(LM2745 / LM2748) Synchronous Buck Controller

www.DataSheet4U.com LM2745/8 Synchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization May 2005 LM2745/8 Synchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization General Description The LM2745/8 are high-speed synchronous buck regulator controllers with an accurate feedback voltage accuracy of ±1.5%. It can provide simple down conversion to output voltages as low as 0.6V. Though the control sections of the IC are rated for 3 to 6V, the driver sections are designed to accept input supply rails as high as 14V. The use of adaptive non-overlapping MOSFET gate drivers helps avoid potential shoot-through problems while maintaining high efficiency. The IC is designed for the more cost-effective option of driving only N-channel MOSFETs in both the high-side and low-side positions. It senses the low-side switch voltage drop for providing a simple, adjustable current limit. The LM2745/8 features a fixed-frequency voltage-mode PWM control architecture which is adjustable from 50 kHz to 1 MHz with one external resistor. In addition, the LM2745 also allows the switching frequency to be synchronized to an external clock signal over the range of 250 kHz to 1 MHz. This wide range of switching frequency gives the power supply designer the flexibility to make better tradeoffs between component size, cost and efficiency. Features include the ability to startup with a pre-biased load on the output, soft-start, input undervoltage lockout (UVLO) and Power Good (based on both undervoltage and overvoltage detection). In addition, the shutdown pin of the IC can be used for providing startup delay, and the soft-start pin can be used for implementing precise tracking, for the purpose of sequencing with respect to an external rail. Features n Switching frequency from 50 kHz to 1 MHz n Switching frequency synchronize range 250 kHz to 1 MHz (LM2745 Only) n Startup with a pre-biased output load n Power stage input voltage from 1V to 14V n Control stage input voltage from 3V to 6V n Output voltage adjustable down to 0.6V n Power Good flag and shutdown n Output overvoltage and undervoltage detection n ±1.5% feedback voltage accuracy over temperature n Low-side adjustable current sensing n Adjustable soft-start n Tracking and sequencing with shutdown and soft start pins n TSSOP-14 package Applications n n n n n Down Conversion from 3.3V Cable Modem, DSL and ADSL Laser Jet and Ink Jet Printers Low Voltage Power Modules DSP, ASIC, Core and I/O Typical Application 20137401 © 2005 National Semiconductor Corporation DS201374 www.national.com LM2745/8 Connection Diagrams 20137490 20137402 14-Lead Plastic TSSOP θJA = 155˚C/W NS Package Number MTC14 14-Lead Plastic TSSOP θJA = 155˚C/W NS Package Number MTC14 Ordering Information Order Number LM2745MTC LM2745MTCX LM2748MTC LM2748MTCX TSSOP-14 MTC14 Package Type NSC Package Drawing 94 Units on Rail 2500 Units on Tape and Reel 94 Units on Rail 2500 Units on Tape and Reel EAO (Pin 8) - Output of the error amplifier. The voltage level on this pin is compared with an internally generated ramp signal to determine the duty cycle. This pin is necessary for compensating the control loop. SS/TRACK (Pin 9) - Soft-start and tracking pin. This pin is internally connected to the non-inverting input of the error amplifier during soft-start, and in fact any time the SS/ TRACK pin voltage happens to be below the internal reference voltage. For the basic soft-start function, a capacitor of minimum value 1 nF is connected from this pin to ground. To track the rising ramp of another power supply’s output, connect a resistor divider from the output of that supply to this pin as described in Application Information. FB (Pin 10) - Feedback pin. This is the inverting input of the error amplifier, which is used for sensing the output voltage and compensating the control loop. FREQ/SYNC (Pin 11) - Frequency adjust pin. The switching frequency is set by connecting a resistor of suitable value between this pin and ground. Some typical values (rounded up to the nearest standard values) are 150 kΩ for 200 kHz, 100 kΩ for 300 kHz, 51.1 kΩ for 500 kHz, 18.7 kΩ for 1 MHz. This pin is also used in the LM2745 to synchronize to an external clock. SD (Pin 12) - IC shutdown pin. Pull this pin to VCC to ensure the IC is enabled. Connect to ground to disable the IC. Under shutdown, both high-side and low-side drives are off. This pin also features a precision threshold for power supply sequencing purposes, as well as a low threshold to ensure minimal quiescent current. HG (Pin 14) - High-gate drive pin. This is the gate drive for the high-side N-channel MOSFET. This signal is interlocked with LG (Pin 2) to