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LM2772 데이터시트(PDF) 9 Page - Texas Instruments |
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LM2772 데이터시트(HTML) 9 Page - Texas Instruments |
9 / 17 page LM2772 www.ti.com SNVS486B – DECEMBER 2006 – REVISED MAY 2013 Thermal Shutdown Protection from damage related to overheating is achieved with a thermal shutdown feature. When the junction temperature rises to 150ºC (typ.), the part switches into shutdown mode. The LM2772 disengages thermal shutdown when the junction temperature of the part is reduced to 140ºC (typ.). Due to the high efficiency of the LM2772, thermal shutdown and/or thermal cycling should not be encountered when the part is operated within specified input voltage, output current, and ambient temperature operating ratings. If thermal cycling is seen under these conditions, the most likely cause is an inadequate PCB layout that does not allow heat to be sufficiently dissipated out of the WSON package. Current Limit Protection The LM2772 charge pump contains current limit protection circuitry that protects the device during VOUT fault conditions where excessive current is drawn. Output current is limited to 500mA (typ). APPLICATION INFORMATION Recommended Capacitor Types The LM2772 requires 5 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2772 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use with the LM2772. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over temperature (X7R: ±15% over -55ºC to 125ºC; X5R: ±15% over -55ºC to 85ºC). Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2772. These types of capacitors typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30ºC to +85ºC range; Z5U: +22%, -56% over +10ºC to +85ºC range). Under some conditions, a 1µF-rated Y5V or Z5U capacitor could have a capacitance as low as 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2772. Net capacitance of a ceramic capacitor decreases with increased DC bias. This degradation can result in lower capacitance than expected on the input and/or output, resulting in higher ripple voltages and currents. Using capacitors at DC bias voltages significantly below the capacitor voltage rating will usually minimize DC bias effects. Consult capacitor manufacturers for information on capacitor DC bias characteristics. Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and capacitor manufacturers. It is strongly recommended that the LM2772 circuit be thoroughly evaluated early in the design-in process with the mass-production capacitors of choice. This will help ensure that any such variability in capacitance does not negatively impact circuit performance. The table below lists some leading ceramic capacitor manufacturers. Manufacturer Contact Information AVX www.avx.com Murata www.murata.com Taiyo-Yuden www.t-yuden.com TDK www.component.tdk.com Vishay-Vitramon www.vishay.com Output Capacitor and Output Voltage Ripple The output capacitor in the LM2772 circuit (COUT) directly impacts the magnitude of output voltage ripple. Other prominent factors also affecting output voltage ripple include input voltage, output current and flying capacitance. Due to the complexity of the regulation topology, providing equations or models to approximate the magnitude of the ripple can not be easily accomplished. But one important generalization can be made: increasing (decreasing) the output capacitance will result in a proportional decrease (increase) in output voltage ripple. Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2772 |
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