Electronic Components Datasheet Search |
|
NCP1547 Datasheet(PDF) 10 Page - ON Semiconductor |
|
NCP1547 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 15 page NCP1547 http://onsemi.com 10 WDRV + 12 mA (VIN * VO ) VO2 VIN ) The base current of a bipolar transistor is equal to collector current divided by beta of the device. Beta of 60 is used here to estimate the base current. The Boost pin provides the base current when the transistor needs to be on. The power dissipated by the IC due to this current is WBASE + VO2 VIN IS 60 where: IS = DC switching current. When the power switch turns on, the saturation voltage and conduction current contribute to the power loss of a non−ideal switch. The power loss can be quantified as WSAT + VO VIN IS VSAT where: VSAT = saturation voltage of the power switch which is shown in Figure 7. The switching loss occurs when the switch experiences both high current and voltage during each switch transition. This regulator has a 30 ns turn−off time and associated power loss is equal to WS + IS VIN 2 30 ns fS The turn−on time is much shorter and thus turn−on loss is not considered here. The total power dissipated by the IC is sum of all the above WIC + WQ ) WDRV ) WBASE ) WSAT ) WS The IC junction temperature can be calculated from the ambient temperature, IC power dissipation and thermal resistance of the package. The equation is shown as follows, TJ + WIC RqJA ) TA Minimum Load Requirement As pointed out in the previous section, a minimum load is required for this regulator due to the pre−driver current feeding the output. Placing a resistor equal to VO divided by 12 mA should prevent any voltage overshoot at light load conditions. Alternatively, the feedback resistors can be valued properly to consume 12 mA current. COMPONENT SELECTION Input Capacitor In a buck converter, the input capacitor witnesses pulsed current with an amplitude equal to the load current. This pulsed current and the ESR of the input capacitors determine the VIN ripple voltage, which is shown in Figure 13. For VIN ripple, low ESR is a critical requirement for the input capacitor selection. The pulsed input current possesses a significant AC component, which is absorbed by the input capacitors. The RMS current of the input capacitor can be calculated using: IRMS + IO D(1 * D) where: D = switching duty cycle which is equal to VO/VIN. IO = load current. Figure 13. Input Voltage Ripple in a Buck Converter To calculate the RMS current, multiply the load current with the constant given by Figure 14 at each duty cycle. It is a common practice to select the input capacitor with an RMS current rating more than half the maximum load current. If multiple capacitors are paralleled, the RMS current for each capacitor should be the total current divided by the number of capacitors. Figure 14. Input Capacitor RMS Current can be Calculated by Multiplying Y Value with Maximum Load Current at any Duty Cycle 0 0.2 0.4 1.0 DUTY CYCLE 0 0.1 0.3 0.4 0.5 0.6 0.2 0.6 0.8 Selecting the capacitor type is determined by each design’s constraint and emphasis. The aluminum electrolytic capacitors are widely available at lowest cost. Their ESR and ESL (equivalent series inductor) are relatively high. Multiple capacitors are usually paralleled to achieve lower ESR. In addition, electrolytic capacitors usually need to be paralleled with a ceramic capacitor for filtering high frequency noises. The OS−CON are solid aluminum electrolytic capacitors, and therefore has a much lower ESR. Recently, the price of the OS−CON capacitors has dropped significantly so that it is now feasible to use |
Similar Part No. - NCP1547_11 |
|
Similar Description - NCP1547_11 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List 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 |