LM25007SDX

LM25007 42V, 0.5A Step-Down Switching Regulator February 5, 2008 LM25007 42V, 0.5A Step-Down Switching Regulator General Description The LM25007 is a monolithic step-down switching regulator featuring all of the functions needed to implement a low cost, efficient, power supply. Capable of driving a 0.5Amp load over a 9V-42V input voltage range, this device is easy to apply and is provided in the small MSOP-8 and the thermally enhanced LLP-8 packages. Ultra-fast transient response is achieved through the use of a constant on-time architecture with Vin feed forward. This feature provides an almost constant operating frequency over load and input voltage variations. The operating frequency can be adjusted with a single resistor. This architecture is easy to use and tolerant of component selection. An intelligent current limit is implemented in the LM25007 with forced off time that is inversely proportional to Vout thus ensuring recovery from fault conditions. The output voltage may be set from 2.5V to >30V. To improve efficiency in light load applications, the Vcc pin can be connected to an external voltage source to eliminate the drop through the internal regulator. Additional features include: thermal shutdown, Vcc under-voltage lockout, gate drive under-voltage lockout, and max duty cycle limiter. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Integrated 0.74 ohm N-Channel MOSFET switch Guaranteed 0.5Amp output current Ultra-Fast Transient Response Up to 800kHz operation No control loop compensation required Vin feed forward provides constant operating frequency 2% accurate 2.5V feedback from -40°C to 125°C Highly efficient operation Intelligent current limit protection External shutdown control Thermal shutdown MSOP-8 and thermally enhanced LLP packages Typical Applications ■ ■ ■ ■ ■ 12VDC and 24VDC distributed rail systems 24VAC systems Automotive body electronics and telematics Industrial systems HB-LED constant current source Package ■ MSOP - 8 ■ LLP - 8 (4mm x 4mm) Basic Step-Down Regulator 20165513 © 2008 National Semiconductor Corporation 201655 www.national.com LM25007 Connection Diagram 20165502 8-Lead MSOP, LLP Ordering Information Order Number LM25007MM LM25007MMX LM25007SD LM25007SDX Package Type MSOP-8 MSOP-8 LLP-8 LLP-8 NSC Package Drawing MUA08A MUA08A SDC08A SDC08A Supplied As 1000 Units on Tape and Reel 3500 Units on Tape and Reel 1000 Units on Tape and Reel 4500 Units on Tape and Reel Pin Descriptions Pin 1 2 Name SW BST Switching Node Boost Boot–strap capacitor input Description Application Information Power switching node. Connect to the inductor, bootstrap capacitor, and free-wheeling diode. An external capacitor is required between the BST and the SW pins. A 0.01µF ceramic capacitor is recommended. A resistor between this pin and RTN sets the off-time when current limit is detected. The off-time is preset to 17 µs if FB = 0V . This pin is connected to the inverting input of the internal regulation comparator. The regulation threshold is 2.5V. A resistor between this pin and VIN sets the switch ontime as a function of Vin. The minimum recommended on-time is 300ns at the maximum input voltage. 3 RCL Current Limit OFF time programming pin 4 5 RTN FB Circuit Ground Feedback Signal from Regulated Output 6 RON/SD On-time set pin 7 VCC Output from the internal high voltage bias regulator. If an auxiliary voltage is available to raise the voltage on this pin above the regulation setpoint (7V), the internal series pass regulator will shutdown, reducing the IC power dissipation. Do not exceed 14V. This output provides gate drive power for the internal Buck switch. An internal diode is provided between this pin and the BST pin. A local 0.1uF decoupling capacitor is recommended. Input supply voltage Recommended operating range: 9V to 42V. Exposed pad, underside of the LLP package option Exposed metal pad on the LLP package underside. It is recommended to connect this pad to the PC board ground plane to aid in heat dissipation. 8 - VIN EP www.national.com 2 LM25007 Typical Application Circuit and Block Diagram 20165501 3 www.national.com LM25007 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to RTN BST to RTN SW to RTN (Steady State) ESD Rating (Note 5) Human Body Model 45V 59V -1V 2kV BST to VCC BST to SW VCC to RTN All Other Inputs to RTN Storage Temperature Range 45V 14V 14V -0.3 to 7V -65°C to +150°C (Note 1) 9V to 42V −40°C to + 125°C Operating Ratings VIN Junction Temperature Electrical Characteristics Specifications with standard type are for TJ = 25°C only; limits in boldface type apply over the full Operating Junction Temperature (TJ) range. 