Electronic Components Datasheet Search |
|
NCV8878 Datasheet(PDF) 9 Page - ON Semiconductor |
|
NCV8878 Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 15 page NCV8878 www.onsemi.com 9 leading edge blanking time, the peak current limit causes the power switch to turn off for the remainder of the cycle. Set the current limit with a resistor from ISNS to GND, with R = VCL / Ilimit. If the voltage across the current sense resistor exceeds the over current threshold voltage the device enters over current hiccup mode. The device will remain off for the hiccup time and then go through the soft−start procedure. UVLO Input Undervoltage Lockout (UVLO) is provided to ensure that unexpected behavior does not occur when VIN is too low to support the internal rails and power the controller. The IC will start up when enabled and VIN surpasses the UVLO threshold plus the UVLO hysteresis and will shut down when VIN drops below the UVLO threshold or the part is disabled. VDRV An internal regulator provides the drive voltage for the gate driver. Bypass with a ceramic capacitor to ground to ensure fast turn on times. The capacitor should be between 0.1 mF and 1 mF, depending on switching speed and charge requirements of the external MOSFET. VDRV uses an internal linear regulator to charge the VDRV bypass capacitor. VOUT must be decoupled at the IC by a capacitor that is equal or larger in value than the VDRV decoupling capacitor. APPLICATION INFORMATION Design Methodology This section details an overview of the component selection process for the NCV8878 in continuous conduction mode boost. It is intended to assist with the design process but does not remove all engineering design work. Many of the equations make heavy use of the small ripple approximation. This process entails the following steps: 1. Define Operational Parameters 2. Select Current Sense Resistor 3. Select Output Inductor 4. Select Output Capacitors 5. Select Input Capacitors 6. Select Compensator Components 7. Select MOSFET(s) 8. Select Diode 9. Design Notes 10. Determine Feedback Loop Compensation Network 1. Define Operational Parameters Before beginning the design, define the operating parameters of the application. These include: VIN(min): minimum input voltage [V] VIN(max): maximum input voltage [V] VOUT: output voltage [V] IOUT(max): maximum output current [A] ICL: desired typical cycle-by-cycle current limit [A] From this the ideal minimum and maximum duty cycles can be calculated as follows: Dmin + 1 * VIN(max) VOUT Dmax + 1 * VIN(min) VOUT Both duty cycles will actually be higher due to power loss in the conversion. The exact duty cycles will depend on conduction and switching losses. If the maximum input voltage is higher than the output voltage, the minimum duty cycle will be negative. This is because a boost converter cannot have an output lower than the input. In situations where the input is higher than the output, the output will follow the input, minus the diode drop of the output diode and the converter will not attempt to switch. If the calculated Dmax is higher the Dmax of the NCV8878, the conversion will not be possible. It is important for a boost converter to have a restricted Dmax, because while the ideal conversion ration of a boost converter goes up to infinity as D approaches 1, a real converter’s conversion ratio starts to decrease as losses overtake the increased power transfer. If the converter is in this range it will not be able to regulate properly. If the following equation is not satisfied, the device will skip pulses at high VIN: Dmin fs w ton(min) Where: fs: switching frequency [Hz] ton(min): minimum on time [s] 2. Select Current Sense Resistor Current sensing for peak current mode control and current limit relies on the MOSFET current signal, which is measured with a ground referenced amplifier. The easiest method of generating this signal is to use a current sense resistor from the source of the MOSFET to device ground. The sense resistor should be selected as follows: RS + VCL ICL Where: RS: sense resistor [W] VCL: current limit threshold voltage [V] ICL: desire current limit [A] 3. Select Output Inductor The output inductor controls the current ripple that occurs over a switching period. A high current ripple will result in excessive power loss and ripple current requirements. A low current ripple will result in a poor control signal and a slow current slew rate in case of load steps. A good starting point for peak to peak ripple is around 20−40% of the inductor current at the maximum load at the worst case VIN, but operation should be verified empirically. The worst case VIN is half of VOUT, or whatever VIN is closest to half of VOUT. After choosing a peak current ripple value, calculate the inductor value as follows: |
Similar Part No. - NCV8878_16 |
|
Similar Description - NCV8878_16 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | 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 |