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LM3551SD датащи(PDF) 11 Page - National Semiconductor (TI) |
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LM3551SD датащи(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Information (Continued) one resistor while Flash-mode is set by utilizes the currents though both current set resistors. The following equations are used to set the LED currents. R TORCH = (1.265V / ITORCH)-RDSON-T R FLASH = (1.265V / (IFLASH-ITORCH))-RDSON-F R DSON-T= 0.98 Ω and R DSON-F = 0.36 Ω Note: Flash LEDs from different manufacturers can have very different continuous and pulse current ratings. See the manufacturers datasheets to ensure that the proper current levels are used to avoid damaging the flash LED. INDUCTOR SELECTION Special care must be taken when selecting an inductor for use in LM3551/2 applications. The inductor should have a current saturation rating that is larger than the worst case peak inductor current of the application to ensure proper operation. Using an inductor with a lower saturation current rating than is required can cause a dramatic drop in the inductance and can derate the maximum output current levels severely. It is worth noting that the output voltage ripple is also affected by the total ripple current in the induc- tor. The following equations can help give a good approxi- mation as to what the peak inductor current will be for a given application at room temperature (T A = +25˚C). I L(average) = [ILED xVOUT-MAX]÷[VIN-MIN x Eff.] ∆I L =[VIN xD]÷[L x FSW] I L(peak) = IL(ave) + [ ∆I L ÷2] V OUT-MAX Maximum Output Voltage. Maximum output volt- age over temperature with OVP used is 11V (12.4V typically). V IN-MIN Minimum Input Voltage. Recommended minimum input voltage is 3.0V. The LM3551/2 will work down to 2.7V however, use at lower input volt- ages will required an inductor with a higher satu- ration current rating. Eff. Converter Efficiency (approx. 85% over input volt- age range). D Duty Cycle=1-[V IN /VOUT] L Inductance. Recommended inductance value is 4.7µH. F SW Switching Frequency = 1.25MHz DIODE SELECTION The output diode for a boost regulator must be chosen correctly depending on the output voltage and output cur- rent. The output diode must have a reverse voltage rating equal to or greater than the output voltage used. The aver- age current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. Using Schottky di- odes with lower forward voltage drop will decrease power dissipation and increase efficiency. CAPACITOR SELECTION Input Capacitor An input capacitor is required to reduce the input ripple and noise for proper operation of the regulator. The size used is dependant on the application and board layout. If the regu- lator will be loaded uniformly, with very little load changes, and at lower current outputs, the input capacitor size can often be reduced. The size can also be reduced if the input of the regulator is very close to the source output. The size will generally need to be larger for applications where the regulator is supplying nearly the maximum rated output or if large load steps are expected. A minimum value of 10µF should be used under normal operating condtions while a 10-22µF capacitor may be required for higher power and dynamic loads. Larger values and/or lower ESR may be needed if the application requires very low ripple on the input source voltage. Output Capacitor A minimum output capacitor value of 4.7µF (Sharp LED) and 10µF (Lumiled) is recommended and may be increased to a larger value. The ESR of the output capacitor is important because it determines the peak to peak output voltage ripple according to the approximate equation: ∆V OUT ) 2x ∆I LxRESR (in Volts) After choosing the output capacitor you can determine a pole-zero pair introduced into the control loop by the follow- ing equations: The zero created by the ESR of the output capacitor is generally at a very high frequency if the ESR is small. If low ESR capacitors are used it can be neglected. The output capacitor pole information is useful in selecting the proper compensation components and is discussed in the COM- PENSATION COMPONENTS section of the datasheet. Capacitor Properties Surface-mount multi-layer ceramic capacitors are recom- mended for both the input and output capacitors. These capacitors are small, inexpensive and have very low equiva- lent series resistance (ESR <20m Ω typ.). Tantalum capaci- tors, OS-CON capacitors, and aluminum electrolytic capaci- tors are not recommended for use with the LM3551/2 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM3551/2. These capacitors have tight capacitance toler- ance (as good as ±10%) and hold their value over tempera- ture (X7R: ±15% over -55˚C to 125˚C; X5R: ±15% over -55˚C to 85˚C). Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM3551/2. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30˚C to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM3551/2. The minimum voltage rating acceptable for the input capaci- tor is 6.3V (10V recommended) and 16V for the output capacitor. In applications that have DC operating points near the maximum voltage rating of the ceramic capacitor, larger capacitor values may be required to compensate for capaci- www.national.com 11 |
Аналогичный номер детали - LM3551SD |
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Аналогичное описание - LM3551SD |
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