Motore di ricerca datesheet componenti elettronici |
|
LM1771UMM Scheda tecnica(PDF) 8 Page - National Semiconductor (TI) |
|
|
LM1771UMM Scheda tecnica(HTML) 8 Page - National Semiconductor (TI) |
8 / 18 page Application Information THEORY OF APPLICATION The LM1771 synchronous buck controller has a control scheme that is referred to as adaptive on-time control. This topology relies on a fixed switch on-time to regulate the output voltage. This on-time is internally set by EEPROM and is available with three different set-points to allow for different frequency options. The LM1771 automatically ad- justs the on-time during operation inversely with the input voltage (VIN) to maintain a constant frequency. Therefore the switching frequency during continuous conduction mode is independent of the inductor and capacitor size unlike hysteretic switchers. At the beginning of the cycle the LM1771 turns on the high side PFET for a fixed duration. This on-time is predetermined (internally set by EEPROM and adjusted by V IN) and the switch will not turn off until the timer has completed its period. The PFET will then turn off for a minimum pre- determined time period. This minimum T OFF of 150ns is internally set and cannot be adjusted. This is to prevent false triggering from occurring on the comparator due to noise from the SW node transition. After the minimum T OFF period has expired, the PFET will remain off until the comparator trip-point has been reached. Upon passing this trip-point (set at 0.8V at the feedback pin), the PFET will turn back on and the process will repeat, thus regulating the output. The NFET control is complementary to the PFET control with the exception of a short dead-time to prevent shoot through from occurring. DEVICE OPERATION Timing Opinion Three versions of the LM1771 are available each with a predetermined T ON set internally by EEPROM. This TON setting will determine the switching frequency for the appli- cation. Derivation and calculation of the switching frequen- cy’s dependence on V IN and TON can be seen in the follow- ing section. In a PWM buck switcher the following equations can be manipulated to obtain the switching frequency. The first equation shows the standard duty-cycle equation given by the volts-seconds balance on the inductor with the following equations defining standard relationships: T ON =Dx TP Using these equations and solving for duty-cycle: D=f SW xTON Frequency can now be expressed as: Or simply written as: where, α =V IN xTON To maintain a set frequency in an application, α is always held constant by varying T ON inversely with VIN. The three versions of the LM1771 are identified by the on times at a V IN of 3.3V for consistency. For clarification see the table below: Product ID T ON @ 3.3V α (V µs) LM1771S 0.5µs 1.65 LM1771T 1.0µs 3.3 LM1771U 2.0µs 6.6 The variation of T ON versus VIN can also be expressed graphically. These graphs can be found in the typical curves section of the datasheet. With α being a constant regardless of the version of the LM1771 used, equation [6] shows that the only dependent variable remaining is V OUT. Since VOUT will be a constant in any application, the frequency will also remain constant. The switching frequency at which the application runs depends upon the V OUT desired and the LM1771 version chosen. For any V OUT, three frequency options (LM1771 versions) can be selected. This can be seen in the table below. The rec- ommended frequency range of operation is 100kHz to 1000kHz. VOUT Timing Options 500ns 1000ns 2000ns 0.8 485 242 121 1 606 303 152 1.2 727 364 182 1.5 909 455 227 1.8 1091 545 273 2.5 1515 758 379 3.3 2000 1000 500 Switching Frequency (kHz) of LM1771 based on output voltage and timing option. SHORT-CIRCUIT PROTECTION The LM1771 has an internal short circuit comparator that constantly monitors the feedback node (except during soft- start). If the feedback voltage drops below 0.55V (equivalent to the output voltage dropping below 68% of nominal), the comparator will trip causing the part to latch off. The LM1771 will not resume switching until the input voltage is taken below the UVLO threshold and then brought back into its normal operating range, or the part is disabled then re- enabled through the enable pin. The purpose of this function is to prevent a severe short circuit from causing damage to the application. Due to the fast transient response of the LM1771 a severe short on the output causing the feedback to drop would only occur if the load applied had an effective resistance that approaches the PMOS R DS(ON). PRECISION ENABLE The LM1771 features a precision enable circuit. If the volt- age on the EN pin is 1.2V or greater, the part is enabled and switching will occur. If the enable voltage falls below 1.2V, www.national.com 8 |
Codice articolo simile - LM1771UMM |
|
Descrizione simile - LM1771UMM |
|
|
Link URL |
Privacy Policy |
ALLDATASHEETIT.COM |
Lei ha avuto il aiuto da alldatasheet? [ DONATE ] |
Di alldatasheet | Richest di pubblicita | contatti | Privacy Policy | scambio Link | Ricerca produttore 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 |