 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
LP3875ET-ADJ Datasheet(PDF) 9 Page - National Semiconductor (TI) |
|
|
||||||||||||||||||||||||||||||
LP3875ET-ADJ Datasheet(HTML) 9 Page - National Semiconductor (TI) |
|
|
9 / 15 page  Application Hints SETTING THE OUTPUT VOLTAGE The output voltage is set using the resistors R1 and R2 (see Typical Application Circuit). The output is also dependent on the reference voltage (typically 1.216V) which is measured at the ADJ pin. The output voltage is given by the equation: V OUT =VADJ x(1+R1/R2) This equation does not include errors due to the bias current flowing in the ADJ pin which is typically about 10 nA. This error term is negligible for most applications. If R1 is > 100k Ω , a small error may be introduced by the ADJ bias current. The tolerance of the external resistors used contributes a significant error to the output voltage accuracy, with 1% resistors typically adding a total error of approximately 1.4% to the output voltage (this error is in addition to the tolerance of the reference voltage at V ADJ). EXTERNAL CAPACITORS Like any low-dropout regulator, external capacitors are re- quired to assure stability. These capacitors must be correctly selected for proper performance. INPUT CAPACITOR: An input capacitor of at least 10µF is required. Ceramic Ceramic, Tantalum, or Electrolytic capaci- tors may be used, and capacitance may be increased with- out limit OUTPUT CAPACITOR: An output capacitor is required for loop stability. It must be located less than 1 cm from the device and connected directly to the output and ground pins using traces which have no other currents flowing through them (see PCB Layout section). The minimum value of output capacitance that can be used for stable full-load operation is 10µF, but it may be increased without limit. The output capacitor must have an ESR value as shown in the stable region of the curve below. Tantalum capacitors are recommended for the output capacitor. ESR Curve 20074670 C FF (Feed Forward Capacitor) The capacitor C FF is required to add phase lead and help improve loop compensation. The correct amount of capaci- tance depends on the value selected for R1 (see Typical Application Circuit). The capacitor should be selected such that the zero frequency as given by the equation shown below is approximately 45 kHz: Fz = 45,000=1/(2x π xR1xC FF ) A good quality ceramic with X5R or X7R dielectric should be used for this capacitor. SELECTING A CAPACITOR It is important to note that capacitance tolerance and varia- tion with temperature must be taken into consideration when selecting a capacitor so that the minimum required amount of capacitance is provided over the full operating tempera- ture range. In general, a good Tantalum capacitor will show very little capacitance variation with temperature, but a ce- ramic may not be as good (depending on dielectric type). Aluminum electrolytics also typically have large temperature variation of capacitance value. Equally important to consider is a capacitor’s ESR change with temperature: this is not an issue with ceramics, as their ESR is extremely low. However, it is very important in Tan- talum and aluminum electrolytic capacitors. Both show in- creasing ESR at colder temperatures, but the increase in aluminum electrolytic capacitors is so severe they may not be feasible for some applications (see Capacitor Character- istics Section). CAPACITOR CHARACTERISTICS CERAMIC: For values of capacitance in the 10 to 100 µF range, ceramics are usually larger and more costly than tantalums but give superior AC performance for bypassing high frequency noise because of very low ESR (typically less than 10 m Ω). However, some dielectric types do not have good capacitance characteristics as a function of voltage and temperature. Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V also exhibit a severe temperature effect, losing more than 50% of nomi- nal capacitance at high and low limits of the temperature range. X7R and X5R dielectric ceramic capacitors are strongly rec- ommended if ceramics are used, as they typically maintain a capacitance range within ±20% of nominal over full operat- ing ratings of temperature and voltage. Of course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance. TANTALUM: Solid Tantalum capacitors are recommended for use on the output because their typical ESR is very close to the ideal value required for loop compensation. They also work well as input capacitors if selected to meet the ESR requirements previously listed. Tantalums also have good temperature stability: a good quality Tantalum will typically show a capacitance value that varies less than 10-15% across the full temperature range of 125˚C to −40˚C. ESR will vary only about 2X going from the high to low temperature limits. The increasing ESR at lower temperatures can cause oscil- lations when marginal quality capacitors are used (if the ESR of the capacitor is near the upper limit of the stability range at room temperature). ALUMINUM: This capacitor type offers the most capaci- tance for the money. The disadvantages are that they are larger in physical size, not widely available in surface mount, and have poor AC performance (especially at higher fre- quencies) due to higher ESR and ESL. www.national.com 9 |
Similar Part No. - LP3875ET-ADJ |
|
Similar Description - LP3875ET-ADJ |
|
|
|
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 |
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