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 stated the following conditions apply: VIN = 24V, RON = 200kΩ. (Note 3). Symbol Startup Regulator VCC Reg VCC Supply VCC undervoltage Lockout Voltage (VCC increasing) VCC Undervoltage Hysteresis VCC UVLO Delay (filter) Operating Current (ICC) Shutdown/Standby Current Switch Characteristics Buck Switch Rds(on) ITEST = 200mA, VBST −VSW = 6.3V (Note 6) 3.4 0.74 1.34 Ω Non-Switching, FB = 3V RON/SD = 0V 6.3 206 3 500 70 675 150 V mV µs µA µA VCC Regulator Output VCC Current Limit (Note 4) 6.6 7 11 7.4 V mA Parameter Conditions Min Typ Max Units Gate Drive UVLO (VBST – VSW) Rising Gate Drive UVLO Hysteresis Current Limit Current Limit Threshold Current Limit Response Time OFF time generator (test 1) OFF time generator (test 2) On Time Generator TON -1 TON -2 Remote Shutdown Threshold Remote Shutdown Hysteresis Vin = 10V Ron = 200K Vin = 40V Ron = 200K Rising Iswitch Overdrive = 0.1A Time to Switch Off FB=0V, RCL = 100K FB=2.3V, RCL = 100K 4.5 400 5.5 V mV 535 725 225 17 2.65 900 mA ns µs µs 2.15 450 0.45 2.77 615 0.7 40 3.5 810 1.1 µs ns V mV www.national.com 4 LM25007 Symbol Minimum Off Time Parameter Minimum Off Timer Conditions FB = 0V Internal reference Trip point for switch ON Trip point for switch OFF Min Typ 300 Max Units ns Regulation and OV Comparators FB Reference Threshold FB Over-Voltage Threshold FB Bias Current Thermal Shutdown Tsd Thermal Shutdown Temp. Thermal Shutdown Hysteresis Thermal Resistance θJA Junction to Ambient MUA Package SDC Package 200 40 °C/W °C/W 165 25 °C °C 2.445 2.5 2.875 100 2.550 V V nA Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: For detailed information on soldering plastic MSOP and LLP packages, refer to the Packaging Data Book available from National Semiconductor Corporation. Note 3: All limits are guaranteed. All electrical characteristics having room temperature limits are tested during production with TA = TJ = 25°C. All hot and cold limits are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control. Note 4: The VCC output is intended as a self bias for the internal gate drive power and control circuits. Device thermal limitations limit external loading. Note 5: The human body model is a 100pF capacitor discharge through a 1.5kΩ resistor into each pin. The human body ESD compliance level for Pin 7 and 8 is 1000V. Note 6: For devices procurred in the LLP-8 package the Rds(on) limits are guaranteed by design characterization data only. 5 www.national.com LM25007 Typical Performance Characteristics Operational Waveforms Operational Waveforms 20165508 LM25007 Operation: VOUT = 10V, VIN = 20V, IOUT = 250mA CH1: Switch Node, CH2: VOUT (AC), CH4: Inductor Current 20165509 LM25007 Operation: VOUT = 10V, VIN = 42V, IOUT = 250mA CH1: Switch Node, CH2: VOUT (AC), CH4: Inductor Current LM25007 10V Output Efficiency Current Limit Off-Time vs VFB 20165510 20165511 VIN vs TON RON = 100k, 200k, 300k 20165512 www.national.com 6 LM25007 Detailed Operating Description The LM25007 is a monolithic step-down switching regulator featuring all of the functions needed to implement low cost, efficient, power supply. Capable of driving a 0.5Amp load over a 9-42V input voltage range, this device is easy to apply and is provided in the small MSOP-8 and the thermally enhanced LLP-8 packages. Ultra-fast transient response is achieved through the use of a constant on-time architecture with Vin feed forward. This feature provides and almost constant operating frequency over load and input voltage variations. The operating frequency may be adjusted up to 800kHz depending on input and output voltages. This architecture is easy to use and tolerant of component selection. An intelligent current limit is implemented in the LM25007 with forced off time that is inversely proportional to Vout to ensure recovery from fault conditions. The output voltage may be set from 2.5V to >30V. To improve efficiency in light load applications, the Vcc pin can be tied to an external voltage source thus eliminating the drop through the integrated internal regulator. Additional protection features include: thermal shutdown, Vcc under-voltage lockout, gate drive under-voltage lockout, and max duty cycle limiter. In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero. In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively constant with load and line variations. The approximate continuous mode operating frequency can be calculated as follows: The output voltage (Vout) can be programmed by two external resistors as shown in Figure 1. The regulated voltage is calculated as follows: VOUT = 2.5 x (R3 + R4) / R4 The feedback comparator in hysteretic regulators depends on the output ripple voltage to switch the output transistor on and off at regular intervals. In order for the internal comparator to respond quickly to changes in output voltage, proportional to inductor current, a minimum amount of capacitor Equivalent Series Resistance (ESR) is required. A ripple voltage of 25mV is recommended at the feedback pin (FB) for stable operation. In cases