поискавой системы для электроныых деталей |
|
FAN5358S712X датащи(PDF) 10 Page - Fairchild Semiconductor |
|
FAN5358S712X датащи(HTML) 10 Page - Fairchild Semiconductor |
10 / 13 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5358 • Rev. 1.0.2 10 Applications Information Selecting the Inductor The output inductor must meet both the required inductance and the energy handling capability of the application. The inductor value affects the average current limit, the PWM-to-PFM transition point, the output voltage ripple, and the efficiency. The ripple current (∆I) of the regulator is: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ • − • ≈ Δ SW OUT IN IN OUT f L V V V V I (1) The maximum average load current, IMAX(LOAD), is related to the peak current limit, ILIM(PK) by the ripple current: 2 I I I ) PK ( LIM ) LOAD ( MAX Δ − = (2) The transition between PFM and PWM operation is determined by the point at which the inductor valley current crosses zero. The regulator DC current when the inductor current crosses zero, IDCM, is: 2 I IDCM Δ = (3) The FAN5358 is optimized for operation with L=2.2µH. The inductor should be rated to maintain at least 70% of its value at ILIM(PK). Efficiency is affected by the inductor DCR and inductance value. Decreasing the inductor value for a given physical size typically decreases the DCR; but since ∆I increases, the RMS current increases, as do core and skin effect losses. 12 I I I 2 2 ) DC ( OUT RMS Δ + = (4) The increased RMS current produces higher losses through the RDS(ON) of the IC MOSFETs as well as the inductor ESR. Increasing the inductor value produces lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance usually results in an inductor with lower saturation current. Table 3 shows the effects of inductance higher or lower than the recommended inductor on regulator performance. Thermal Considerations The FAN5358 is designed to supply a maximum of 500mA, at the specified output voltage, with an operating junction temperature of up to 125°C. Once the power dissipation and thermal resistance is known, the maximum junction temperature of the device can be calculated. The power dissipation by the IC can be calculated from the power efficiency diagram Figure 5 and subtracting the power dissipated by the inductor due to its serial resistance (ESR). The inductor ESR is dependent, not only upon the size and type of inductor, but also upon the switching frequency, which depends on the load and VIN. Some inductor manufacturers provide full information regarding the variation of the inductor ESR with the switching frequency. This information can be used to show that, at high switching frequency (~2 MHz) and maximum load, the power dissipated by the inductor can exceed the power dissipated by the IC package itself. The actual thermal resistance depends upon the thermal characteristics of the SC-70 surface-mount package and the surrounding printed circuit board (PCB) copper to which it is mounted. This can be improved by providing a heat sink of surrounding copper ground on the PCB. Depending on the size of the copper area, the resulting θJA can be reduced below 280°C/W. The addition of backside copper with through holes, stiffeners, and other enhancements can also help reduce thermal resistance. The heat contributed by the dissipation of other devices, particularly the inductor, located nearby, must be included in the design considerations. Once the limiting parameters are determined, the design can be modified to ensure that the device remains within specified operating conditions even if the maximum load is applied permanently. In short circuit VOUT-to-GND condition, the FAN5358 is fully protected and the power dissipated is internally reduced below 100mW. Overload conditions at minimum VIN should be considered as worst case, when it is possible for the device to enter a thermal cycling loop in which the circuit enters a shutdown condition, cools, re-enables, and again overheats and shuts down repeatedly due to an unmanaged fault condition. The diagram in Figure 20 was determined experimentally, using the recommended two-layer PCB in still air, to be used as a thermal guide. 55 60 65 70 75 80 85 90 2.7 2.9 3.1 3.3 3.5 Input Voltage (V) Safe Operating Area for 500mA Load Area Where Thermal Protection May Trigger Figure 20. Maximum Ambient Temperature vs. Input Voltage at 500mA |
Аналогичный номер детали - FAN5358S712X |
|
Аналогичное описание - FAN5358S712X |
|
|
ссылки URL |
Конфиденциальность |
ALLDATASHEETRU.COM |
Вашему бизинису помогли Аллдатащит? [ DONATE ] |
Что такое Аллдатащит | реклама | контакт | Конфиденциальность | обмен ссыками | поиск по производителю 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 |