Step-Down Switching

LM5576 SIMPLE SWITCHER® 75V, 3A Step-Down Switching Regulator March 2007 LM5576 SIMPLE SWITCHER® 75V, 3A Step-Down Switching Regulator General Description The LM5576 is an easy to use SIMPLE SWITCHER® buck regulator which allows design engineers to design and optimize a robust power supply using a minimum set of components. Operating with an input voltage range of 6 - 75V, the LM5576 delivers 3A of continuous output current with an integrated 170mΩ N-Channel MOSFET. The regulator utilizes an Emulated Current Mode architecture which provides inherent line regulation, tight load transient response, and ease of loop compensation without the usual limitation of low-duty cycles associated with current mode regulators. The operating frequency is adjustable from 50kHz to 500kHz to allow optimization of size and efficiency. To reduce EMI, a frequency synchronization pin allows multiple IC’s from the LM(2)557x family to self-synchronize or to synchronize to an external clock. The LM5576 guarantees robustness with cycle-by-cycle current limit, short-circuit protection, thermal shut-down, and remote shut-down. The device is available in a power enhanced TSSOP-20 package featuring an exposed die attach pad for thermal dissipation. The LM5576 is supported by the full suite of WEBENCH® On-Line design tools. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Integrated 75V, 170mΩ N-channel MOSFET Ultra-wide input voltage range from 6V to 75V Adjustable output voltage as low as 1.225V 1.5% feedback reference accuracy Operating frequency adjustable between 50kHz and 500kHz with single resistor Master or slave frequency synchronization Adjustable soft-start Emulated current mode control architecture Wide bandwidth error amplifier Built-in protection Package ■ TSSOP-20EP (Exposed Pad) Simplified Application Schematic 20189901 WEBENCH® is a registered trademark of National Semiconductor Corporation. © 2007 National Semiconductor Corporation 201899 www.national.com LM5576 Connection Diagram 20189902 Top View 20-Lead TSSOP Ordering Information Order Number LM5576MH LM5576MHX 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 LM5576 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: 6V to 75V 2 SD Shutdown or UVLO input 3, 4 5 Vin SYNC Input supply voltage Oscillator synchronization input or output The internal oscillator can be synchronized to an external clock with an external pull-down device. Multiple LM5576 devices can be synchronized together by connection of their SYNC pins. Output of the internal error amplifier Feedback signal from the regulated output Internal oscillator frequency set input Ramp control signal 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. 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 precharged before the LM5576 is enabled. An internal precharge MOSFET is turned on for 265ns each cycle just prior to the on-time interval of the buck switch. 6 7 8 9 COMP FB RT RAMP 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 LM5576 Pin(s) 20 Name BST Description Boost input for bootstrap capacitor 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. NA EP Exposed Pad www.national.com 4 LM5576 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 SD, VCC to GND 76V 90V 76V -1.5V 76V 14V BST to SW OUT to GND SYNC, SS, FB, RAMP to GND ESD Rating (Note 2) Human Body Model Storage Temperature Range 14V Limited to Vin 7V 2kV -65°C to +150°C (Note 1) 6V to 75V −40°C to + 125°C Operating Ratings VIN Operation Junction Temperature Electrical Characteristics Symbol STARTUP REGULATOR VccReg Vcc Regulator Output Vcc LDO Mode turn-off Vcc Current Limit VCC SUPPLY Vcc UVLO Threshold Specifications with standard typeface are for TJ = 25°C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 48V, RT = 32.4kΩ unless otherwise stated. (Note 3) Parameter Conditions Min 6.85 Vcc = 0V (Vcc increasing) FB = 1.3V SD = 0V (SD Increasing) (Standby Increasing) 0.47 1.17 5.03 Typ 7.15 9 25 5.35 0.25 3.4 57 0.7 0.1 1.225 0.1 5 170 3.8 0.56 70 265 RAMP = 0V RAMP = 2.5V 7 180 RT = 11kΩ 425 3.6 4.2 100 10 200 485 11 110 1.3 RT = 11kΩ 550 15 14 220 545 5.1 340 1.28 4.5 85 0.9 5.67 Max 7.45 Units V V mA V V mA µA V V V V µA mΩ V V Ω ns A ns µA kHz kHz kΩ Ω V kHz ns 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) SYNC Frequency SYNC Pulse Width Minimum 5 www.national.com LM5576 Symbol RAMP GENERATOR Parameter Ramp Current 1 Ramp Current 2 Conditions Vin = 60V, Vout=10V Vin = 10V, Vout=10V Min 235 18 416 Typ 275 25 500 80 0.7 Max 315 32 575 Units µA µA ns ns V 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 DIODE SENSE RESISTANCE DSENSE THERMAL SHUTDOWN Tsd Thermal Shutdown Threshold Thermal Shutdown Hysteresis THERMAL RESISTANCE θJC θJA Junction to Case Junction to Ambient 6 40 °C/W °C/W 165 25 °C °C 42 mΩ 3 3 Vfb = COMP 1.207 1.225 17 70 1.243 V nA dB mA MHz 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). Typical Performance Characteristics Oscillator Frequency vs RT Oscillator Frequency vs Temperature FOSC = 200kHz 20189920 20189921 www.national.com 6 LM5576 Soft Start Current vs Temperature VCC vs ICC VIN = 12V 20189922 20189923 VCC vs VIN RL = 7kΩ Error Amplifier Gain/Phase AVCL = 101 20189924 20189925 Demoboard Efficiency vs IOUT and VIN 20189926 7 www.national.com LM5576 Typical Application Circuit and Block Diagram 20189903 www.national.com 8 FIGURE 1. LM5576 Detailed Operating Description The LM5576 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 integrates a 75V N-Channel buck switch with an output current capability of 3 Amps. The regulator control method is based on current mode control utilizing an emulated current ramp. Peak current mode control provides inherent