поискавой системы для электроныых деталей |
|
LM2655 датащи(PDF) 11 Page - National Semiconductor (TI) |
|
|
LM2655 датащи(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page Design Procedure (Continued) within 30ns. An input capacitance of less than 1000pF is recommended. Several suitable MOSFETs are shown in Table 1. EXTERNAL SCHOTTKY DIODE (Syncronous) A Schottky diode is recommended to prevent the intrinsic body diode of the low-side MOSFET from conducting during the deadtime in PWM operation. If the body diode turns on, there is extra power dissipation in the body diode because of the reverse-recovery current and higher forward voltage drop. In addition, the high-side MOSFET has more switching loss because the diode reverse-recovery current adds to the high-side MOSFET turn-on current. These losses degrade the efficiency by 1-2%. The improved efficiency and noise immunity with the Schottky diode become more obvious with increasing input voltage and load current. It is important to place the diode very close to the switch pin of the LM2655. Extra parasitic impedance due to the trace between the switch pin and the cathode of the diode will cause the current limit to decrease. The breakdown voltage rating of the diode is preferred to be 25% higher than the maximum input voltage. Since it is on for a short period of time, the diode’s average current rating need only be 30% of the maximum output current. EXTERNAL SCHOTTKY DIODE (Asyncronous) In asyncronous mode, the output current commutates throught the schottky diode when the high-side MOSFET is turned off. Using a schottky diode with low forward voltage drop will minimize the effeciency loss in the diode. However, to achieve the greatest efficiency, the LM2655 should be operated in syncronous mode using a low-side MOSFET. Since the Schottky diode conducts for the entire second half of the duty cycle in asyncronous mode, it should be rated higher than the full load current. BOOST CAPACITOR The boost capacitor provides the extra votage needed to turn the high-side, n-channel MOSFET on. A 0.1 µF ceramic capacitor is recommended for the boost capacitor. The typi- cal voltage across the boost capacitor is 6.7V. SOFT-START CAPACITOR A soft-start capacitor is used to provide the soft-start feature. When the input voltage is first applied, or when the SD(SS) pin is allowed to go high, the soft-start capacitor is charged by a current source (approximately 2 µA). When the SD(SS) pin voltage reaches 0.6V (shutdown threshold), the internal regulator circuitry starts to operate. The current charging the soft-start capacitor increases from 2 µA to approximately 10 µA. With the SD(SS) pin voltage between 0.6V and 1.3V, the level of the current limit is zero, which means the output voltage is still zero. When the SD(SS) pin voltage increases beyond 1.3V, the current limit starts to increase. The switch duty cycle, which is controlled by the level of the current limit, starts with narrow pulses and gradually gets wider. At the same time, the output voltage of the converter increases towards the nominal value, which brings down the output voltage of the error amplifier. When the output of the error amplifier is less than the current limit voltage, it takes over the control of the duty cycle. The converter enters the normal current-mode PWM operation. The SD(SS) pin voltage is eventually charged up to about 2V. The soft-start time can be estimated as: T SS =CSS * 0.6V/2 µA + CSS * (2V−0.6V)/10 µA During start-up, the internal circuit is monitoring the soft-start voltage. When the softstart voltage reaches 2V, the under- voltage and overvoltage protections are enabled. If the output voltage doesn’t rise above 80% of the normal value before the soft-start reaches 2V, undervoltage protec- tion shut down the device. You can avoid this by either increasing the value of the soft-start capacitor, or using a LDELAY capacitor. LDELAY CAPACITOR The LDELAY capacitor (CDELAY) provides a means to con- trol undervoltage latch protection. By changing CDELAY, the user can adjust the time delay between the output voltage dropping below 80% of its nominal value and the part shut- ting off due to undervoltage latch protection. The LDELAY circuit consists ofa5µA current source in series with a user defined capacitor, CDELAY. The 5 µA current source is turned on whenever the output voltage is below 80% of its nominal value, otherwise this current source is off. With the output voltage below 80% of its nominal value, the 5 µA current source begins to charge CDELAY, as shown in Fig- ure 2. If the potential across CDELAY reaches 2V, undervolt- age latch protection will be enabled and the part will shut- down. If the output voltage recovers to above 80% of its nominal value before the potential across CDELAY reaches 2V, undervoltage latch protection will remain disabled. Hence, CDELAY sets a time delay by the following equation: T DELAY (ms) = CDELAY (nF) * 2V/5A Undervoltage latch protection can be disabled by tying the LDELAY pin to the ground. COMPENSATION COMPONENTS In the control to output transfer function, the first pole F p1 can be estimated as 1/(2 πR OUTCOUT); The ESR zero Fz1 of the output capacitor is 1/(2 πESRC OUT); Also, there is a high frequency pole F p2 in the range of 45kHz to 150kHz: F p2 =Fs/( πn(1−D)) whereD=V OUT/VIN, n = 1+0.348L/(VIN−VOUT)(L isinµHs and V IN and VOUT in volts). The total loop gain G is approximately 1000/I OUT where IOUT is in amperes. A Gm amplifier is used inside the LM2655. The output resis- tor R o of the Gm amplifier is about 80k Ω.C c1 and RC together with R o give a lag compensation to roll off the gain: F pc1 = 1/(2 πC c1(Ro+Rc)), Fzc1 = 1/2 πC c1Rc. 10128422 FIGURE 2. Undervoltage latch protection. www.national.com 11 |
Аналогичный номер детали - LM2655 |
|
Аналогичное описание - LM2655 |
|
|
ссылки URL |
Конфиденциальность |
ALLDATASHEETRU.COM |
Вашему бизинису помогли Аллдатащит? [ DONATE ] |
Что такое Аллдатащит | реклама | контакт | Конфиденциальность | обмен ссыками | поиск по производителю 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 |