LM2852_06

LM2852 2A 500/1500kHz Synchronous SIMPLE SWITCHER ® Buck Regulator October 2006 LM2852 2A 500/1500kHz Synchronous SIMPLE SWITCHER ® Buck Regulator General Description The LM2852 SIMPLE synchronous buck regulator is a high frequency step-down switching voltage regulator capable of driving up to a 2A load with excellent line and load regulation. The LM2852 can accept an input voltage between 2.85V and 5.5V and deliver an output voltage that is factory programmable from 0.8V to 3.3V in 100mV increments. The LM2852 is available with a choice of two switching frequencies - 500kHz (LM2852Y) or 1.5MHz (LM2852X). It also features internal, type-three compensation to deliver a low component count solution. The exposed-pad TSSOP-14 package enhances the thermal performance of the LM2852. SWITCHER ® Features n Input voltage range of 2.85 to 5.5V n Factory EEPROM set output voltages from 0.8V to 3.3V in 100mV increments n Maximum load current of 2A n Voltage Mode Control n Internal type-three compensation n Switching frequency of 500kHz or 1.5MHz n Low standby current of 10µA n Internal 60 mΩ MOSFET switches n Standard voltage options 0.8/1.0/1.2/1.5/1.8/2.5/3.3 volts Applications n Low voltage point of load regulation n Local solution for FPGA/DSP/ASIC core power n Broadband networking and communications infrastructure n Portable computing Typical Application Circuit 20127001 20127002 SIMPLE SWITCHER ® SIMPLE SWITCHERreg; is a registered trademark of National Semiconductor Corporation © 2006 National Semiconductor Corporation DS201270 www.national.com LM2852 Connection Diagram TOP VIEW 20127003 MXA14A ETSSOP-14 Pin Descriptions AVIN (Pin 1): Chip bias input pin. This provides power to the logic of the chip. Connect to the input voltage or a separate rail. EN (Pin 2): Enable. Connect this pin to ground to disable the chip; connect to AVIN or leave floating to enable the chip; enable is internally pulled up. SGND (Pin 3): Signal ground. SS (Pin 4): Soft-start pin. Connect this pin to a small capacitor to control startup. The soft-start capacitance range is restricted to values 1 nF to 50 nF. NC (Pins 5, 12 and 13): No connect. These pins must be tied to ground or left floating in the application. PVIN (Pins 6, 7): Input supply pin. PVIN is connected to the input voltage. This rail connects to the source of the internal power PFET. SW (Pins 8, 9): Switch pin. Connect to the output inductor. PGND (Pins 10, 11): Power ground. Connect this to an internal ground plane or other large ground plane. SNS (Pin 14): Output voltage sense pin. Connect this pin to the output voltage as close to the load as possible. Exposed Pad: Connect to ground. www.national.com 2 LM2852 Ordering Information Order Number LM2852YMXA-0.8 LM2852YMXAX-0.8 LM2852YMXA-1.0 LM2852YMXAX-1.0 LM2852YMXA-1.2 LM2852YMXAX-1.2 LM2852YMXA-1.5 LM2852YMXAX-1.5 LM2852YMXA-1.8 LM2852YMXAX-1.8 LM2852YMXA-2.5 LM2852YMXAX-2.5 LM2852YMXA-3.0 LM2852YMXAX-3.0 LM2852YMXA-3.3 LM2852YMXAX-3.3 LM2852XMXA-0.8 LM2852XMXAX-0.8 LM2852XMXA-1.0 LM2852XMXAX-1.0 LM2852XMXA-1.2 LM2852XMXAX-1.2 LM2852XMXA-1.5 LM2852XMXAX-1.5 LM2852XMXA-1.8 LM2852XMXAX-1.8 LM2852XMXA-2.5 LM2852XMXAX-2.5 LM2852XMXA-3.0 LM2852XMXAX-3.0 LM2852XMXA-3.3 LM2852XMXAX-3.3 Note: Contact factory for other voltage options. Frequency Voltage Option 0.8 1.0 1.2 1.5 Package Type Package Drawing Supplied As 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 500kHz 1.8 2.5 3.0 3.3 0.8 1.0 1.2 1.5 TSSOP-14 exposed pad MXA14A 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 94 Units, Rail 2500 Units, Tape and Reel 1500kHz 1.8 2.5 3.0 3.3 3 www.national.com LM2852 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. PVIN, AVIN, EN, SNS ESD Susceptibility (Note 2) Power Dissipation Storage Temperature Range Maximum Junction Temp. 14-Pin Exposed Pad TSSOP Package Infrared (15 sec) Vapor Phase (60 sec) Soldering (10 sec) −0.3V to 6.5V 2kV Internally Limited −65˚C to +150˚C 150˚C 220˚C 215˚C 260˚C Operating Ratings