LM2853MHX-1.2

LM2853 3A 550 kHz Synchronous SIMPLE SWITCHER ® Buck Regulator October 2006 LM2853 3A 550 kHz Synchronous SIMPLE SWITCHER ® Buck Regulator General Description The LM2853 synchronous SIMPLE buck regulator is a 550 kHz step-down switching voltage regulator capable of driving up to a 3A load with excellent line and load regulation. The LM2853 accepts an input voltage between 3.0V and 5.5V and delivers a customizable output voltage that is factory programmable from 0.8V to 3.3V in 100mV increments. Internal type-three compensation enables a low component count solution and greatly simplifies external component selection. The exposed-pad TSSOP-14 package enhances the thermal performance of the LM2853. SWITCHER ® Features n Input voltage range of 3.0V to 5.5V n Factory EEPROM set output voltages from 0.8V to 3.3V in 100 mV increments n Maximum load current of 3A n Voltage Mode Control n Internal type-three compensation n Switching frequency of 550 kHz n Low standby current of 12 µA n Internal 40 mΩ MOSFET switches n Standard voltage options 0.8/1.0/1.2/1.5/1.8/2.5/3.0/3.3 volts n Exposed pad TSSOP-14 package Applications n Low voltage point of load regulation n Local solution for FPGA/DSP/ASIC core power n Broadband networking and communications infrastructure Typical Application Circuit 20201502 Efficiency vs Load Current (VOUT = 3.3V) 20201501 SIMPLE SWITCHER ® is a Registered Trademark of National Semiconductor Corporation. © 2006 National Semiconductor Corporation DS202015 www.national.com LM2853 Connection Diagram 20201503 Ordering Information Order Number LM2853MH-0.8 LM2853MHX-0.8 LM2853MH-1.0 LM2853MHX-1.0 LM2853MH-1.2 LM2853MHX-1.2 LM2853MH-1.5 LM2853MHX-1.5 LM2853MH-1.8 LM2853MHX-1.8 LM2853MH-2.5 LM2853MHX-2.5 LM2853MH-3.0 LM2853MHX-3.0 LM2853MH-3.3 LM2853MHX-3.3 Voltage Option 0.8 1.0 1.2 1.5 1.8 2.5 3.0 3.3 Package Marking LM2853-0.8 LM2853-1.0 LM2853-1.2 LM2853-1.5 LM2853-1.8 LM2853-2.5 LM2853-3.0 LM2853-3.3 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 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 Note: Contact factory for other voltage options. Pin Descriptions Pin # 1 2 3 4 5 6,7 8,9 10,11 12,13 14 Exposed Pad Name AVIN EN SGND SS NC PVIN SW PGND NC SNS EP Enable. Low noise ground. Soft-Start Pin. No Connect. This pin must be tied to ground. Input Voltage for Power Circuitry. Switch Pin. Power Ground. No-Connect. These pins must be tied to ground. Output Voltage Sense Pin. The exposed pad is internally connected to GND, but it cannot be used as the primary GND connection. The exposed pad should be soldered to an external GND plane. Function Input Voltage for Control Circuitry. www.national.com 2 LM2853 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. AVIN, PVIN, EN, SNS, SW, SS ESD Susceptibility (Note 2) Power Dissipation Storage Temperature Range Maximum Junction Temp. −0.3V to 6.0V 2kV Internally Limited −65˚C to +150˚C 150˚C 14-Pin Exposed Pad TSSOP Package Infrared (15 sec) Vapor Phase (60 sec) Soldering (10 sec) 220˚C 215˚C 260˚C Operating Ratings (Note 1) PVIN to GND AVIN to GND Junction Temperature 1.5V to 5.5V 3.0V to 5.5V −40˚C to +125˚C Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those in bold face type apply over the full Junction Temperature Range (−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. Unless otherwise specified AVIN = PVIN = 5V. Symbol SYSTEM PARAMETERS VOUT Voltage Tolerance (Note 3) 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 (Note 3) VOUT = 0.8V, 1.0V, 1.2V, 1.5V, 1.8V or 2.5V 3.0V ≤ AVIN ≤ 5.5V VOUT = 3.0V or 3.3V 3.5V ≤ AVIN ≤ 5.5V ∆VOUT/∆IO VON RDS(ON)-P RDS(ON)-N RSS ICL IQ ISD RSNS PWM fosc Drange VIH VIL IEN Switching Frequency Duty Cycle Range EN Pin Minimum High Input EN Pin Maximum Low Input EN Pin Pullup Current EN = 0V 1.5 . 