Electronic Components Datasheet Search |
|
LM2636MTC Datasheet(PDF) 10 Page - National Semiconductor (TI) |
|
|
LM2636MTC Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Applications Information (Continued) result in a smaller C 1,C2 and a larger R1. However, too large an R 1 can also bring error due to the bias current required by the inverting input pin of the error amplifier. Calculations show that the following combination is a good one: R 2 =51Ω, C 1 = 0.022 µF, R1 = 5.6 kΩ,C2 = 820 pF. For a different application or different type of output capaci- tors, a different compensation scheme may be necessary. The user can either follow the steps above to figure the ap- propriate component values or contact the factory for help. MOSFET SELECTION The selection of MOSFET switches affects both the effi- ciency of the whole converter and the current limit setting. From an efficiency point of view it is suggested that for the high-side switch, only logic level MOSFETs be used. Stan- dard MOSFETs can be used for the low side switch when 12V is used to power the BOOTV pin. The lower loss asso- ciated with the MOSFETs is two-fold — Ohmic loss and switching loss. The Ohmic loss is easy to calculate whereas the switching loss is much more difficult to estimate. In gen- eral the switching loss is directly proportional to the switching frequency. As the power MOSFET technology advances, lower and lower gate charge devices will be available. That should allow the user to go to higher switching frequencies without the penalty of losing too much efficiency. As an example, let us select the MOSFETs for a converter with a target efficiency of 80% at a load of 2.8V, 14A. As- sume the inductors lose 1W, the capacitors lose 0.75W and the total switching loss at 300 kHz is 3.2W. The total allowed power loss is 9.8W, so the MOSFET Ohmic loss should not exceed 4.9W. Assume the two switches have the same con- duction loss, i.e., 2.5W each, then the ON resistance for the two switches is: The low side switch ON resistance is much higher than the high side because at 2.8V the duty cycle is higher than 50% and becomes even larger at full load. For the high side switch, an IRL3202 (TO-220 package) or IRL3202S (D 2PAK) should be sufficient. For the low side switch, an IRL3303 (TO-220 package) or IRL3303S (D 2PAK) should be suffi- cient. Since each FET is dissipating 3.2W/2 + 2.5W = 4.1W, it is suggested that appropriate heat sinks be used in the case of TO-220 package or large enough copper area be connected to the drain in the case of surface mount pack- age. CAPACITOR SELECTION The selection of capacitors is an extremely important step when designing a converter for a load such as the Pentium II. Since the typical slew rate of the load current dur- ing a large load transient is around 20A/µs to 30A/µs, the switching converter has to rely on the output capacitors to take care of the first few microseconds. Under such a current slew rate, ESR of the output capacitors is more of a concern than the ESL. Depending on the kind of capacitors being used, capacitance of the output capacitors may or may not be an important factor. When the output capacitance is too low, the converter may have to have a small output inductor to quickly supply current to the output capacitors when the load suddenly kicks in and to quickly stop supplying current when the load is suddenly removed. Multilayer ceramic (MLC) capacitors can have very low ESR but also a low capacitance value compared to other kinds of capacitors. Low ESR aluminum electrolytic capacitors tend to have large sizes and capacitances. Tantalum electrolytic capacitors can have a fairly low ESR with a much smaller size and capacitance than the aluminum capacitors. Certain OSCON capacitors present ultra low ESR and long life span. By the time the total ESR of the output capacitor bank reaches around 9 m Ω, the capacitance of the aluminum/ tantalum/OSCON capacitors is usually already in the milli- farad range. For those capacitors, ESR is the only factor to consider. MLCs can have the same amount of total ESR with much less capacitance, most probably under 100 µF. A very small inductor, ultra fast control loop and a high switching frequency become necessary in such a case to deal with the fast charging/discharging rate of the output capacitor bank. From a cost savings point of view, aluminum electrolytic ca- pacitors are the most popular choice for output capacitors. They have reasonably long life span and they tend to have huge capacitance to withstand the charging or discharging process during a load transient for a fairly long period. Sanyo MV-GX series gives good performance when enough of the capacitors are paralleled. The 6MV1500GX capacitor has a typical ESR of 44 m Ω. Five of these capacitors should be sufficient in the case of on-board power supply for a Pentium II motherboard. The challenge for input capacitors is the ripple current. The large ripple current drawn by the high side switch tends to generate quite some heat due to the capacitor ESR. The ripple current ratings in the capacitor catalogs are usually specified under the highest allowable temperature. In the case of desktop applications, those ratings seem too conser- vative. A good way to ensure enough number of capacitors is through lab evaluation. The input current RMS ripple value can be determined by the following equation: and the power loss in each input capacitor is: In the case of Pentium II power supply, the maximum output current is around 14A. Under the worst case when duty cycle is 50%, the maximum input capacitor RMS ripple current is half of output current, i.e., 7A. It is found that three Sanyo 16MV820GX capacitors are enough under room tempera- ture. The typical ESR of those capacitors is 44 m Ω.Sothe power loss in each of them is around (7A) 2 x44m Ω/32 = 0.24W. Note that the power loss in each capacitor is in- versely proportional to the square of the total number of ca- pacitors, which means the power loss in each capacitor quickly drops when the number of capacitors increases. INDUCTOR SELECTION The size of the output is determined by a number of param- eters. Basically the larger the inductor, the smaller the output ripple voltage, but the slower the converter’s response speed during a load transient. On the other hand, a smaller inductor requires higher switching frequency to maintain the same level of output ripple, and probably results in a more lossy converter, but has less inertia responding to load tran- www.national.com 10 |
Similar Part No. - LM2636MTC |
|
Similar Description - LM2636MTC |
|
|
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 |