PVIN to GND AVIN to GND Junction Temperature θJA 1.5V to 5.5V 2.85V to 5.5V −40˚C to +125˚C 38˚C/W Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column. Limits in standard type are for TJ = 25˚C only; limits in boldface type apply over the junction temperature (TJ) range of -40˚C to +125˚C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25˚C, and are provided for reference purposes only. Symbol VOUT Parameter Conditions Min 0.782 0.9775 1.1730 1.4663 1.7595 2.4437 2.9325 3.2257 0.2 Typ Max 0.818 1.0225 1.2270 1.5337 1.8405 2.5563 3.0675 3.3743 0.6 % Units V SYSTEM PARAMETERS Voltage Tolerance3 VOUT = 0.8V option VOUT = 1.0V option VOUT = 1.2V option VOUT = 1.5V option VOUT = 1.8V option VOUT = 2.5V option VOUT = 3.0V option VOUT = 3.3V option ∆VOUT/∆AVIN Line Regulation 3 VOUT = 0.8V, 1.0V, 1.2V, 1.5V, 1.8V or 2.5V 2.85V ≤ AVIN ≤ 5.5V VOUT = 3.3V 3.5V ≤ AVIN ≤ 5.5V 0.2 8 2.47 85 150 75 55 400 2.75 2.25 4 3 0.85 10 400 0.6 % mV/A ∆VOUT/∆IO VON rDSON-P rDSON-N RSS ICL IQ ISD RSNS PWM fosc Load Regulation UVLO Threshold (AVIN) PFET On Resistance NFET On Resistance Soft-start resistance Normal operation Rising Falling Hysteresis Isw = 2A Isw = 2A 2.85 210 140 120 V mV mΩ mΩ kΩ Peak Current Limit LM2852X Threshold LM2852Y Operating Current Non-switching Shutdown EN = 0V Quiescent Current Sense pin resistance LM2852X LM2852Y 1500kHz option. 500kHz option. 4.95 3.65 2 25 A mA µA kΩ 1050 325 1500 500 1825 625 kHz kHz www.national.com 4 LM2852 Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column. Limits in standard type are for TJ = 25˚C only; limits in boldface type apply over the junction temperature (TJ) range of -40˚C to +125˚C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25˚C, and are provided for reference purposes only. (Continued) Symbol Drange VIH VIL IEN ENABLE CONTROL4 EN Pin Minimum High Input EN Pin Maximum Low Input EN Pin Pullup Current TJ for Thermal Shutdown Hysteresis for Thermal Shutdown EN = 0V 1.2 75 25 % of AVIN % of AVIN µA Parameter Duty Cycle Range Conditions Min 0 Typ Max 100 Units % THERMAL CONTROLS TSD TSD-HYS 165 10 ˚C ˚C Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating Range indicates conditions for which the device is intended to be functional, but does not guarantee specfic performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: Human body model: 1.5kΩ in series with 100pF. SW and PVIN pins are derated to 1.5kV Note 3: VOUT measured in a non-switching, closed-loop configuration at the SNS pin. Note 4: The enable pin is internally pulled up, so the LM2852 is automatically enabled unless an external enable voltage is applied. 5 www.national.com LM2852 LM2852Y Typical Performance Characteristics (500kHz) Efficiency vs ILoad VOUT = 1.5V Efficiency vs ILoad VOUT = 2.5V 20127024 20127004 Efficiency vs ILoad VOUT = 3.3V Frequency vs Temperature 20127006 20127009 www.national.com 6 LM2852 LM2852X Typical Performance Characteristics (1500kHz) Efficiency vs ILoad VOUT = 1.5V Efficiency vs ILoad VOUT = 2.5V 20127025 20127026 Efficiency vs ILoad VOUT = 3.3V Frequency vs Temperature 20127028 20127027 7 www.national.com LM2852 LM2852 Typical Performance Characteristics (Both Y and X Versions) Shutdown Current vs VIN Quiescent Current (Non-Switching) vs VIN 20127008 20127007 NMOS Switch RDSON vs Temperature PMOS Switch RDSON vs Temperature 20127010 20127011 www.national.com 8 LM2852 Block Diagram 20127012 Applications Information The LM2852 is a DC-DC synchronous buck regulator belonging to National Semiconductor’s SIMPLE SWITCHER ® family. Integration of the PWM controller, power switches and compensation network greatly reduces the component count required to implement a switching power supply. A typical application requires only four components: an input capacitor, a soft-start capacitor, an output filter capacitor and an output filter inductor. INPUT CAPACITOR (CIN) Fast switching of large currents in the buck converter places a heavy demand on the voltage source