STEP-DOWN SWITCHING REGULATOR

www.DataSheet4U.com February 2006 LM25005 42V, 2.5A Step-Down Switching Regulator LM25005 42V, 2.5A Step-Down Switching Regulator General Description The LM25005 switching regulator features all of the functions necessary to implement an efficient, high voltage buck regulator using a minimum of external components. This easy to use regulator includes a 42V, 160mΩ, N-channel MOSFET, with an output current capability of 2.5 Amps. The regulator control method is based upon current mode control utilizing an emulated current ramp. Current mode control provides inherent line voltage feed-forward, cycle-by-cycle current limiting and ease of loop compensation. The use of an emulated control ramp reduces noise sensitivity of the pulse-width modulation circuit, allowing reliable control of very small duty cycles necessary in high input voltage applications. The operating frequency is programmable from 50kHz to 500kHz. An oscillator synchronization pin allows multiple LM25005 regulators to self-synchronize or to be synchronized to an external clock. Additional protection features include: current limit, thermal shutdown and remote shutdown capability. The device is available in a power enhanced TSSOP-20 package featuring an exposed die attach pad to aid thermal dissipation. Features n n n n n n n n n n n n Integrated 42V, 160mΩ N-channel MOSFET Ultra-wide input voltage range from 7V to 42V Internal bias regulator Adjustable output voltage from 1.225V 1.5% feedback reference accuracy Current mode control with emulated inductor current ramp Single resistor oscillator frequency setting Oscillator synchronization input Programmable soft-start Shutdown / Standby input Wide bandwidth error amplifier Thermal Shutdown Package n TSSOP-20EP (Exposed Pad) Simplified Application Schematic 20170001 © 2006 National Semiconductor Corporation DS201700 www.national.com LM25005 Connection Diagram 20170002 Top View 20-Lead TSSOP Ordering Information Order Number LM25005MH LM25005MHX Package Type Exposed Pad TSSOP-20 Exposed Pad TSSOP-20 NSC Package Drawing MXA20A MXA20A Supplied As 73 Units in Rail 2500 Units on Tape and Reel www.national.com 2 LM25005 Pin Descriptions Pin(S) 1 Name VCC Description Output of the bias regulator Application Information Vcc tracks Vin up to 9V. Beyond 9V, Vcc is regulated to 7 Volts. A 0.1uF to 1uF ceramic decoupling capacitor is required. An external voltage (7.5V – 14V) can be applied to this pin to reduce internal power dissipation. If the SD pin voltage is below 0.7V the regulator will be in a low power state. If the SD pin voltage is between 0.7V and 1.225V the regulator will be in standby mode. If the SD pin voltage is above 1.225V the regulator will be operational. An external voltage divider can be used to set a line undervoltage shutdown threshold. If the SD pin is left open circuit, a 5µA pull-up current source configures the regulator fully operational. Nominal operating range: 7V to 42V The internal oscillator can be synchronized to an external clock with an external pull-down device. Multiple LM25005 devices can be synchronized together by connection of their SYNC pins. The loop compensation network should be connected between this pin and the FB pin. This pin is connected to the inverting input of the internal error amplifier. The regulation threshold is 1.225V. The internal oscillator is set with a single resistor, connected between this pin and the AGND pin. The recommended frequency range is 50KHz to 500KHz. An external capacitor connected between this pin and the AGND pin sets the ramp slope used for current mode control. Recommended capacitor range 50pF to 2000pF. Internal reference for the regulator control functions An external capacitor and an internal 10µA current source set the time constant for the rise of the error amp reference. The SS pin is held low during standby, Vcc UVLO and thermal shutdown. Connect directly to the regulated output voltage. Low side reference for the PRE switch and the IS sense resistor. Current measurement connection for the re-circulating diode. An internal sense resistor and a sample/hold circuit sense the diode current near the conclusion of the off-time. This current measurement provides the DC level of the emulated current ramp. The source terminal of the internal buck switch. The SW pin should be connected to the external Schottky diode and to the buck inductor. This open drain output can be connected to SW pin to aid charging the bootstrap capacitor during very light load conditions or in applications where the output may be pre-charged before the LM25005 is enabled. An internal pre-charge MOSFET is turned on for 250ns each cycle just prior to the on-time interval of the buck switch. 