LM2852 2A 500/1500kHz SIMPLE SYNCHRONOUS™ Buck Regulator General Description Features The LM2852 SIMPLE SYNCHRONOUS™ buck regulator is 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 1.0/1.2/1.5/1.8/2.5/3.3 volts 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 a customizable 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 compensation to deliver a low component count solution. The exposed-pad TSSOP-14 package enhances the thermal performance of the LM2852. Applications n Low voltage point of load regulators n Local solution for FPGA/DSP/ASIC core power n Broadband networking and communications infrastructure n Portable computing Typical Application Circuit 20127001 20127002 © 2005 National Semiconductor Corporation DS201270 www.national.com LM2852 2A 500/1500kHz SIMPLE SYNCHRONOUS Buck Regulator January 2005 LM2852 Connection Diagram TOP VIEW 20127003 MXA14A NC (Pins 5, 12 and 13): No-connect. These pins must be tied to ground or left floating in the application. 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. 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. 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): Low-noise ground. SS (Pin 4): Soft-start pin. Connect this pin to a small capacitor to control startup and soften inrush current. The soft-start capacitance range is restricted to values 1 nF to 50 nF. www.national.com 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. 2 Order Number Frequency LM2852YMXA-1.0 Voltage Option Package Type Package Drawing 1.0 94 Units, Rail LM2852YMXAX-1.0 2500 Units, Tape and Reel LM2852YMXA-1.2 1.2 94 Units, Rail LM2852YMXAX-1.2 2500 Units, Tape and Reel LM2852YMXA-1.5 1.5 94 Units, Rail LM2852YMXAX-1.5 LM2852YMXA-1.8 Supplied As 500kHz 2500 Units, Tape and Reel 1.8 94 Units, Rail LM2852YMXAX-1.8 2500 Units, Tape and Reel LM2852YMXA-2.5 2.5 94 Units, Rail LM2852YMXAX-2.5 2500 Units, Tape and Reel LM2852YMXA-3.3 3.3 TSSOP-14 exposed pad LM2852YMXAX-3.3 MXA14A 94 Units, Rail 2500 Units, Tape and Reel LM2852XMXA-1.0 1.0 LM2852XMXAX-1.0 LM2852XMXA-1.2 1.2 LM2852XMXAX-1.2 LM2852XMXA-1.5 LM2852XMXAX-1.5 LM2852XMXA-1.8 1.5 1500kHz Coming Soon 1.8 LM2852XMXAX-1.8 LM2852XMXA-2.5 2.5 LM2852XMXAX-2.5 LM2852XMXA-3.3 3.3 LM2852XMXAX-3.3 Note: Contact factory for other voltage options. 3 www.national.com LM2852 Ordering Information LM2852 Absolute Maximum Ratings (Note 1) Operating Ratings If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. PVIN to GND 1.5V to 5.5V AVIN to GND 2.85V to 5.5V PVIN, AVIN, EN, SNS Junction Temperature −0.3V to 6.5V ESD Susceptibility (Note 2) θJA 2kV Power Dissipation Internally Limited Storage Temperature Range −65˚C to +150˚C Maximum Junction Temp. 150˚C 14-Pin Exposed Pad TSSOP Package Infrared (15 sec) Vapor Phase (60 sec) Soldering (10 sec) 220˚C 215˚C 260˚C −40˚C to +125˚C 37.6˚C/W Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column. Typicals and limits appearing in plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over full Operating Junction Temperature Range (−40˚C to +125˚C). Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Symbol Parameter Conditions Min Typ Max Units V SYSTEM PARAMETERS VOUT ∆VOUT/∆AVIN Voltage Tolerance3 VOUT = 1.0V option 0.9775 1.0225 Line Regulation3 VOUT = 1.2V option 1.1730 1.2270 VOUT = 1.5V option 1.4663 1.5337 VOUT = 1.8V option 1.7595 1.8405 VOUT = 2.5V option 2.4437 2.5563 VOUT = 3.3V option 3.2257 3.3743 VOUT = 0.8V, 1.0V, 1.2V, 1.5V, 1.8V or 2.5V 2.85V ≤ AVIN ≤ 5.5V 0.2 0.6 % VOUT = 3.3V 3.5V ≤ AVIN ≤ 5.5V 0.2 0.6 % ∆VOUT/∆IO Load Regulation Normal operation VON UVLO Threshold (AVIN) Rising 8 Falling Hysteresis 85 mV/A 2.47 2.85 V 150 210 mV rDSON-P PFET On Resistance Isw = 2A 75 140 mΩ rDSON-N NFET On Resistance Isw = 2A 55 120 mΩ RSS Soft-start resistance ICL Peak Current Limit Threshold IQ Operating Current Non-switching ISD Shut Down EN = 0V Quiescent Current RSNS 400 2.25 Sense pin resistance 3 kΩ 3.65 A 0.85 2 mA 10 25 µA 400 kΩ PWM fosc Drange www.national.com LM2852X 1500kHz option. TBD 1500 TBD kHz LM2852Y 500kHz option. 325 500 625 kHz 100 % Duty Cycle Range 0 4 Symbol Parameter Conditions Min Typ Max Units ENABLE CONTROL4 VIH EN Pin Minimum High Input VIL EN Pin Maximum Low Input IEN EN Pin Pullup Current 75 % of AVIN 25 EN = 0V % of AVIN 1.2 µA THERMAL CONTROLS TSD TJ for Thermal Shutdown 165 ˚C TSD-hys Hysteresis for Thermal Shutdown 10 ˚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 Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column. Typicals and limits appearing in plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over full Operating Junction Temperature Range (−40˚C to +125˚C). Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. (Continued) LM2852 Typical Performance Characteristics Efficiency vs ILoad VOUT = 2.5V Efficiency vs ILoad VOUT = 1.5V 20127024 20127004 Efficiency vs ILoad VOUT = 3.3V Quiescent Current (Non-Switching) vs VIN and Temp. 20127006 20127007 Shut-Down Current vs VIN and Temp. Frequency vs Temperature and VIN 20127009 20127008 www.national.com 6 (Continued) NMOS Switch RDSON vs Temperature and PVIN PMOS Switch RDSON vs Temperature and PVIN 20127010 20127011 7 www.national.com LM2852 Typical Performance Characteristics LM2852 Block Diagram 20127012 Applications Information The LM2852 is a DC-DC buck converter belonging to National Semiconductor’s SIMPLE SYNCHRONOUS ® 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. permanently attached to the reference voltage node which is 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. 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 to be the load current times the square root of the duty cycle: 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. 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) 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 www.national.com 8 type three, is included on-chip. The benefit to integrated compensation is straight-forward, 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. (Continued) 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 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. 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. COMPENSATION The LM2852 provides a highly integrated solution to power supply design. The compensation of the LM2852, which is TABLE 1. Output Filter Values L (µH) Frequency Option LM2852Y (500kHz) CESR (mΩ) C (µF) VOUT (V) PVIN (V) Min Max Min Max Min Max 0.8 3.3 10 15 100 220 70 200 0.8 5 10 15 100 120 70 200 1 3.3 10 15 100 180 70 200 1 5 10 15 100 180 70 200 1.2 3.3 10 15 100 180 70 200 1.2 5 15 22 100 120 70 200 1.5 3.3 10 15 100 120 70 200 1.5 5 22 22 100 120 70 200 1.8 3.3 10 15 100 120 100 200 1.8 5 22 33 100 120 100 200 2.5 3.3 6.8 10 68 120 95 275 2.5 5 15 22 68 120 95 275 3.3 5 15 22 68 100 100 275 CHOOSING AN INDUCTANCE VALUE 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. where ∆IL is the peak to peak current ripple, D is the duty cycle VOUT/VIN, VIN is the input voltage applied to the output stage, 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, OUTPUT FILTER INDUCTORS 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. 