supplying PVIN. The input capacitor, CIN, supplies extra charge when the switcher needs to draw a burst of current from the supply. The RMS current rating and the voltage rating of the CIN capacitor are therefore important in the selection of CIN. The RMS current specification can be approximated by: also connected to the soft-start pin, SS. Adding a soft-start capacitor externally increases the time it takes for the output voltage to reach its final level. The charging time required for the reference voltage can be estimated using the RC time constant of the DAC resistor and the capacitance connected to the SS pin. Three RC time constant periods are needed for the reference voltage to reach 95% of its final value. The actual start-up time will vary with differences in the DAC resistance and higher-order effects. If little or no soft-start capacitance is connected, then the start-up time may be determined by the time required for the current limit current to charge the output filter capacitance. The capacitor charging equation I = C ∆V/∆t can be used to estimate the start-up time in this case. For example, a part with a 3V output, a 100 µF output capacitance and a 3A current limit threshold would require a time of 100 µs: where D is the duty cycle, VOUT/VIN. CIN also provides filtering of the supply. Trace resistance and inductance degrade the benefits of the input capacitor, so CIN should be placed very close to PVIN in the layout. A 22 µF or 47 µF ceramic capacitor is typically sufficient for CIN. In parallel with the large input capacitance a smaller capacitor may be added such as a 1µF ceramic for higher frequency filtering. SOFT-START CAPACITOR (CSS) The DAC that sets the reference voltage of the error amp sources a current through a resistor to set the reference voltage. The reference voltage is one half of the output voltage of the switcher due to the 200kΩ divider connected to the SNS pin. Upon start-up, the output voltage of the switcher tracks the reference voltage with a two to one ratio as the DAC current charges the capacitance connected to the reference voltage node. Internal capacitance of 20pF is permanently attached to the reference voltage node which is 9 Since it is undesirable for the power supply to start up in current limit, a soft-start capacitor must be chosen to force the LM2852 to start up in a more controlled fashion based on the charging of the soft-start capacitance. In this example, suppose a 3 ms start time is desired. Three time constants are required for charging the soft-start capacitor to 95% of the final reference voltage. So in this case RC=1ms. The DAC resistor, R, is 400 kΩ so C can be calculated to be 2.5nF. A 2.7nF ceramic capacitor can be chosen to yield approximately a 3ms start-up time. SOFT-START CAPACITOR (CSS) AND FAULT CONDITIONS Various fault conditions such as short circuit and UVLO of the LM2852 activate internal circuitry designed to control the voltage on the soft-start capacitor. For example, during a short circuit current limit event, the output voltage typically www.national.com LM2852 Applications Information (Continued) falls to a low voltage. During this time, the soft-start voltage is forced to track the output so that once the short is removed, the LM2852 can restart gracefully from whatever voltage the output reached during the short circuit event. The range of soft-start capacitors is therefore restricted to values 1nF to 50nF. COMPENSATION The LM2852 provides a highly integrated solution to power supply design. The compensation of the LM2852, which is type-three, is included on-chip. The benefit to integrated compensation is straightforward, simple power supply design. Since the output filter capacitor and inductor values impact the compensation of the control loop, the range of L, C and CESR values is restricted in order to ensure stability. OUTPUT FILTER VALUES Table 1 details the recommended inductor and capacitor ranges for the LM2852 that are