avoid shoot-through. Supplied As Pin Description BOOT (Pin 1) - Bootstrap pin. This is the supply rail for the high-side gate driver. When the high-side MOSFET turns on, the voltage on this pin should be at least one gate threshold above the regulator input voltage VIN to properly turn on the MOSFET. See MOSFET Gate Drivers in the Application Information section for more details on how to select MOSFETs. LG (Pin 2) - Low-gate drive pin. This is the gate drive for the low-side N-channel MOSFET. This signal is interlocked with the high-side gate drive HG (Pin 14), so as to avoid shootthrough. PGND (Pins 3, 13) - Power ground. This is also the ground for the low-side MOSFET driver. Both the pins must be connected together on the PCB and form a ground plane, which is usually also the system ground. SGND (Pin 4) - Signal ground. It should be connected appropriately to the ground plane with due regard to good layout practices in switching power regulator circuits. VCC (Pin 5) Supply rail for the control sections of the IC. PWGD (Pin 6) - Power Good pin. This is an open drain output, which is typically meant to be connected to VCC or any other low voltage source through a pull-up resistor. Choose the pull-up resistor so that the current going into this pin is kept below 1 mA. A recommended value for the pull-up resistor is 100 kΩ for most applications. The voltage on this pin is thus pulled low under output undervoltage or overvoltage fault conditions and also under input UVLO. ISEN (Pin 7) - Current limit threshold setting pin. This sources a fixed 40 µA current. A resistor of appropriate value should be connected between this pin and the drain of the low-side MOSFET (switch node). The minimum value for this resistor is 1 kΩ. www.national.com 2 LM2745/8 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VCC BOOT Voltage ISEN FREQ/SYNC Voltage All other pins Junction Temperature Storage Temperature -0.3 to 7V -0.3 to 18V -0.3 to 14V -0.5 to VCC + 0.3V -0.3 to VCC + 0.3V 150˚C −65˚C to 150˚C Soldering Information Lead Temperature (soldering, 10sec) Infrared or Convection (20sec) ESD Rating (Note 3) 260˚C 235˚C 2kV Operating Ratings Supply Voltage Range, VCC (Note 2) BOOT Voltage Range Junction Temperature Range (TJ) Thermal Resistance (θJA) 3V to 6V 1V to 17V −40˚C to +125˚C 155˚C/W Electrical Characteristics VCC = 3.3V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA= TJ= 25˚C. Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Symbol VFB VON Parameter FB Pin Voltage UVLO Thresholds VCC Rising VCC Falling VCC = 3.3V, VSD = 3.3V fSW = 600 kHz VCC = 5V, VSD = 3.3V fSW = 600 kHz VCC = 3.3V, VSD = 0V VFB Rising VFB Falling VSS = 0V VSS = 2.0V 7 LM2745 LM2748 LM2745 LM2748 1.1 1.0 1.3 1.0 Conditions VCC = 3V to 6V Min 0.591 Typ 0.6 2.79 2.42 1.7 1.5 2 1.8 1 10 10 10 90 25 Sourcing 40 20 55 14 2.3 2.1 mA 2.6 2.6 3 µA µs µs µA µA µA nA Max 0.609 Units V V IQ_VCC Operating VCC Current Shutdown VCC Current tPWGD1 tPWGD2 ISS-ON ISS-OC PWGD Pin Response Time PWGD Pin Response Time SS Pin Source Current SS Pin Sink Current During Over Current ISEN Pin Source Current Trip Point FB Pin Current Error Amplifier Unity Gain Bandwidth Error Amplifier DC Gain Error Amplifier Slew Rate EAO Pin Current Sourcing and Sinking Capability Error Amplifier Output Voltage ISEN-TH IFB GBW G SR IEAO VEAO ERROR AMPLIFIER 9 118 2 14 16 Minimum Maximum 1 2.2 MHz dB V/µs mA V V 3 www.national.com LM2745/8 Electrical Characteristics (Continued) VCC = 3.3V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA= TJ= 25˚C. Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Symbol Parameter BOOT Pin Quiescent Current High-Side MOSFET Driver Pull-Up ON resistance High-Side MOSFET Driver Pull-Down ON resistance Conditions VBOOT = 12V, VSD = 0 VBOOT = 5V @ 350 mA Sourcing 350 mA Sinking Min Typ 18 2.7 0.8 2.7 0.8 Max 90 Units µA Ω Ω Ω Ω GATE DRIVE IQ-BOOT RHG_UP RHG_DN RLG_UP RLG_DN OSCILLATOR RFADJ = 750 kΩ PWM Frequency fSW LM2745 External Synchronizing Signal Frequency SYNCL SYNCH DMAX LM2745 Synchronization Signal Low Threshold LM2745 Synchronization Signal High Threshold Max High-Side Duty Cycle RFADJ = 100 kΩ RFADJ = 42.2 kΩ RFADJ = 18.7 kΩ Voltage Swing = 0V to VCC fSW = 250 kHz to 1 MHz fSW = 250 kHz to 1 MHz fSW = 300 kHz fSW = 600 kHz fSW = 1 MHz VFB = 0.575V, VBOOT = 3.3V VSD Rising VFB = 0.575V, VBOOT = 3.3V VSD Falling VSD Rising VSD Falling VFB Falling VFB Rising VFB Falling VFB Rising 0.8 0.408 0.677 0.434 0.710 60 90 0.457 0.742 0.232 1.3 2 86 78 67 1.1 250 475 50 300 600 1000 1000 1 V V 725 kHz Low-Side MOSFET Driver Pull-Up VBOOT = 5V @ 350 mA Sourcing ON resistance Low-Side MOSFET Driver Pull-Down ON resistance 350 mA Sinking % LOGIC INPUTS AND OUTPUTS VSTBY-IH VSTBY-