công cụ tìm kiếm bảng dữ liệu linh kiện điện tử |
|
LM2622 bảng dữ liệu(PDF) 9 Page - National Semiconductor (TI) |
|
|
LM2622 bảng dữ liệu(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page Operation (Continued) The LM2622 is a current mode PWM boost converter. The signal flow of this control scheme has two feedback loops, one that senses switch current and one that senses output voltage. To keep a current programmed control converter stable above duty cycles of 50%, the inductor must meet certain criteria. The inductor, along with input and output voltage, will determine the slope of the current through the inductor (see Figure 2 (a)). If the slope of the inductor current is too great, the circuit will be unstable above duty cycles of 50%. A 10µH inductor is recommended for most 600 kHz applica- tions, while a 4.7µH inductor may be used for most 1.25 MHz applications. If the duty cycle is approaching the maximum of 85%, it may be necessary to increase the inductance by as much as 2X. See Inductor and Diode Selection for more detailed inductor sizing. The LM2622 provides a compensation pin (V C) to customize the voltage loop feedback. It is recommended that a series combination of R C and CC be used for the compensation network, as shown in the typical application circuit. For any given application, there exists a unique combination of R C and C C that will optimize the performance of the LM2622 circuit in terms of its transient response. The series combi- nation of R C and CC introduces a pole-zero pair according to the following equations: where R O is the output impedance of the error amplifier, approximately 1Meg Ω. For most applications, performance can be optimized by choosing values within the range 5k Ω≤ R C ≤ 20kΩ (R C can be up to 200k Ω if C C2 is used, see High Output Capacitor ESR Compensation) and 680pF ≤ C C ≤ 4.7nF. Refer to the Applications Information section for rec- ommended values for specific circuits and conditions. Refer to the Compensation section for other design requirement. COMPENSATION This section will present a general design procedure to help insure a stable and operational circuit. The designs in this datasheet are optimized for particular requirements. If differ- ent conversions are required, some of the components may need to be changed to ensure stability. Below is a set of general guidelines in designing a stable circuit for continu- ous conduction operation (loads greater than approximately 75mA), in most all cases this will provide for stability during discontinuous operation as well. The power components and their effects will be determined first, then the compensation components will be chosen to produce stability. INDUCTOR AND DIODE SELECTION Although the inductor sizes mentioned earlier are fine for most applications, a more exact value can be calculated. To ensure stability at duty cycles above 50%, the inductor must have some minimum value determined by the minimum input voltage and the maximum output voltage. This equa- tion is: where fs is the switching frequency, D is the duty cycle, and R DSON is the ON resistance of the internal switch taken from the graph "R DSON vs. VIN"inthe Typical Performance Char- acteristics section. This equation is only good for duty cycles greater than 50% (D>0.5), for duty cycles less than 50% the recommended values may be used. The corresponding in- ductor current ripple as shown in Figure 2 (a) is given by: The inductor ripple current is important for a few reasons. One reason is because the peak switch current will be the average inductor current (input current or I LOAD/D’) plus ∆i L. As a side note, discontinuous operation occurs when the inductor current falls to zero during a switching cycle, or ∆i L is greater than the average inductor current. Therefore, con- tinuous conduction mode occurs when ∆i L is less than the average inductor current. Care must be taken to make sure that the switch will not reach its current limit during normal operation. The inductor must also be sized accordingly. It should have a saturation current rating higher than the peak inductor current expected. The output voltage ripple is also affected by the total ripple current. The output diode for a boost regulator must be chosen correctly depending on the output voltage and the output current. The typical current waveform for the diode in con- tinuous conduction mode is shown in Figure 2 (b). The diode must be rated for a reverse voltage equal to or greater than the output voltage used. The average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. During short circuit testing, or if short circuit condi- tions are possible in the application, the diode current rating must exceed the switch current limit. Using Schottky diodes with lower forward voltage drop will decrease power dissipa- tion and increase efficiency. DC GAIN AND OPEN-LOOP GAIN Since the control stage of the converter forms a complete feedback loop with the power components, it forms a closed- loop system that must be stabilized to avoid positive feed- back and instability. A value for open-loop DC gain will be required, from which you can calculate, or place, poles and zeros to determine the crossover frequency and the phase margin. A high phase margin (greater than 45˚) is desired for the best stability and transient response. For the purpose of stabilizing the LM2622, choosing a crossover point well be- low where the right half plane zero is located will ensure sufficient phase margin. A discussion of the right half plane zero and checking the crossover using the DC gain will follow. INPUT AND OUTPUT CAPACITOR SELECTION The switching action of a boost regulator causes a triangular voltage waveform at the input. A capacitor is required to reduce the input ripple and noise for proper operation of the regulator. The size used is dependant on the application and board layout. If the regulator will be loaded uniformly, with www.national.com 9 |
Số phần tương tự - LM2622 |
|
Mô tả tương tự - LM2622 |
|
|
Link URL |
Chính sách bảo mật |
ALLDATASHEET.VN |
Cho đến nay ALLDATASHEET có giúp ích cho doanh nghiệp của bạn hay không? [ DONATE ] |
Alldatasheet là | Quảng cáo | Liên lạc với chúng tôi | Chính sách bảo mật | Trao đổi link | Tìm kiếm theo nhà sản xuất 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 |