suggested for various typical output voltages. Values slightly different than those recommended may be used, however the phase margin of the power supply may be degraded. TABLE 1. Output Filter Values Frequency Option L (µH) VOUT (V) 0.8 0.8 1.0 1.0 1.2 LM2852Y (500kHz) 1.2 1.5 1.5 1.8 1.8 2.5 2.5 3.3 0.8 0.8 1.0 1.0 1.2 LM2852X (1500kHz) 1.2 1.5 1.5 1.8 1.8 2.5 2.5 3.3 PVIN (V) 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 5.0 1 10 Min 10 10 10 10 10 15 10 22 10 22 6.8 15 15 Max 15 15 15 15 15 22 15 22 15 33 10 22 22 Min 100 100 100 100 100 100 100 100 100 100 68 68 68 C (µF) Max 220 120 180 180 180 120 120 120 120 120 120 120 100 Min 70 70 70 70 70 70 70 70 100 100 95 95 100 CESR (mΩ) Max 200 200 200 200 200 200 200 200 200 200 275 275 275 The 1500kHz version is designed for ceramic output capacitors which typically have very low ESR ( < 10mΩ.) www.national.com 10 LM2852 Applications Information CHOOSING AN INDUCTANCE VALUE (Continued) The current ripple present in the output filter inductor is determined by the input voltage, output voltage, switching frequency and inductance according to the following equation: The maximum inductor current for a 2A load would therefore be 2A plus 60.8 mA, 2.0608A. As shown in the ripple equation, the current ripple is inversely proportional to inductance. OUTPUT FILTER INDUCTORS where ∆IL is the peak-to-peak current ripple, D is the duty cycle VOUT/VIN, VIN is the input voltage applied to the PVIN pin, VOUT is the output voltage of the switcher, f is the switching frequency and L is the inductance of the output filter inductor. Knowing the current ripple is important for inductor selection since the peak current through the inductor is the load current plus one half the ripple current. Care must be taken to ensure the peak inductor current does not reach a level high enough to trip the current limit circuitry of the LM2852. As an example, consider a 5V to 1.2V conversion and a 500kHz switching frequency. According to Table 1, a 15µH inductor may be used. Calculating the expected peak-topeak ripple, Once the inductance value is chosen, the key parameter for selecting the output filter inductor is its saturation current (Isat) specification. Typically Isat is given by the manufacturer as the current at which the inductance of the coil falls to a certain percentage of the nominal inductance. The Isat of an inductor used in an application should be greater than the maximum expected inductor current to avoid saturation. Below is a table of inductors that may be suitable in LM2852 applications. TABLE 2. LM2852 Output Filter Inductors Inductance (µH) 1 1 6.8 7 10 10 12 15 15 18 22 22 22 27 33 33 Part Number DO1608C-102 DO1813P-102HC DO3316P-682 MSS1038-702NBC DO3316P-103 MSS1038-103NBC MSS1038-123NBC D03316P-153 MSS1038-153NBC MSS1038-183NBC DO3316P-223 MSS1038-223NBC DO3340P-223 MSS1038-273NBC MSS1038-333NBC DO3340P-333 Vendor Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft 11 www.national.com LM2852 Applications Information OUTPUT FILTER CAPACITORS (Continued) Table 1. Below are some examples of capacitors that can typically be used in an LM2852 application. The capacitors that may be used in the output filter with the LM2852 are limited in value and ESR range according to TABLE 3. LM2852 Output Filter Capacitors Capacitance (µF) 10 10 68 68 100 100 100 100 120 120 150 150 150 150 220 220 220 Part Number GRM31MR61A106KE19 GRM32DR61E106K 595D686X_010C2T 595D686X_016D2T 595D107X_6R3C2T 595D107X_016D2T NOSC107M004R0150 NOSD107M006R0100 595D127X_004C2T 595D127X_010D2T 595D157X_004C2T 595D157X_016D2T NOSC157M004R0150 NOSD157M006R0100 595D227X_004D2T NOSD227M004R0100 NOSE227M006R0100 Chemistry Ceramic Ceramic Tantalum Tantalum Tantalum Tantalum Niobium Oxide Niobium Oxide Tantalum Tantalum Tantalum Tantalum Niobium Oxide Niobium Oxide Tantalum Niobium Oxide Niobium Oxide Vendor Murata Murata Vishay - Sprague Vishay - Sprague Vishay - Sprague Vishay - Sprague AVX AVX Vishay - Sprague Vishay - Sprague Vishay - Sprague Vishay - Sprague AVX AVX Vishay - Sprague AVX AVX SPLIT-RAIL OPERATION The LM2852 can be powered using two separate voltages for AVIN and PVIN. AVIN is the supply for the control logic; PVIN is the supply for the power FETs. The output filter components need to be chosen based on the value of PVIN. For PVIN levels lower than 3.3V, use output filter component values recommended for 3.3V. PVIN must always be equal to or less than AVIN. 20127014 SWITCH NODE PROTECTION The LM2852 includes protection circuitry that monitors the voltage on the switch pin. Under certain conditions, switching is disabled in order to protect the switching devices. One result of the protection circuitry may be observed when power to the LM2852 is applied with no or light load on the output. The output regulates to the rated voltage, but no switching may be observed. As soon as the output is loaded, the LM2852 begins normal switching operation. www.national.com 12 LM2852 Applications Information LAYOUT HINTS (Continued) These are several guidelines to follow while designing the PCB layout for an LM2852 application. 1. The input bulk capacitor, CIN, should be placed very close to the PVIN pin to keep the resistance as low as possible between the capacitor and the pin. High current levels will be present in this connection. 2. All ground connections must be tied together. Use a broad ground plane, for example a completely filled back plane, to establish the lowest resistance possible between all ground connections. 3. The sense pin connection should be made as close to the load as possible so that the voltage at the load is the expected regulated value. The sense line should not run too close to nodes with high EMI (such as the switch node) to minimize interference. 4. The switch node connections should be low resistance to reduce power losses. Low resistance means the trace between the switch pin and the inductor should be wide. However, the area of the switch node should not be too large since EMI increases with greater area. So connect the inductor to the switch pin with a short, but wide trace. Other high current connections in the application such as PVIN and VOUT assume the same trade off between low resistance and EMI. 5. Allow area under the chip to solder the entire exposed die attach pad to ground for improved thermal and electrical performance. LM2852 Example Circuit Schematic 20127020 FIGURE 1. Bill of Materials for 500kHz (LM2852Y) 3.3VIN to 1.8 VOUT Conversion ID U1 LO CO* CIN CINX CSS Rf Cf Part Number LM2852YMXA-1.8 DO3316P-153 595D107X_6R3C2T GRM32ER60J476ME20B GRM21BR71C105KA01B VJ0805Y272KXXA CRCW060310R0F GRM21BR71C105KA01B Type 2A Buck Inductor Capacitor Capacitor Capacitor Capacitor Resistor Capacitor Case Code “C” 1210 0805 0805 0603 0805 Size ETSSOP-14 15 µH 100 µF ± 20% 47µF/X5R/6.3V 1µF/X7R/16V 2.7nF ± 10% 10Ω ± 10% 1µF/X7R/16V Parameters Qty 1 1 1 1 1 1 1 1 Vendor NSC Coilcraft Vishay-Sprague Murata Murata Vishay-Vitramon Vishay-Dale Murata * If a “non-tantalum” solution is desired use an NOSC107M004R0150, 100 µF capacitor from AVX for CO. Bill of Materials for 1500kHz (LM2852X) 3.3V to 1.8V Conversion ID U1 L0 C0 CIN CINX CSS Rf Cf Part Number LM2852XMXA-1.8 DO1813P-102HC GRM32DR61E106K GRM32ER60J476ME20B GRM21BR71C105KA01B VJ0805Y272KXXA CRCW060310R0F GRM21BR71C105KA01B Type 2A Buck Inductor Capacitor Capacitor Capacitor Capacitor Resistor Capacitor 1210 1210 0805 0805 0603 0805 Size ETSSOP-14 1 µH 10 µF/X5R/25V 47µF/X5R/6.3V 1µF/X7R/16V 2.7nF ± 10% 10Ω ± 10% 1µF/X7R/16V Parameters Qty 1 1 1 1 1 1 1 1 Vendor NSC Coilcraft Murata Murata Murata Vishay-Vitramon Vishay-Dale Murata 13 www.national.com LM2852 2A 500/1500kHz Synchronous SIMPLE SWITCHER ® Buck Regulator Physical Dimensions inches (millimeters) unless otherwise noted 14-Lead ETSSOP Package NS Package Number MXA14A 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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