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LM5575QMH/NOPB bảng dữ liệu(PDF) 10 Page - Texas Instruments |
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10 / 31 page LM5575-Q1 SNOSB23C – OCTOBER 2008 – REVISED DECEMBER 2014 www.ti.com 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 pullup 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 and 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. 8.3.2 Current Limit The LM5575-Q1 contains a unique current monitoring scheme for control and overcurrent protection. When set correctly, the emulated current sense signal provides a signal which is proportional to the buck switch current with a scale factor of 1.0 V/A. The emulated ramp signal is applied to the current limit comparator. If the emulated ramp signal exceeds 2.1 V (2.1 A) the present current cycle is terminated (cycle-by-cycle current limiting). In applications with small output inductance and high-input voltage, the switch current may overshoot due to the propagation delay of the current limit comparator. If an overshoot should occur, the diode current sampling circuit will detect the excess inductor current during the off-time of the buck switch. If the sample and hold DC level exceeds the 2.1-V current limit threshold, the buck switch will be disabled and skip pulses until the diode current sampling circuit detects the inductor current has decayed below the current limit threshold. This approach prevents current runaway conditions due to propagation delays or inductor saturation since the inductor current is forced to decay following any current overshoot. 8.3.3 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 (overtemperature, 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. 8.3.4 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 180°C, 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. 8.4 Device Functional Modes 8.4.1 High-Voltage Start-Up Regulator The LM5575-Q1 contains a dual-mode internal high-voltage start-up regulator that provides the VCC bias supply for the PWM controller and bootstrap 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 V or the SD pin falls below 1.125 V. 10 Submit Documentation Feedback Copyright © 2008–2014, Texas Instruments Incorporated Product Folder Links: LM5575-Q1 |
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