Moteur de recherche de fiches techniques de composants électroniques |
|
LM1770 Fiches technique(PDF) 8 Page - National Semiconductor (TI) |
|
|
LM1770 Fiches technique(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page Application Information THEORY OF APPLICATION The LM1770 synchronous buck switcher has a control scheme that is referred to as constant 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 LM1770 automatically ad- justs the on-time during operation inversely with the input voltage (V IN) to maintain a constant frequency. Therefore the switching frequency during continuous conduction mode is independent of the inductor and capacitor size unlike hyster- etic switchers. At the beginning of the cycle the LM1770 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 LM1770 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 this 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 LM1770 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) LM1770S 0.5µs 1.65 LM1770T 1.0µs 3.3 LM1770U 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 LM1770 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 LM1770 version chosen. For any V OUT, three frequency options (LM1770 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 LM1770 based on output voltage and timing option. SHORT-CIRCUIT PROTECTION The LM1770 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 LM1770 will not resume switching until the input voltage is taken below the UVLO threshold and then brought back into its normal operating range. 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 LM1770 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). SOFT-START To limit in-rush current and allow for a controlled startup the LM1770 incorporates an internal soft-start scheme. Every time the input voltage rises through the UVLO threshold the LM1770 goes through an adaptive soft-start that limits the www.national.com 8 |
Numéro de pièce similaire - LM1770 |
|
Description similaire - LM1770 |
|
|
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 |