325 0 75 25 550 725 100 kHz % % of AVIN % of AVIN µA Load Regulation UVLO Threshold (AVIN) PFET On Resistance NFET On Resistance Soft-Start Resistance Peak Current Limit Threshold Operating Current Shutdown Quiescent Current Sense Pin Resistance Non-switching EN = 0V 3.6 Normal operation Rising Falling Hysteresis Isw = 3A Isw = 3A 50 0.782 0.9775 1.1730 1.4663 1.7595 2.4437 2.9325 3.2257 0.8 1.0 1.2 1.5 1.8 2.5 3.0 3.3 0.2 0.818 1.0225 1.227 1.5337 1.8405 2.5563 3.0675 3.3743 1.1 % V Parameter Conditions Min Typ Max Units 0.2 2 2.47 155 40 32 450 5 0.85 12 432 1.1 % mV/A 3.0 260 120 100 V mV mΩ mΩ kΩ A 2 50 mA µA kΩ ENABLE CONTROL (Note 4) 3 www.national.com LM2853 Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those in bold face type apply over the full Junction Temperature Range (−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. Unless otherwise specified AVIN = PVIN = 5V. (Continued) Symbol THERMAL CONTROLS TSD TSD-HYS Thermal Shutdown Threshold Hysteresis for Thermal Shutdown Junction to Ambient MXA14A 165 10 ˚C ˚C Parameter Conditions Min Typ Max Units THERMAL RESISTANCE θJA 38 ˚C/W 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 specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test Method is per JESD22-AI14. 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 LM2853 is automatically enabled unless an external enable voltage is applied. www.national.com 4 LM2853 Typical Performance Characteristics = AVIN = PVIN = 5V, TJ = 25˚C. Efficiency vs. ILOAD VOUT = 1.8V Unless otherwise specified, the following conditions apply: VIN NFET RDS(ON) vs. Temperature 20201507 20201505 Efficiency vs. ILOAD VOUT = 2.5V PFET RDS(ON) vs. Temperature 20201509 20201504 Efficiency vs. ILOAD VOUT = 3.3V Switching Frequency vs. Temperature 20201508 20201506 5 www.national.com LM2853 Typical Performance Characteristics Unless otherwise specified, the following conditions apply: VIN = AVIN = PVIN = 5V, TJ = 25˚C. (Continued) IQ vs. VIN and Temperature ISD vs. VIN and Temperature 20201510 20201511 www.national.com 6 LM2853 Block Diagram 20201512 Applications Information The LM2853 is a DC-DC buck regulator belonging to National Semiconductor’s synchronous 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: load regulation and transient performance, the use of a small 1 µF ceramic capacitor is also recommended as a local bypass for the AVIN pin. SOFT-START CAPACITOR (CSS) The DAC that sets the reference voltage of the error amplifier 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 200 kΩ 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 20 pF is 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. 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 5A current limit threshold would require a time of 60 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 should be added such as a 1 µF ceramic for higher frequency filtering. Ceramic capacitors with high quality dielectrics such as X5R or X7R should be used to provide a constant capacitance across temperature and line variations. For improved 7 www.national.com LM2853 Applications Information (Continued) voltage the output reached during the short circuit event. The range of soft-start capacitors is therefore restricted to values 1 nF to 50 nF. COMPENSATION The LM2853 provides a highly integrated solution to power supply design. The compensation of the LM2853, which is type-three, is included on-chip. The benefit of 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 LO, CO and CESR values is restricted in order to ensure stability. OUTPUT FILTER VALUES Table 1 details the recommended inductor and capacitor ranges for the LM2853 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. For best performance when output voltage ripple is a concern, ESR values near the minimum of the recommended range should be paired with capacitance values near the maximum. If a minimum output voltage ripple solution from a 5V input voltage is desired, a 6.8 µH inductor can be paired with a 220 µF (50 mΩ) capacitor without degraded phase margin. Since it is undesirable for the power supply to start up in current limit, a soft-start capacitor must be chosen to force the LM2853 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 = 1 ms. The DAC resistor, R, is 450 kΩ so C can be calculated to be 2.2 nF. A 2.2 nF ceramic capacitor can be chosen to yield approximately a 3 ms start-up time. SOFT-START CAPACITOR (CSS) AND FAULT CONDITIONS Various fault conditions such as short circuit and UVLO of the LM2853 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 LM2853 can restart gracefully from whatever TABLE 1. Recommended LO and CO Values LO (µH) VOUT (V) 0.8 0.8 1 1 1.2 1.2 1.5 1.5 1.8 1.8 2.5 2.5 3.0 3.0 3.3 VIN (V) 5 3.3 5 3.3 5 3.3 5 3.3 5 3.3 5 3.3 5 3.3 5 Min 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 Max 6.8 4.7 6.8 4.7 6.8 4.7 6.8 4.7 6.8 4.7 6.8 4.7 6.8 4.7 6.8 Min 120 150 120 150 120 120 120 120 120 100 120 100 120 100 120 CO (µF) Max 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 Min 70 50 70 50 70 60 70 60 70 70 70 80 70 80 70 CESR (mΩ) Max 100 100 100 100 100 100 100 100 120 120 150 150 150 150 150 www.national.com 8 LM2853 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 3A load would therefore be 3A plus 177 mA, 3.177A. As shown in the ripple equation, the current ripple is inversely proportional to inductance. 