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LM5576MH/NOPB bảng dữ liệu(PDF) 10 Page - Texas Instruments |
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LM5576MH/NOPB bảng dữ liệu(HTML) 10 Page - Texas Instruments |
10 / 34 page 10 LM5576, LM5576-Q1 SNVS447J – JANUARY 2007 – REVISED NOVEMBER 2014 www.ti.com Product Folder Links: LM5576 LM5576-Q1 Submit Documentation Feedback Copyright © 2007–2014, Texas Instruments Incorporated Feature Description (continued) An external set-point voltage divider from VIN to GND can be used to set the operational input range of the regulator. The divider must be designed such that the voltage at the SD pin will be greater than 1.225 V when VIN is in the desired operating range. The internal 5 µA pull-up current source must be included in calculations of the external set-point divider. Hysteresis of 0.1 V is included for both the shutdown and standby thresholds. The SD pin is internally clamped with a 1 k Ω resistor and an 8 V zener clamp. The voltage at the SD pin should never exceed 14 V. If the voltage at the SD pin exceeds 8 V, the bias current will increase at a rate of 1 mA/V. The SD pin can also be used to implement various remote enable / disable functions. Pulling the SD pin below the 0.7 V threshold totally disables the controller. If the SD pin voltage is above 1.225 V the regulator will be operational. 7.3.2 Soft-Start The soft-start feature allows the regulator to gradually reach the initial steady state operating point, thus reducing start-up stresses and surges. The internal soft-start current source, set to 10 µA, gradually increases the voltage of an external soft-start capacitor connected to the SS pin. The soft-start capacitor voltage is connected to the reference input of the error amplifier. Various sequencing and tracking schemes can be implemented using external circuits that limit or clamp the voltage level of the SS pin. In the event a fault is detected (over-temperature, VCC UVLO, SD) the soft-start capacitor will be discharged. When the fault condition is no longer present a new soft-start sequence will commence. 7.3.3 Thermal Protection Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction temperature is exceeded. When activated, typically at 165°C (Q1 version) and 180°C (Q0 version), the controller is forced into a low power reset state, disabling the output driver and the bias regulator. This feature is provided to prevent catastrophic failures from accidental device overheating. 7.4 Device Functional Modes 7.4.1 High Voltage Start-Up Regulator The LM5576 contains a dual-mode internal high voltage startup regulator that provides the CC bias supply for the PWM controller and boot-strap MOSFET gate driver. The input pin (VIN) can be connected directly to the input voltage, as high as 75 Volts. For input voltages below 9 V, a low dropout switch connects VCC directly to VIN. In this supply range, VCC is approximately equal to VIN. For VIN voltage greater than 9 V, the low dropout switch is disabled and the VCC regulator is enabled to maintain VCC at approximately 7 V. The wide operating range of 6 V to 75 V is achieved through the use of this dual mode regulator. The output of the VCC regulator is current limited to 25 mA. Upon power up, the regulator sources current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds the VCC UVLO threshold of 5.35 V and the SD pin is greater than 1.225 V, the output switch is enabled and a soft-start sequence begins. The output switch remains enabled until VCC falls below 5.0 V or the SD pin falls below 1.125 V. An auxiliary supply voltage can be applied to the VCC pin to reduce the IC power dissipation. If the auxiliary voltage is greater than 7.3 V, the internal regulator will essentially shut off, reducing the IC power dissipation. The VCC regulator series pass transistor includes a diode between VCC and VIN that should not be forward biased in normal operation. Therefore the auxiliary VCC voltage should never exceed the VIN voltage. In high voltage applications extra care should be taken to ensure the VIN pin does not exceed the absolute maximum voltage rating of 76 V. During line or load transients, voltage ringing on the VIN line that exceeds the Absolute Maximum Ratings can damage the IC. Both careful PC board layout and the use of quality bypass capacitors located close to the VIN and GND pins are essential. |
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