2 SD Shutdown or UVLO input 3, 4 5 Vin SYNC Input supply voltage Oscillator synchronization input or output 6 7 8 COMP FB RT Output of the internal error amplifier Feedback signal from the regulated output Internal oscillator frequency set input 9 RAMP Ramp control signal 10 11 AGND SS Analog ground Soft-start 12 13, 14 15, 16 OUT PGND IS Output voltage connection Power ground Current sense 17, 18 SW Switching node 19 PRE Pre-charge assist for the bootstrap capacitor 3 www.national.com LM25005 Pin Descriptions Pin(S) 20 Name BST (Continued) Description Application Information An external capacitor is required between the BST and the SW pins. A 0.022µF ceramic capacitor is recommended. The capacitor is charged from Vcc via an internal diode during the off-time of the buck switch. Exposed metal pad on the underside of the device. It is recommended to connect this pad to the PWB ground plane, in order to aid in heat dissipation. Boost input for bootstrap capacitor NA EP Exposed Pad www.national.com 4 LM25005 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 GND BST to GND PRE to GND SW to GND (Steady State) BST to VCC VCC to GND BST to SW 45V 60V 45V -1.5V 45V 14V 14V OUT to GND SD, SYNC, SS, FB to GND ESD Rating (Note 2) Human Body Model Storage Temperature Range Limited to Vin 7V 2kV -65˚C to +150˚C Operating Ratings (Note 1) VIN Operation Junction Temperature 7V to 42V −40˚C to + 125˚C Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 24V, RT = 32.4kΩ unless otherwise stated. (Note 3) Symbol STARTUP REGULATOR VccReg Vcc Regulator Output Vcc LDO Mode turn-off Vcc Current Limit VCC SUPPLY Vcc UVLO Threshold Vcc Undervoltage Hysteresis Bias Current (Iin) Shutdown Current (Iin) SHUTDOWN THRESHOLDS Shutdown Threshold Shutdown Hysteresis Standby Threshold Standby Hysteresis SD Pull-up Current Source SWITCH CHARACTERSICS Buck Switch Rds(on) BOOST UVLO BOOST UVLO Hysteresis Pre-charge Switch Rds(on) Pre-charge Switch on-time CURRENT LIMIT Cycle by Cycle Current Limit Cycle by Cycle Current Limit Delay SOFT-START SS Current Source OSCILLATOR Frequency1 Frequency2 SYNC Source Impedance SYNC Sink Impedance SYNC Threshold (falling) Upper SYNC Frequency SYNC Pulse Width Minimum 15 RT = 11kΩ 180 425 200 485 10 160 1.4 550 220 525 KHz KHz kΩ Ω V KHz ns 7 10 14 µA RAMP = 0V RAMP = 2.5V 3 3.5 100 4.25 A ns 160 3.8 0.56 75 275 320 mΩ V V Ω ns (Standby Increasing) 1.18 (SD Increasing) 0.5 0.7 0.1 1.225 0.1 5 1.27 0.9 V V V V µA FB = 1.3V SD = 0V (Vcc increasing) 5.95 6.35 1 3 50 4.5 85 6.75 V V mA µA Vcc = 0V 6.85 7.15 9 20 7.45 V V mA Parameter Conditions Min Typ Max Units 5 www.national.com LM25005 Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 24V, RT = 32.4kΩ unless otherwise stated. (Note 3) (Continued) Symbol RAMP GENERATOR Ramp Current 1 Ramp Current 2 PWM COMPARATOR Forced Off-time Min On-time COMP to PWM Comparator Offset ERROR AMPLIFIER Feedback Voltage FB Bias Current DC Gain COMP Sink / Source Current Unity Gain Bandwidth THERMAL SHUTDOWN Tsd Thermal Shutdown Threshold Thermal Shutdown Hysteresis THERMAL RESISTANCE θJC θJA Junction to Case Junction to Ambient 4 40 ˚C/W ˚C/W 165 25 ˚C ˚C 3 3 Vfb = COMP 1.207 1.225 10 70 1.243 V nA dB mA MHz 500 80 0.7 ns ns V Vin = 36V, Vout=10V Vin = 10V, Vout=10V 136 18 160 25 184 32 µA µA Parameter Conditions Min Typ Max Units 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: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 3: Min and Max limits are 100% production tested at 25˚C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL). www.national.com 6 LM25005 Typical Performance Characteristics Oscillator Frequency vs RT Oscillator Frequency vs Temperature FOSC = 200kHz 20170020 20170021 Soft Start Current vs Temperature VCC vs ICC VIN = 12V 20170022 20170023 VCC vs VIN RL = 7kΩ Error Amplifier Gain/Phase AVCL = 101 20170024 20170025 7 www.national.com LM25005 Typical Performance Characteristics Demoboard Efficiency vs IOUT and VIN (Continued) 20170026 www.national.com 8 Typical Application Circuit and Block Diagram LM25005 9 20170003 FIGURE 1. www.national.com LM25005 Detailed Operating Description The LM25005 switching regulator features all of the functions necessary to implement an efficient high voltage buck regulator using a minimum of external components. This easy to use regu