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LM2770SDX-12157 Arkusz danych(PDF) 11 Page - National Semiconductor (TI) |
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LM2770SDX-12157 Arkusz danych(HTML) 11 Page - National Semiconductor (TI) |
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11 / 14 page  Operation Description (Continued) source. This can be done, but the voltage on the output pin of the LM2770 must not be brought above the input voltage. The output pin will draw a small amount of current when driven externally due the internal feedback resistor divider connected between V OUT and GND. SOFT START The LM2770 employs soft start circuitry to prevent excessive input inrush currents during startup. At startup, the output voltage gradually rises from 0V to the nominal output volt- age. This occurs in 200µs (typ.). Soft-start is engaged when the part is enabled, including situations where voltage is established simultaneously on the V IN and EN pins. THERMAL SHUTDOWN Protection from overheating-related damage is achieved with a thermal shutdown feature. When the junction tem- perature rises to 150oC (typ.), the part switches into shut- down mode. The LM2770 disengages thermal shutdown when the junction temperature of the part is reduced to 140oC (typ.). Due to the high efficiency of the LM2770, thermal shutdown and/or thermal cycling should not be en- countered 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 LLP pack- age. SHORT-CIRCUIT AND CURRENT LIMIT PROTECTION The LM2770 charge pump contains circuitry that protects the device from destructive failure in the event of a direct short to ground on the output. This short-circuit protection circuit limits the output current to 400mA (typ.) when the output voltage is below 165mV (typ.). The sleep-mode LDO con- tains a 60mA (typ.) current limit circuit. Application Information RECOMMENDED CAPACITOR TYPES The LM2770 requires 4 external capacitors for proper opera- tion. Surface-mount multi-layer ceramic capacitors are rec- ommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and alumi- num electrolytic capacitors generally are not recommended for use with the LM2770 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 LM2770. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over temperature (X7R: ±15% over -55oCto125oC; X5R: ±15% over -55oCto 85oC). Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2770. These types of capacitors typically have wide capacitance toler- ance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30oCto+85oC range; Z5U: +22%, -56% over +10oCto+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 capaci- tance requirements of the LM2770. Net capacitance of a ceramic capacitor decreases with in- creased DC bias. This degradation can result in lower ca- pacitance 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 capaci- tor manufacturers for information on capacitor DC bias char- acteristics. Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and capaci- tor manufacturers. It is strongly recommended that the LM2770 circuit be thoroughly evaluated early in the design-in process with the mass-production capacitors of choice. This will help to ensure that any such variability in capacitance does not negatively impact circuit performance. The table below lists some leading ceramic capacitor manu- facturers. 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 VOTAGE RIPPLE The output capacitor in the LM2770 circuit (C OUT) directly impacts the magnitude of output voltage ripple. Other promi- nent factors also affecting output voltage ripple include input voltage, output current and flying capacitance. Due to the complexity of multi-gain and PFM switching, providing equa- tions or models to approximate the magnitude of the ripple can not be easily accomplished. But one important generali- zation can be made: increasing (decreasing) the output ca- pacitance will result in a proportional decrease (increase) in output voltage ripple. This can be observed in the output voltage ripple waveforms in the Typical Performance Char- acteristics section. In typical high-current applications, a 10µF low-ESR ceramic output capacitor is recommended. Different output capaci- tance values can be used to reduce ripple, shrink the solu- tion size, and/or cut the cost of the solution. But changing the output capacitor may also require changing the flying capaci- tors and/or input capacitor to maintain good overall circuit performance. Performance of the LM2770 with different ca- pacitor setups in discussed in the section Recommended Capacitor Configurations. High ESR in the output capacitor increases output voltage ripple. If a ceramic capacitor is used at the output, this is usually not a concern because the ESR of a ceramic capaci- tor is typically vey low and has only a minimal impact on ripple magnitudes. If a different capacitor type with higher ESR is used (tantalum, for example), the ESR could result in high ripple. To eliminate this effect, the net output ESR can be significantly reduced by placing a low-ESR ceramic ca- pacitor in parallel with the primary output capacitor. The low ESR of the ceramic capacitor will be in parallel with the higher ESR, resulting in a low net ESR based on the prin- ciples of parallel resistance reduction. Due to the PFM nature of the LM2770, output voltage ripple is highest at light loads. To eliminate this ripple, consider running the LM2770 in sleep mode when load currents are 20mA or less. Sleep mode disables the charge pump and enables the internal low-noise bypass linear regulator (LDO). www.national.com 11 |
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Podobny opis - LM2770SDX-12157 |
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