where the intrinsic capacitor ESR is too small, additional series resistance may be added. For applications where lower output voltage ripple is required the load can be connected directly to the low ESR output capacitor, as shown in Figure 1. The series resistor (R5) will degrade the load regulation. Another technique for enhancing the ripple voltage at the FB pin is to place a capacitor in parallel with the feedback divider resistor R3. The addition of the capacitor reduces the attenuation of the ripple voltage from the feedback divider. Hysteretic Control Circuit Overview The LM25007 is a Buck DC-DC regulator that uses a constant on-time control scheme. The on-time is programmed by an external resistor and varies inversely with line input voltage (Vin). The core regulation elements of the LM25007 are the feedback comparator and the on-time one-shot. The regulator output voltage is sensed at the feedback pin (FB) and is compared to an internal reference voltage (2.5V). If the FB signal is below the reference voltage, the buck switch is turned on for a fixed time pulse determined by the line voltage and a programming resistor (RON). Following the on period the switch will remain off for at least the minimum off timer period of 300ns. If the FB pin voltage is still below the reference after the 300ns off-time, the switch will turn on again for another on time period. This switching behavior will continue until the FB pin voltage reaches the reference voltage level. The LM25007 operates in discontinuous conduction mode at light load currents or continuous conduction mode at heavier load currents. In discontinuous conduction mode, current through the output inductor starts at zero and ramps up to a peak value during the buck switch on-time and then back to zero during the off-time. The inductor current remains at zero until the next on-time period starts when FB falls below the internal reference. In discontinuous mode the operating frequency is relatively low and will vary with load. Therefore at light loads the conversion efficiency is maintained, since the switching losses decrease with the reduction in load current and switching frequency. The approximate discontinuous mode operating frequency can be calculated as follows: High Voltage Bias Regulator (VCC) The LM25007 contains an internal high voltage bias regulator. The input pin (Vin) can be connected directly to line voltages from 9 to 42 Volts. To avoid supply voltage transients due to long lead inductances on the input pin (VIN), it is always recommended to connect a low ESR ceramic chip capacitor (≊ 0.1µF) between VIN and RTN pins located close to the LM25007. The regulator is internally current limited to 11mA. Upon power up, the regulator is enabled and sources current into an external capacitor connected to the VCC pin. When the voltage on the VCC pin reaches its UVLO level (6.3V), the controller output is enabled. An external auxiliary supply voltage can be diode connected to the Vcc pin. If the auxiliary voltage is greater than 7 Volts the internal regulator will shutoff, thus reducing internal power dissipation. Do not exceed 14V at VCC. See Figure 2. 7 www.national.com LM25007 20165505 FIGURE 1. Low Ripple Output Configuration 20165506 FIGURE 2. Self Biased Configuration www.national.com 8 LM25007 Over-Voltage Comparator The over-voltage comparator is provided to protect the output from overvoltage conditions due to sudden input line voltage changes or output loading changes. The over-voltage comparator monitors the FB pin versus an internal 2.875V reference. If the voltage at FB rises above 2.875V the comparator immediately terminates the buck switch on-time pulse. tional to Vin and used internally to control the on-timer. This scheme of input voltage feed-forward hysteretic operation achieves nearly constant operational frequency over varying line and load conditions. The on-time equation for the LM25007 is : Ton = 1.42 x 10-10 x RON / VIN The RON/SD pin of the LM25007 also provides a shutdown function which disables the regulator and significantly decreases quiescent power dissipation. By pulling the pin below 0.7V activates the low power shutdown mode. The VIN quiescent current in the shutdown mode is approximately 100µA internal to the LM25007 plus the current in the RON resistor. See Figure 3 ON Time generator and Shutdown The on-time of the LM25007 is set inversely proportional to the input voltage by an external resistor connected between RON/SD and VIN. The RON/SD terminal is a low impedance input biased at approximately 1.5V. The current through the resistor and into the RON/SD pin is approximately propor- 20165507 FIGURE 3. Shutdown Implementation Current Limit The LM25007 contains an intelligent current limit off-timer intended to reduce the foldback characteristic inherent with fixed off-time over-current protection. If