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LM22675QMR-5.0 数据表(PDF) 10 Page - Texas Instruments |
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LM22675QMR-5.0 数据表(HTML) 10 Page - Texas Instruments |
10 / 23 page 0.0 0.2 0.4 0.6 0.8 1.0 1.2 5 10 15 20 25 30 35 40 45 SHORT CIRCUIT VOLTAGE (v) SAFE OPERATING AREA LM22675 SNVS591K – SEPTEMBER 2008 – REVISED MARCH 2013 www.ti.com Figure 13. SOA Soft-Start The soft-start feature allows the regulator to gradually reach steady-state operation, thus reducing start-up stresses. The internal soft-start feature brings the output voltage up in about 500 µs. This time is fixed and can not be changed. Soft-start is reset any time the part is shut down or a thermal overload event occurs. Boot-Strap Supply The LM22675 incorporates a floating high-side gate driver to control the power MOSFET. The supply for this driver is the external boot-strap capacitor connected between the BOOT pin and SW. A good quality 10 nF ceramic capacitor must be connected to these pins with short, wide PCB traces. One reason the regulator imposes a minimum off-time is to ensure that this capacitor recharges every switching cycle. A minimum load of about 5 mA is required to fully recharge the boot-strap capacitor in the minimum off-time. Some of this load can be provided by the output voltage divider, if used. Thermal Protection Internal thermal shutdown circuitry protects the LM22675 should the maximum junction temperature be exceeded. This protection is activated at about 150°C, with the result that the regulator will shutdown until the temperature drops below about 135°C. Internal Loop Compensation The LM22675 has internal loop compensation designed to provide a stable regulator over a wide range of external power stage components. The internal compensation of the -ADJ option is optimized for output voltages below 5V. If an output voltage of 5V or greater is needed, the -5.0 option with an external resistor divider can be used. Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The LM22675 stability can be verified using the WEBENCH Designer online circuit simulation tool at www.ti.com. A quick start spreadsheet can also be downloaded from the online product folder. The complete transfer function for the regulator loop is found by combining the compensation and power stage transfer functions. The LM22675 has internal type III loop compensation, as detailed in Figure 14. This is the approximate "straight line" function from the FB pin to the input of the PWM modulator. The power stage transfer function consists of a D.C. gain and a second order pole created by the inductor and output capacitor(s). Due to the input voltage feedforward employed in the LM22675, the power stage D.C. gain is fixed at 20dB. The second order pole is characterized by its resonant frequency and its quality factor (Q). For a first pass design, the product of inductance and output capacitance should conform to the following equation: (8) 10 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Product Folder Links: LM22675 |
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类似说明 - LM22675QMR-5.0 |
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