전자부품 데이터시트 검색엔진 |
|
LM2717MT-ADJ 데이터시트(PDF) 11 Page - National Semiconductor (TI) |
|
|
LM2717MT-ADJ 데이터시트(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page Buck Operation (Continued) the DC resistance. The inductance is related to the peak-to- peak inductor ripple current, the input and the output volt- ages (for 300kHz operation): A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, and current stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple current increases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resis- tance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss equal 2% of the output power. OUTPUT CAPACITOR The selection of C OUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant fre- quency, PWM mode is approximated by: The ESR term usually plays the dominant role in determining the voltage ripple. Low ESR ceramic, aluminum electrolytic, or tantalum capacitors (such as MuRata MLCC, Taiyo Yuden MLCC, Nichicon PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX TPS, and CDE polymer aluminum) is recom- mended. An aluminum electrolytic capacitor is not recom- mended for temperatures below −25˚C since its ESR rises dramatically at cold temperatures. Ceramic or tantalum ca- pacitors have much better ESR specifications at cold tem- perature and is preferred for low temperature applications. BOOTSTRAP CAPACITOR A 4.7nF ceramic capacitor or larger is recommended for the bootstrap capacitor. For applications where the input voltage is less than twice the output voltage a larger capacitor is recommended, generally 0.1µF to 1µF to ensure plenty of gate drive for the internal switches and a consistently low R DSON. SOFT-START CAPACITOR (BOTH REGULATORS) The LM2717-ADJ contains circuitry that can be used to limit the inrush current on start-up of the DC/DC switching regu- lators. This inrush current limiting circuitry serves as a soft- start. The external SS pins are used to tailor the soft-start for a specific application. A current (I SS) charges the external soft-start capacitor, C SS. The soft-start time can be esti- mated as: T SS =CSS*0.6V/ISS When programming the soft-start time use the equation given in the Soft-Start Capacitor section above. The soft- start function is used simply to limit inrush current to the device that could stress the input voltage supply. The soft- start time described above is the time it takes for the current limit to ramp to maximum value. When this function is used the current limit starts at a low value and increases to nominal at the set soft-start time. Under maximum load conditions the output voltage may rise at the same rate as the soft-start, however at light or no load conditions the output voltage will rise much faster as the switch will not need to conduct much current to charge the output capacitor. SHUTDOWN OPERATION (BOTH REGULATORS) The shutdown pins of the LM2717-ADJ are designed so that they may be controlled using 1.8V or higher logic signals. If the shutdown function is not to be used the pin may be left open. The maximum voltage to the shutdown pin should not exceed 7.5V. If the use of a higher voltage is desired due to system or other constraints it may be used, however a 100k or larger resistor is recommended between the applied volt- age and the shutdown pin to protect the device. SCHOTTKY DIODE The breakdown voltage rating of D 1 and D2 is preferred to be 25% higher than the maximum input voltage. The current rating for the diode should be equal to the maximum output current for best reliability in most applications. In cases where the input voltage is much greater than the output voltage the average diode current is lower. In this case it is possible to use a diode with a lower average current rating, approximately (1-D)*I OUT however the peak current rating should be higher than the maximum load current. LOOP COMPENSATION The general purpose of loop compensation is to meet static and dynamic performance requirements while maintaining stability. Loop gain is what is usually checked to determine small-signal performance. Loop gain is equal to the product of control-output transfer function and the output-control transfer function (the compensation network transfer func- tion). The DC loop gain of the LM2717 is usually around 55dB to 60dB when loaded. Generally speaking it is a good idea to have a loop gain slope that is -20dB /decade from a very low frequency to well beyond the crossover frequency. The crossover frequency should not exceed one-fifth of the switching frequency, i.e. 60kHz in the case of 300kHz switching frequency. The higher the bandwidth is, the faster the load transient response speed will potentially be. How- ever, if the duty cycle saturates during a load transient, further increasing the small signal bandwidth will not help. Since the control-output transfer function usually has very limited low frequency gain, it is a good idea to place a pole in the compensation at zero frequency, so that the low fre- quency gain will be relatively large. A large DC gain means high DC regulation accuracy (i.e. DC voltage changes little with load or line variations). The rest of the compensation scheme depends highly on the shape of the control-output plot. www.national.com 11 |
유사한 부품 번호 - LM2717MT-ADJ |
|
유사한 설명 - LM2717MT-ADJ |
|
|
링크 URL |
개인정보취급방침 |
ALLDATASHEET.CO.KR |
ALLDATASHEET 가 귀하에 도움이 되셨나요? [ DONATE ] |
Alldatasheet는? | 광고문의 | 운영자에게 연락하기 | 개인정보취급방침 | 링크교환 | 제조사별 검색 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 |