9 www.national.com LM2852 Applications Information LM2852 Applications Information (Continued) TABLE 2. (LM2852Y Output Filter Inductors (500kHz) Inductance (µH) Part Number Vendor 6.8 DO3316P-682 Coilcraft 7 MSS1038-702NBC Coilcraft 10 DO3316P-103 Coilcraft 10 MSS1038-103NBC Coilcraft 12 MSS1038-123NBC Coilcraft 15 D03316P-153 Coilcraft 15 MSS1038-153NBC Coilcraft 18 MSS1038-183NBC Coilcraft 22 DO3316P-223 Coilcraft 22 MSS1038-223NBC Coilcraft 22 DO3340P-223 Coilcraft 27 MSS1038-273NBC Coilcraft 33 MSS1038-333NBC Coilcraft 33 DO3340P-333 Coilcraft OUTPUT FILTER CAPACITORS The capacitors that may be used in the output filter with the LM2852 are limited in value and ESR range according to Table 1. Below are some examples of capacitors that can typically be used in an LM2852 application. TABLE 3. LM2852Y Output Filter Capacitors (500kHz) Capacitance (µF) Part Number Chemistry Vendor 68 595D686X_010C2T Tantalum Vishay - Sprague 68 595D686X_016D2T Tantalum Vishay - Sprague 100 595D017X_6R3C2T Tantalum Vishay - Sprague 100 595D107X_016D2T Tantalum Vishay - Sprague 100 NOSC107M004R0150 Niobium Oxide AVX 100 NOSD107M006R0100 Niobium Oxide AVX 120 595D127X_004C2T Tantalum Vishay - Sprague 120 595D127X_010D2T Tantalum Vishay - Sprague 150 595D157X_004C2T Tantalum Vishay - Sprague 150 595D157X_016D2T Tantalum Vishay - Sprague 150 NOSC157M004R0150 Niobium Oxide AVX 150 NOSD157M006R0100 Niobium Oxide AVX 220 595D227X_004D2T Tantalum Vishay - Sprague 220 NOSD227M004R0100 Niobium Oxide AVX 220 NOSE227M006R0100 Niobium Oxide AVX www.national.com 10 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. (Continued) SPLIT-RAIL OPERATION The LM2852 can be powered using two separate voltages for AVIN and PVIN. AVIN is the supply for the control logic; 20127014 Layout Hints node) to minimize interference. 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 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. Lab measurements show improved regulation performance when the exposed pad is well grounded. LM2852Y Example Circuit Schematic (500kHz) 20127020 FIGURE 1. Bill of Materials for 3.3VIN to 1.8 VOUT Conversion ID Part Number U1 LM2852YMXA-1.8 Type Size 2A Buck ETSSOP-14 Parameters Qty Vendor 1 NSC LO DO3316P-153 15 µH 1 Coilcraft CO* 595D107X_6R3C2T Capacitor Case Code “C” 100 µF ± 20% 1 Vishay-Sprague CIN GRM32ER60J476ME20B Capacitor 1210 47µF/X5R/6.3V 1 Murata CINX GRM21BR71C105KA01B Capacitor 0805 1µF/X7R/16V 1 Murata CSS VJ0805Y272KXXA Capacitor 0805 2.7nF ± 10% 1 Vishay-Vitramon Rf CRCW060310R0F Resistor 0603 10Ω ± 10% 1 Vishay-Dale Cf GRM21BR71C105KA01B Capacitor 0805 1µF/X7R/16V 1 Murata Inductor * If a “non-tantalum” solution is desired use an NOSC107M004R0150, 100 µF capacitor from AVX for CO. 11 www.national.com LM2852 Applications Information LM2852 www.national.com 12 inches (millimeters) 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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