Moteur de recherche de fiches techniques de composants électroniques |
|
LM2797MM-1.8 Fiches technique(PDF) 11 Page - National Semiconductor (TI) |
|
|
LM2797MM-1.8 Fiches technique(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Operation Description (Continued) TABLE 4. Typical BATOK functionality, with 1M Ω pull-up resistor connected between BATOK and V IN V IN EN BATOK State Internal BATOK Transistor State V(BATOK) > 2.85V H HIGH OFF V IN > 1.1V, < 2.65V H LOW ON 0V > 1.1V L LOW ON 0V ≤ 1.1V X LOW OFF V IN, ≤ 1.1V BATOK: INPUT VOLTAGE STATUS INDICATOR The BATOK pin is an NMOS-open-drain-logic signal that indicates the status of the input voltage. To function properly, the BATOK pin must be connected to a pull-up resistor, or other pull-up device. With a pull-up in place, V(BATOK) will be HIGH when V IN is at or above 2.85V. If the output falls below 2.65V (typ.), V(BATOK) will be 0V. There is hysteresis of 20mV (typ.) between the thresholds. The BATOK function is disabled and V(BATOK) is pulled down to 0V when the LM2797/98 is in shutdown (EN = 0V). Table 4 is a complete list of the typical BATOK regions of operation. SHUTDOWN The LM2797/98 is in shutdown mode when the voltage on the active-low logic enable pin (EN) is low. In shutdown, the LM2797/98 draws virtually no supply current. When in shut- down, the output of the LM2797/98 is completely discon- nected from the input, and will be 0V unless driven by an outside source. In some applications, it may be desired to disable the LM2797/98 and drive the output pin with another voltage source. This can be done, but the voltage on the output pin of the LM2797/98 must not be brought above the input voltage. The output pin will draw a small amount of current when driven externally due the internal feedback resistor divider connected between V OUT and GND. SOFT START The LM2797/98 employs soft start circuitry to prevent exces- sive input inrush currents during startup. At startup, the output voltage gradually rises from 0V to the nominal output voltage. This occurs in 400µs (typ.) with the LM2798. Turn-on time of the LM2797 is 100µs (typ.). Soft-start is engaged when the part is enabled, including situations where voltage is established simultaneously on the V IN and EN pins. THERMAL SHUTDOWN Protection from overheating-related damage is achieved with a thermal shutdown feature. When the junction tem- perature rises to 150oC (typ.), the part switches into shut- down mode. The LM2797/98 disengages thermal shutdown when the junction temperature of the part is reduced to 130oC (typ.). Due to its high efficiency, the LM2797/98 should not activate thermal shutdown (or exhibit related thermal cycling) when the part is operated within specified input voltage, output current, and ambient temperature op- erating ratings. SHORT-CIRCUIT PROTECTION The LM2797/98 short-circuit protection circuitry protects the device in the event of excessive output current and/or output shorts to ground. A graph of "Short-Circuit Current vs. Input Voltage" is provided in the Performance Characteristics section. Application Information OUTPUT VOLTAGE RIPPLE The voltage ripple on the output of the LM2797/98 is highly dependent on application conditions. The output capacitor, the input voltage, and the output current each play a signifi- cant part in determining the output voltage ripple. Due to the complexity of LM2797/98 operation, providing equations or models to approximate the magnitude of the ripple cannot be easily accomplished. The following general statements can be made, however The output capacitor will have a significant effect on output voltage ripple magnitude. Ripple magnitude will typically be linearly proportional to the output capacitance present. A low-ESR ceramic capacitor is recommended on the output to keep output voltage ripple low. Placing multiple capacitors in parallel can reduce ripple significantly. Doing this increases capacitance and reduces ESR (the effective net ESR is governed by the properties of parallel resistance). Placing two identical capacitors in parallel have twice the capaci- tance and half the ESR, as compared to one of these ca- pacitors all by itself. Similarly, if a large-value, high-ESR capacitor (tantalum, for example) is to be used as the pri- mary output capacitor, the net output ESR can be signifi- cantly reduced by placing a low-ESR ceramic capacitor in parallel with this primary output capacitor. Ripple is increased when the LM2797/98 is gain hopping. With high output currents, ripple is likely to vary significantly with input voltage, depending on whether on not the part is gain hopping. CAPACITORS The LM2797/98 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and alumi- num electrolytic capacitors generally are not recommended for use with the LM2797/98 due to their high ESR, as com- pared to ceramic capacitors. For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use with the LM2797/98. These capacitors have tight capacitance toler- ance (as good as ±10%) and hold their value over tempera- ture (X7R: ±15% over -55oCto125oC; X5R: ±15% over -55oCto85oC). www.national.com 11 |
Numéro de pièce similaire - LM2797MM-1.8 |
|
Description similaire - LM2797MM-1.8 |
|
|
Lien URL |
Politique de confidentialité |
ALLDATASHEET.FR |
ALLDATASHEET vous a-t-il été utile ? [ DONATE ] |
À propos de Alldatasheet | Publicité | Contactez-nous | Politique de confidentialité | Echange de liens | Fabricants 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 |