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LM22680 Datasheet(PDF) 11 Page - Texas Instruments |
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LM22680 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 27 page LM22680 LM22680 RT/SYNC RT/SYNC Vout 1 k: 1 k: 2N7002 EN EN ENABLE LM22680, LM22680-Q1 www.ti.com SNVS595L – SEPTEMBER 2008 – REVISED NOVEMBER 2014 Feature Description (continued) 7.3.4 Self Synchronization It is possible to synchronize multiple LM22680 regulators together to share the same switching frequency. This can be done by tying the RT/SYNC pins together through a MOSFET and connecting a 1 k Ω resistor to ground at each pin. Figure 14 shows this connection. The gate of the MOSFET should be connected to the regulator with the highest output voltage. Also, the EN pins of both regulators should be tied to the common system enable, in order to properly initialize both regulators. The operation is as follows: When the regulators are enabled, the outputs are low and the MOSFET is off. The 1 k Ω resistors pull the RT/SYNC pins low, thus enabling the synchronization mode. These resistors are small enough to pull the RT/SYNC pin low, rather than activate the frequency adjust mode. Once the output voltage of one of the regulators is sufficient to turn on the MOSFET, the two RT/SYNC pins are tied together and the regulators will run in synchronized mode. The two regulators will be clocked at the same frequency but slightly phase shifted according to the minimum off-time of the regulator with the fastest internal oscillator. The slight phase shift helps to reduce stress on the input capacitors of the regulator. It is important to choose a MOSFET with a low gate threshold voltage so that the MOSFET will be fully enhanced. Also, a MOSFET with low inter-electrode capacitance is required. The 2N7002 is a good choice. Figure 14. Self Synchronizing Setup 7.3.5 Boot-Strap Supply The LM22680 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. 7.3.6 Internal Loop Compensation The LM22680 has internal loop compensation designed to provide a stable regulator over a wide range of external power stage components. Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The LM22680 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 LM22680 has internal type III loop compensation, as detailed in Figure 15. 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 LM22680, the power stage dc 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 Equation 4. (4) Alternatively, this pole should be placed between 1.5 kHz and 15 kHz and is given by Equation 5. Copyright © 2008–2014, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM22680 LM22680-Q1 |
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