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LM22679-Q1 Datenblatt(PDF) 9 Page - Texas Instruments |
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LM22679-Q1 Datenblatt(HTML) 9 Page - Texas Instruments |
9 / 27 page LM22679, LM22679-Q1 www.ti.com SNVS581L – FEBRUARY 2013 – REVISED NOVEMBER 2014 7.3 Feature Description 7.3.1 UVLO The LM22679 also incorporates an input undervoltage lock-out (UVLO) feature. This prevents the regulator from turning on when the input voltage is not great enough to properly bias the internal circuitry. The rising threshold is 4.3 V (typ) while the falling threshold is 3.9 V (typ). 7.3.2 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 can be extended by using an external capacitor connected to the SS pin. Values in the range of 100 nF to 1 µF are recommended. The approximate soft-start time can be estimated from Equation 1. TSS ≈ 26 × 10 3 × C SS (1) Soft-start is reset any time the part is shut down or a thermal overload event occurs. 7.3.3 Boot-Strap Supply The LM22679 device 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. 7.3.4 Internal Compensation The LM22679 device 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 5 V. If an output voltage of 5 V 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 LM22679 stability can be verified using the WEBENCH Designer online circuit simulation tool. 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 LM22679 has internal type III loop compensation, as detailed in Internal Loop Compensation section. 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 dc gain and a second order pole created by the inductor and output capacitor(s). Due to the input voltage feedforward employed in the LM22679, the power stage dc gain is fixed at 20 dB. 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 Equation 2. (2) Alternatively, this pole should be placed between 1.5 kHz and 15 kHz and is given by Equation 3. (3) The Q factor depends on the parasitic resistance of the power stage components and is not typically in the control of the designer. Of course, loop compensation is only one consideration when selecting power stage components (see Application and Implementation for more details). Copyright © 2013–2014, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM22679 LM22679-Q1 |
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