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LM2748 Datasheet(PDF) 2 Page - National Semiconductor (TI) |
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LM2748 Datasheet(HTML) 2 Page - National Semiconductor (TI) |
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2 / 23 page  Connection Diagrams 20137490 14-Lead Plastic TSSOP θ JA = 155˚C/W NS Package Number MTC14 20137402 14-Lead Plastic TSSOP θ JA = 155˚C/W NS Package Number MTC14 Ordering Information Order Number Package Type NSC Package Drawing Supplied As LM2745MTC TSSOP-14 MTC14 94 Units on Rail LM2745MTCX 2500 Units on Tape and Reel LM2748MTC 94 Units on Rail LM2748MTCX 2500 Units on Tape and Reel 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 V IN to properly turn on the MOSFET. See MOSFET Gate Drivers in the Application Information section for more details on how to select MOS- FETs. 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 shoot- through. 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. V CC (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 V CC 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 k Ω for most applications. The voltage on this pin is thus pulled low under output undervoltage or overvolt- age fault conditions and also under input UVLO. I SEN (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 is1k Ω. 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 refer- ence 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, con- nect 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 k Ω for 200 kHz, 100 k Ω 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 V CC 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. www.national.com 2 |
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