 |
Moteur de recherche de fiches techniques de composants électroniques |
|
|
Moteur de recherche de fiches techniques de composants électroniques |
|
LM2758 Fiches technique(PDF) 11 Page - National Semiconductor (TI) |
|
|
||||||||||||||||||||||||||||||
LM2758 Fiches technique(HTML) 11 Page - National Semiconductor (TI) |
|
|
11 / 14 page  POWER EFFICIENCY Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (P LED) to the power drawn at the input of the part (P IN). With a 1x/1.5x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM2758 can be predicted as follows: P LED = VLED × ILED P IN = VIN × IIN P IN = VIN × (Gain × ILED + IQ) E = (P LED ÷ PIN) For a simple approximation, the current consumed by internal circuitry (I Q) can be neglected, and the resulting efficiency will become: E = V LED ÷ (VIN × Gain) Neglecting I Q will result in a slightly higher efficiency predic- tion, but this impact will be negligible due to the value of I Q being very low compared to the typical Torch and Flash cur- rent levels (100-500 mA). It is also worth noting that efficiency as defined here is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power con- sumed by the circuit (V IN x IIN) be evaluated rather than power efficiency. THERMAL PROTECTION Internal thermal protection circuitry disables the LM2758 when the junction temperature exceeds 150°C (typ.). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive pow- er dissipation. The device will recover and operate normally when the junction temperature falls below 140°C (typ.). It is important that the board layout provide good thermal conduc- tion to keep the junction temperature within the specified operating ratings. POWER DISSIPATION The power dissipation (P DISSIPATION) and junction temperature (T J) can be approximated with the equations below. PIN is the power generated by the 1x/1.5x charge pump, P LED is the power consumed by the LEDs, T A is the ambient temperature, and θ JA is the junction-to-ambient thermal resistance for the 12–bump micro SMD package. V IN is the input voltage to the LM2758, V LED is the nominal LED forward voltage, and ILED is the programmed LED current. P DISSIPATION = PIN - PLED = (Gain × V IN × ILED) − (VLED × ILED) T J = TA + (PDISSIPATION × θJA) The junction temperature rating takes precedence over the ambient temperature rating. The LM2758 may be operated outside the ambient temperature rating, so long as the junc- tion temperature of the device does not exceed the maximum operating rating of 125°C. The maximum ambient tempera- ture rating must be derated in applications where high power dissipation and/or poor thermal resistance causes the junc- tion temperature to exceed 125°C. MICRO SMD PACKAGE ASSEMBLY AND USE Use of the micro SMD package requires specialized board layout, precision mounting and careful re-flow techniques as detailed in National Semiconductor Application Note 1112. Refer to the section "Surface Mount Technology (SMD) As- sembly Considerations". For best results in assembly, align- ment ordinals on the PC board should be used to facilitate placement of the device. The pad style used with the micro SMD package must be the NSMD (non-solder mask defined) typ. This means that the solder-mask opening is larger than the pad size. This prevents a lip that otherwise forms if the solder mask and pad overlap, from holding the device off the surface of the board and interfering with mounting. See Ap- plication Note 1112 for specific instructions how to do this. The 12-bump package used for LM2758 has 300 micron sol- der balls and requires 10.82 mils pads for mounting on the circuit board. The trace to each pad should enter the pad with a 90° entry angle to prevent debris from being caught in deep corners. Initially, the trace to each pad should be 7 mil. wide, for a section approximately 7 mil. long or longer, as a thermal relief. Then each trace should neck up or down to its optimal width. The important criteria is symmetry. This ensures the solder bumps on the LM2758 re-flow evenly and that the de- vice solders level to the board. In particular, special attention must be paid to the pads for bumps C1 and D3, because V IN and GND are typically connected to large copper planes, thus inadequate thermal relief can result in late or inadequate re-flow of these bumps. The micro SMD package is optimized for the smallest possi- ble size in applications with red or infrared opaque cases. Because the micro SMD package lacks the plastic encapsu- lation characteristic of larger devices, it is vulnerable to light. Backside metallization and/or epoxy coating, along with front side shading by the printed circuit board, reduce this sensi- tivity. However, the package has exposed die edges. In par- ticular, micro SMD devices are sensitive to light, in the red and infrared range, shining on the package’s exposed die edges. BOARD LAYOUT CONSIDERATIONS PC board layout is an important part of DC-DC converter de- sign. Poor board layout can disrupt the performance of a DC- DC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss in the traces. These can send erroneous signals to the DC-DC converter IC, re- sulting in poor regulation or instability. Poor layout can also result in re-flow problems leading to poor solder joints be- tween the micro SMD package and board pads. Poor solder joints can result in erratic or degraded performance. 11 www.national.com |
Numéro de pièce similaire - LM2758 |
|
Description similaire - LM2758 |
|
|
|
Lien URL |
| Politique de confidentialité |
| ALLDATASHEET.FR |
| ALLDATASHEET vous a-t-il été utile ? [ DONATE ] |
À propos de Alldatasheet | Publicité | Contactez-nous | Politique de confidentialité | Echange de liens | Fabricants All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
French ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
