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LM22678-Q1 Scheda tecnica(PDF) 10 Page - Texas Instruments |
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LM22678-Q1 Scheda tecnica(HTML) 10 Page - Texas Instruments |
10 / 26 page 100 1k 10k 100k 1M 10M 0 5 10 15 20 25 30 35 40 FREQUENCY (Hz) -ADJ -5.0 LM22678, LM22678-Q1 SNVS585L – SEPTEMBER 2008 – REVISED NOVEMBER 2014 www.ti.com Feature Description (continued) 7.3.3 Boot-Strap Supply The LM22678 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 Loop Compensation The LM22678 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 LM22678 stability can be verified using the WEBENCH Designer online circuit simulation tool at WEBENCH Designer. 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 LM22678 has internal type III loop compensation, as detailed in Figure 11. 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 feed forward employed in the LM22678, 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 3. (3) Alternatively, this pole should be placed between 1.5 kHz and 15 kHz and is determined by Equation 4. (4) 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). Figure 11. Compensator Gain 10 Submit Documentation Feedback Copyright © 2008–2014, Texas Instruments Incorporated Product Folder Links: LM22678 LM22678-Q1 |
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