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 are suitable in LM2853 applications. 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 LO 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 LM2853. As an example, consider a 5V to 1.2V conversion and a 550 kHz switching frequency. According to Table 1, a 4.7 µH inductor may be used. Calculating the expected peak-to-peak ripple, TABLE 2. Recommended Inductors Inductance 4.7 µF 4.7 µF 4.7 µF 5.2 µF 5.6 µF 6.8 µF 6.8 µF Part Number DO3308P-472ML DO3316P-472ML MSS1260-472ML MSS1038-522NL MSS1260-562ML DO3316P-682ML MSS1260-682ML Vendor Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Coilcraft Below are some examples of capacitors that can typically be used in an LM2853 application. OUTPUT FILTER CAPACITORS The recommended capacitors that may be used in the output filter with the LM2853 are limited in value and ESR range according to Table 1. TABLE 3. Recommended Capacitors Capacitance (µF) 100 100 100 100 100 120 150 150 150 150 150 150 220 220 220 220 Part Number 594D107X_010C2T 593D107X_010D2_E3 TPSC107M006#0075 NOSD107M006#0080 NOSC107M004#0070 594D127X_6R3C2T 594D157X_010C2T 595D157X_010D2T 591D157X_6R3C2_20H TPSD157M006#0050 TPSC157M004#0070 NOSD157M006#0070 594D227X_6R3D2T 591D227X_6R3D2_20H 591D227X_010D2_20H 593D227X_6R3D2_E3 Chemistry Tantalum Tantalum Tantalum Niobium Oxide Niobium Oxide Tantalum Tantalum Tantalum Tantalum Tantalum Tantalum Niobium Oxide Tantalum Tantalum Tantalum Tantalum Vendor Vishay-Sprague Vishay-Sprague AVX AVX AVX Vishay-Sprague Vishay-Sprague Vishay-Sprague Vishay-Sprague AVX AVX AVX Vishay-Sprague Vishay-Sprague Vishay-Sprague Vishay-Sprague 9 www.national.com LM2853 Applications Information Capacitance (µF) 220 220 (Continued) TABLE 3. Recommended Capacitors (Continued) Part Number TPSD227M006#0050 NOSD227M0040060 Chemistry Tantalum Niobium Oxide Vendor AVX AVX SPLIT-RAIL OPERATION The LM2853 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. 20201513 SWITCH NODE PROTECTION The LM2853 includes protection circuitry that monitors the voltage on the switch pin. Under certain fault conditions, switching is disabled in order to protect the switching devices. One side effect of the protection circuitry may be observed when power to the LM2853 is applied with no or light load on the output. The output will regulate to the rated voltage, but no switching may be observed. As soon as the output is loaded, the LM2853 will begin normal switching operation. LAYOUT GUIDELINES These are several guidelines to follow while designing the PCB layout for an LM2853 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 be- tween 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 dV/dt or dl/dt (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 performance. Lab measurements also show improved regulation performance when the exposed pad is well grounded. LM2853 Example Circuit Schematic 20201514 FIGURE 1. www.national.com 10 LM2853 LM2853 Example Circuit Schematic (Continued) Bill of Materials for 5V to 3.3V Conversion ID U1 CIN CBYP CSS LO CO Part Number LM2853MH-3.3 GRM31CR60J476ME19 GRM21BR71C105KA01 VJ0805Y222KXXA DO3316P-682 594D127X06R3C2T Type 3A Buck Capacitor Capacitor Capacitor Inductor Capacitor Size ETSSOP-14 1206 0805 0603 DO3316P C Case Parameters 3.3V 47 µF 1 µF 2.2 nF 6.8 µH 120µF (85mΩ) Qty 1 1 1 1 1 1 Vendor NSC Murata Murata Vishay-Vitramon Coilcraft Vishay-Sprague Bill of Materials for 3.3V to 1.2V Conversion ID U1 CIN CBYP CSS LO CO Part Number LM2853MH-1.2 GRM31CR60J476ME19 GRM21BR71C105KA01 VJ0805Y222KXXA DO3316P-472 NOSD157M006R0070 Type 3A Buck Capacitor Capacitor Capacitor Inductor Capacitor Size ETSSOP-14 1206 0805 0603 DO3316P D Case Parameters 1.2V 47 µF 1 µF 2.2 nF 4.7 µH 150 µF (70 mΩ) Qty 1 1 1 1 1 1 Vendor NSC Murata Murata Vishay-Vitramon Coilcraft AVX 11 www.national.com LM2853 3A 550 kHz 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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