the current in the Buck switch exceeds 725mA the present cycle on-time is immediately terminated (cycle by cycle current limit). Following the termination of the cycle a non-resetable current limit off timer is initiated. The duration of the off time is a function of the external resistor (RCL) and the FB pin voltage. When the FB pin voltage equals zero, the current limit off-time is internally preset to 17µs. This condition occurs in short circuit operation when a maximum amount of off-time is required. In cases of overload (not complete short circuit) the current limit off-time is reduced as a function of the output voltage (measured at the FB pin). Reducing the off-time with smaller overloads reduces the amount of foldback and also reduces the initial start-up time. The current limit off time for a given FB pin voltage and RCL resistor can be calculated by the following equation: Toff = 10-5 / (0.59 + (VFB / 7.22 x 10-6 x RCL)) Applications utilizing low resistance inductors and/or a low voltage drop rectifier may require special evaluation at high line, short circuit conditions. In this special case the preset 17µs (FB = 0V) off-time may be insufficient to balance the inductor volt*time product. Additional inductor resistance, output resistance or a larger voltage drop rectifier may be necessary to balance the inductor cycle volt*time product and limit the short circuit current. N - Channel Buck Switch and Driver The LM25007 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. The bootstrap capacitor is charged by VCC through the internal high voltage diode. A 0.01uF ceramic capacitor connected between the BST pin and SW pin is recommended. During each cycle when the Buck switch turns off, the SW pin is approximately 0V. When the SW pin voltage is low, the bootstrap capacitor is charged from Vcc through the internal diode. The minimum off timer, set to 300ns, ensures that there will be a minimum interval every cycle to recharge the bootstrap capacitor. An external re-circulating diode from the SW pin to ground is necessary to carry the inductor current after the internal Buck switch turns off. This external diode must be of the Ultra-fast or Schottky type to reduce turn-on losses and current over- 9 www.national.com LM25007 shoot. The reverse voltage rating of the re-circulating diode must be greater than the maximum line input voltage. Minimum Load Current A minimum load current of 1 mA is required to maintain proper operation. If the load current falls below that level, the bootstrap capacitor may discharge during the long off-time, and the circuit will either shutdown, or cycle on and off at a low frequency. If the load current is expected to drop below 1 mA in the application, the feedback resistors should be chosen low enough in value so they provide the minimum required current at nominal Vout. Thermal Protection Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction temperature is exceeded. When thermal protection is activated, typically at 165 degrees Celsius, the controller is forced into a low power reset state, disabling the output driver. This feature is provided to prevent catastrophic failures from accidental device overheating. 10V, 400mA Demo Board Bill of Materials (See Typical Application Circuit) ITEM C1 C2 C3 C4 C5 D1 L1 R1 R2 R3 R4 R5 U1 PART NUMBER C4532X7R2A105M C4532X7R1E156M C1206C104K5RAC C1206C103K5RAC C3216X7R2A104KT MURA110T3 SLF7045T-101MR60-1 CRCW12062003F CRCW12061003F CRCW12063011F CRCW12061001F CRCW12061R00F LM25007 DESCRIPTION CAPACITOR, CER, TDK CAPACITOR, CER, TDK CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, TDK DIODE, 100V, ON SEMI BUCK INDUCTOR, TDK RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR REGULATOR, NATIONAL 100µH 200K 100K 3.01K 1K 1 VALUE 1µ, 100V 15µ, 25V 0.1µ, 50V 0.01µ, 50V 0.1µ, 100V www.national.com 10 LM25007 Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead MSOP Package NS Package Number MUA08A 8-Lead LLP Package NS Package Number SDC08A 11 www.national.com LM25007 42V, 0.5A Step-Down Switching Regulator For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock Conditioners Data Converters Displays Ethernet Interface LVDS Power Management Switching Regulators LDOs LED Lighting PowerWise Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/displays www.national.com/ethernet www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH Analog University App Notes Distributors Green Compliance Packaging Design Support www.national.com/webench www.national.com/AU www.national.com/appnotes www.national.com/contacts www.national.com/quality/green www.national.com/packaging www.national.com/quality www.national.com/refdesigns www.national.com/feedback Quality and Reliability Reference Designs Feedback THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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