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LM2731 Datasheet(PDF) 10 Page - Texas Instruments |
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LM2731 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 22 page LM2731 SNVS217E – MAY 2004 – REVISED APRIL 2010 www.ti.com The current limit comparator feeds directly into the flip-flop that drives the switch FET. If the FET current reaches the limit threshold, the FET is turned off and the cycle terminated until the next clock pulse. The current limit input terminates the pulse regardless of the status of the output of the PWM comparator. Application Hints SELECTING THE EXTERNAL CAPACITORS The best capacitors for use with the LM2731 are multi-layer ceramic capacitors. They have the lowest ESR (equivalent series resistance) and highest resonance frequency which makes them optimum for use with high frequency switching converters. When selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage, they may provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from Taiyo-Yuden, AVX, and Murata. SELECTING THE OUTPUT CAPACITOR A single ceramic capacitor of value 4.7 µF to 10 µF will provide sufficient output capacitance for most applications. If larger amounts of capacitance are desired for improved line support and transient response, tantalum capacitors can be used. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used, but are usually prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies above 500 kHz due to significant ringing and temperature rise due to self-heating from ripple current. An output capacitor with excessive ESR can also reduce phase margin and cause instability. In general, if electrolytics are used, it is recommended that they be paralleled with ceramic capacitors to reduce ringing, switching losses, and output voltage ripple. SELECTING THE INPUT CAPACITOR An input capacitor is required to serve as an energy reservoir for the current which must flow into the coil each time the switch turns ON. This capacitor must have extremely low ESR, so ceramic is the best choice. We recommend a nominal value of 2.2 µF, but larger values can be used. Since this capacitor reduces the amount of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other circuitry. FEED-FORWARD COMPENSATION Although internally compensated, the feed-forward capacitor Cf is required for stability (see Basic Application Circuit). Adding this capacitor puts a zero in the loop response of the converter. The recommended frequency for the zero fz should be approximately 6 kHz. Cf can be calculated using the formula: Cf = 1 / (2 X π X R1 X fz) (2) SELECTING DIODES The external diode used in the typical application should be a Schottky diode. A 20V diode such as the MBR0520 is recommended. The MBR05XX series of diodes are designed to handle a maximum average current of 0.5A. For applications exceeding 0.5A average but less than 1A, a Microsemi UPS5817 can be used. LAYOUT HINTS High frequency switching regulators require very careful layout of components in order to get stable operation and low noise. All components must be as close as possible to the LM2731 device. It is recommended that a 4- layer PCB be used so that internal ground planes are available. As an example, a recommended layout of components is shown: 10 Submit Documentation Feedback Copyright © 2004–2010, Texas Instruments Incorporated Product Folder Links: LM2731 |
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