Motor de Búsqueda de Datasheet de Componentes Electrónicos |
|
LTC1704EGN Datasheet(PDF) 13 Page - Linear Technology |
|
LTC1704EGN Datasheet(HTML) 13 Page - Linear Technology |
13 / 28 page LTC1704/LTC1704B 13 1704bfa APPLICATIO S I FOR ATIO low, indicating that the output is out of regulation. For PGOOD to go high, both the outputs must be in regulation for more than 20 µs. PGOOD remains active during soft- start and current limit. Upon power-up, PGOOD is forced low. As soon as the RUN/SS and REGILM pins rise above the shutdown thresholds, the two pairs of power good comparators take over and control the transistor MPG directly. The 1 µs and 20µs delay ensures that short output transient glitches that are successfully “caught” by the power good comparators don’t cause momentary glitches at the PGOOD pin. Shutdown/Soft-Start The RUN/SS pin performs two functions: when pulled to ground, it shuts down the switcher drivers, and acts as a conventional soft-start pin, enforcing a maximum duty cycle limit proportional to the voltage at RUN/SS. An internal 3 µA current source pull-up is connected to the RUN/SS pin, allowing a soft-start ramp to be generated with a single external capacitor to ground. The 3 µA current source is active even when the LTC1704 is shut down, ensuring the device will start when any external pull-down at RUN/SS is released. The RUN/SS pin shuts down the switcher drivers when it falls below 0.5V (Figure 4). Between 0.5V and about 1V, the LTC1704 wakes up and the duty cycle is kept to minimum. As the potential at RUN/SS goes higher, the duty cycle increases linearly between 1V and 2V, reaching its final value of 90% when RUN/SS is above 2V. Some- where before this point, the feedback amplifier will as- sume control of the loop and the output will come into regulation. When RUN/SS rises to 1V below VCC , the MIN feedback comparator is enabled, and the LTC1704 voltage feedback loop is in full operation. Switcher Supply Current Limit The LTC1704 switcher supply includes an onboard cur- rent limit circuit that limits the maximum output current to a user-programmed level. It works by sensing the voltage drop across QB during the time that QB is on and compar- ing that voltage to a user-programmed voltage at IMAX. Since QB looks like a low value resistor during its on-time, the voltage drop across it is proportional to the current flowing in it. In a buck converter, the average current in the inductor is equal to the output current. This current also flows through QB during its on-time. Thus, by watching the voltage across QB, the LTC1704 can monitor the output current. Any time QB is on and the current flowing to the output is reasonably large, the SW node at the drain of QB will be somewhat negative with respect to PGND. The LTC1704 senses this voltage and inverts it to allow it to compare the sensed voltage with a positive voltage at the IMAX pin. The IMAX pin includes a trimmed 10µA pull-up, enabling the user to set the voltage at IMAX with a single resistor, RIMAX, to ground. The LTC1704 compares the two inputs and begins limiting the output current when the magnitude of the negative voltage at the SW pin is greater than the voltage at IMAX. The current limit detector is connected to an internal gm amplifier that pulls a current from the RUN/SS pin propor- tional to the difference in voltage magnitudes between the SW and IMAX pins. This current begins to discharge the soft-start capacitor at RUN/SS, reducing the duty cycle and controlling the output voltage until the current drops below the limit. The soft-start capacitor needs to move a fair amount before it has any effect on the duty cycle, adding a delay until the current limit takes effect (Figure 4). This allows the LTC1704 to experience brief overload conditions without affecting the output voltage regulation. Figure 4. Soft-Start Operation in Start Up and Current Limit 0V VOUT 5V 4V 2V 1V 0.5V MINIMUM DUTY CYCLE 0V 1704 F04 DRIVER DISABLE MODE LTC1704 ENABLE MIN COMPARATOR ENABLE RUN/SS CONTROLS DUTY CYCLE COMP CONTROLS DUTY CYCLE START-UP NORMAL OPERATION CURRENT LIMIT HARD CURRENT LIMIT |
Número de pieza similar - LTC1704EGN |
|
Descripción similar - LTC1704EGN |
|
|
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 |