 |
Motor de Búsqueda de Datasheet de Componentes Electrónicos |
|
|
Motor de Búsqueda de Datasheet de Componentes Electrónicos |
|
LM2717-ADJ Datasheet(PDF) 10 Page - National Semiconductor (TI) |
|
|
||||||||||||||||||||||||||||||
LM2717-ADJ Datasheet(HTML) 10 Page - National Semiconductor (TI) |
|
|
10 / 16 page  Buck Operation PROTECTION (BOTH REGULATORS) The LM2717-ADJ has dedicated protection circuitry running during normal operation to protect the IC. The Thermal Shut- down circuitry turns off the power devices when the die temperature reaches excessive levels. The UVP comparator protects the power devices during supply power startup and shutdown to prevent operation at voltages less than the minimum input voltage. The OVP comparator is used to prevent the output voltage from rising at no loads allowing full PWM operation over all load conditions. The LM2717- ADJ also features a shutdown mode for each converter decreasing the supply current to approximately 10µA (both in shutdown mode). CONTINUOUS CONDUCTION MODE The LM2717-ADJ contains current-mode, PWM buck regu- lators. A buck regulator steps the input voltage down to a lower output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady state), the buck regulator operates in two cycles. The power switch is connected between V IN and SW1 and SW2. In the first cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and the load current is supplied by C OUT and the rising current through the inductor. During the second cycle the transistor is open and the diode is forward biased due to the fact that the inductor current cannot instantaneously change direction. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined approximately as: where D is the duty cycle of the switch, D and D' will be required for design calculations. The LM2717-ADJ has a minimum switch ON time which corresponds to a minimum duty cycle of approximately 10% at 600kHz operation and approximately 5% at 300kHz op- eration. In the case of some high voltage differential appli- cations (low duty cycle operation) this minimum duty cycle may be exceeded causing the feedback pin over-voltage protection to trip as the output voltage rises. This will put the device into a PFM type operation which can cause an un- predictable frequency spectrum and may cause the average output voltage to rise slightly. If this is a concern the switch- ing frequency may be lowered and/or a pre-load added to the output to keep the device full PWM operation. Note that the OVP function monitors the FB pin so it will not function if the feedback resistor is disconnected from the output. Due to slight differences between the two converters it is recom- mended that Buck 1 be used for the lower of the two output voltages for best operation. DESIGN PROCEDURE This section presents guidelines for selecting external com- ponents. SETTING THE OUTPUT VOLTAGE The output voltage is set using the feedback pin and a resistor divider connected to the output as shown in Figure 4. The feedback pin voltage (V FB) is 1.258V, so the ratio of the feedback resistors sets the output voltage according to the following equation: INPUT CAPACITOR A low ESR aluminum, tantalum, or ceramic capacitor is needed between the input pin and power ground. This ca- pacitor prevents large voltage transients from appearing at the input. The capacitor is selected based on the RMS current and voltage requirements. The RMS current is given by: The RMS current reaches its maximum (I OUT/2) when V IN equals 2VOUT. This value should be calculated for both regulators and added to give a total RMS current rating. For an aluminum or ceramic capacitor, the voltage rating should be at least 25% higher than the maximum input voltage. If a tantalum capacitor is used, the voltage rating required is about twice the maximum input voltage. The tantalum ca- pacitor should be surge current tested by the manufacturer to prevent being shorted by the inrush current. The minimum capacitor value should be 47µF for lower output load current applications and less dynamic (quickly changing) load con- ditions. For higher output current applications or dynamic load conditions a 68µF to 100µF low ESR capacitor is rec- ommended. It is also recommended to put a small ceramic capacitor (0.1µF to 4.7µF) between the input pins and ground to reduce high frequency spikes. INDUCTOR SELECTION The most critical parameter for the inductor in a current mode switcher is the minimum value required for stable operation. To prevent subharmonic oscillations and achieve good phase margin a target minimum value for the inductor is: Where V IN is the minimum input voltage and RDSON is the maximum switch ON resistance. For best stability the induc- tor should be in the range of 0.5L MIN (absolute minimum) and 2L MIN. Using an inductor with a value less than 0.5LMIN can cause subharmonic oscillations. The inductor should meet this minimum requirement at the peak inductor current expected for the application regardless of what the inductor ripple current and output ripple voltage requirements are. A value larger than 2L MIN is acceptable if the ripple require- ments of the application require it but it may reduce the phase margin and increase the difficulty in compensating the circuit. The most important parameters for the inductor from an applications standpoint are the inductance, peak current and www.national.com 10 |
Número de pieza similar - LM2717-ADJ |
|
Descripción similar - LM2717-ADJ |
|
|
|
Enlace URL |
| Política de Privacidad |
| ALLDATASHEET.ES |
| ¿ALLDATASHEET es útil para Ud.? [ DONATE ] |
Todo acerca de Alldatasheet | Publicidad | Contáctenos | Política de Privacidad | Intercambio de Enlaces | Lista de Fabricantes 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 |
Spanish ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
