency. SLEEP MODE BYPASS LDO The LM2770 offers a bypass low-dropout linear regulator (LDO) for low-noise performance under light loads. Capable of delivering up to 20mA of output current, this LDO has low ground pin current and is ideal for stand-by operation. The LDO is activated with the SLEEP logic input pin. When SLEEP is active, the charge pump is disabled and the LDO supplies all load current. SHUTDOWN The LM2770 is in shutdown mode when the voltage on the enable pin (EN) is logic-low. In shutdown, the LM2770 draws virtually no supply current. When in shutdown, the output of the LM2770 is completely disconnected from the input. The internal feedback resistors will pull the output voltage down to 0V (unless the output is driven by an outside source). In some applications, it may be desired to disable the LM2770 and drive the output pin with another voltage 10 www.national.com LM2770 Operation Description (Continued) source. This can be done, but the voltage on the output pin of the LM2770 must not be brought above the input voltage. The output pin will draw a small amount of current when driven externally due the internal feedback resistor divider connected between VOUT and GND. SOFT START The LM2770 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 200µs (typ.). Soft-start is engaged when the part is enabled, including situations where voltage is established simultaneously on the VIN and EN pins. THERMAL SHUTDOWN Protection from overheating-related damage is achieved with a thermal shutdown feature. When the junction temperature rises to 150oC (typ.), the part switches into shutdown mode. The LM2770 disengages thermal shutdown when the junction temperature of the part is reduced to 140oC (typ.). Due to the high efficiency of the LM2770, 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. SHORT-CIRCUIT AND CURRENT LIMIT PROTECTION The LM2770 charge pump contains circuitry that protects the device from destructive failure in the event of a direct short to ground on the output. This short-circuit protection circuit limits the output current to 400mA (typ.) when the output voltage is below 165mV (typ.). The sleep-mode LDO contains a 60mA (typ.) current limit circuit. 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 LM2770 circuit be thoroughly evaluated early in the design-in process with the mass-production capacitors of choice. This will help to ensure that any such variability in capacitance does not negatively impact circuit performance. The table below lists some leading ceramic capacitor manufacturers. Manufacturer AVX Murata Taiyo-Yuden TDK Vishay-Vitramon Contact Information www.avx.com www.murata.com www.t-yuden.com www.component.tdk.com www.vishay.com Application Information RECOMMENDED CAPACITOR TYPES The LM2770 requires 4 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 LM2770 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 LM2770. 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 LM2770. 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 LM2770. 11 OUTPUT CAPACITOR AND OUTPUT VOTAGE RIPPLE The output capacitor in the LM2770 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 multi-gain and PFM switching, 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. This can be observed in the output voltage ripple waveforms in the Typical Performance Characteristics section. In typical high-current applications, a 10µF 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 capacitors and/or input capacitor to maintain good overall circuit performance. Performance of the LM2770 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 vey 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 capacitor 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. Due to the PFM nature of the LM2770, output voltage ripple is highest at light loads. To eliminate this ripple, consider running the LM2770 in sleep mode when load currents are 20mA or less. Sleep mode disables the charge pump and enables the internal low-noise bypass linear regulator (LDO). www.national.com LM2770 Application Information (Continued) INPUT CAPACITOR AND INPUT VOTLAGE RIPPLE The input capacitor (CIN) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying capacitors 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. This can be observed in the input voltage ripple waveforms in the Typical Performance Characteristics section. Input voltage, output current, and flying capacitance also will affect input ripple levels to some degree. In typical high-current applications, a 10µF 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 capacitors and/or output capacitor to maintain good overall circuit performance. Performance of the LM2770 with different capacitor setups is discussed below in Recommended Capacitor Configurations. FLYING CAPACITORS The flying capacitors (C1 and C2) transfer 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 LM2770 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 capacitors might overwhelm the input and output capacitors, resulting in increased input and output ripple. The flying capacitors should be identical. As a general guideline, the capacitance value of each flying capacitor should be 1/10th that of the output capacitor, up to a maximum of 1µF. This is a recommendation, not a requirement. Polarized capacitors (tantalum, aluminum electrolytic, etc.) must not be used for the flying capacitors, however, as they could become reverse-biased during LM2770 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 (ripple voltages, output current capability, etc.). As previously discussed, input and output ripple voltages and frequencies will vary considerably with output current and input voltage. The numbers provided show expected ripple voltage when VIN = 3.6V and load currents are between 100mA and 250mA. 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. The columns that provide minimum input voltage recommendations illustrate the effect that smaller flying capacitors have on charge pump output current capability. Using smaller flying capacitors increases the output resistance of the charge pump. As a result, the minimunm input voltage of an application using small flying capacitance may need to be set slightly higher to prevent the output from falling out of regulation when loaded. TABLE 1. LM2770 Performance with Different Capacitor Configurations (Note 13) CAPACITOR CONFIGURATION CIN = COUT = 2x10µF, C1 = C2 = 1µF CIN = COUT = 10µF, C1 = C2 = 1µF CIN = COUT = 4.7µF, C1 = C2 = 0.47µF CIN = COUT = 2.2µF, C1 = C2 = 0.22µF TYPICAL OUTUT RIPPLE (VIN = 3.6V) 25mV 50mV 130mV 200mV TYPICAL INPUT RIPPLE (VIN = 3.6V) 35mV 70mV 150mV 260mV Recommended Minimum VIN for Different Output Currents IOUT = 50mA 3.0V 3.0V 3.0V 3.0V IOUT = 150mA 3.0V 3.0V 3.1V 3.1V IOUT = 250mA 3.1V 3.1V 3.2V 3.2V Note 13: Refer to the text in the Recommended Capacitor Configurations section for detailed information on the data in this table www.national.com 12 LM2770 Layout Guidelines Proper board layout will help to ensure optimal performance of the LM2770 circuit. The following guidelines are recommended: • Place capacitors as close to the LM2770 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 LM2770 to minimize trace resistance and inductance. • Use a low resistance connection between ground and the GND pin of the LM2770. Using wide traces and/or multiple vias to connect GND to a ground plane on the board is most advantageous. 20126015 FIGURE 1. Recommended Board Layout of a LM2770 Circuit Unlabelled vias connect to an internal ground plane. 13 www.national.com LM2770 High Efficiency Switched Capacitor Step-Down DC/DC Regulator with Sleep Mode 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. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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