Electronic Components Datasheet Search |
|
LTC1439 Datasheet(PDF) 10 Page - Linear Technology |
|
LTC1439 Datasheet(HTML) 10 Page - Linear Technology |
10 / 32 page 10 LTC1438/LTC1439 OPERATION Main Control Loop The LTC1438/LTC1439 use a constant frequency, current mode step-down architecture. During normal operation, the top MOSFET is turned on each cycle when the oscillator sets the RS latch and turned off when the main current comparator I1 resets the RS latch. The peak inductor current at which I1 resets the RS latch is controlled by the voltage on the ITH1 (ITH2) pin, which is the output of each error amplifier (EA). The VPROG1 pin, described in the Pin Functions, allows the EA to receive a selectively attenuated output feedback voltage VFB1 from the SENSE – 1 pin while VPROG2 and VOSENSE2 allow EA to receive an output feed- back voltage VFB2 from either internal or external resistive dividers on the second controller. When the load current increases, it causes a slight decrease in VFB relative to the 1.19V reference, which in turn causes the ITH1 (ITH2) voltage to increase until the average inductor current matches the new load current. After the large top MOSFET has turned off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by current comparator I2, or the beginning of the next cycle. The top MOSFET drivers are biased from floating boot strap capacitor CB, which normally is recharged during each Off cycle. When VIN decreases to a voltage close to VOUT, however, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detec- tor counts the number of oscillator cycles that the top MOSFET remains on and periodically forces a brief off period to allow CB to recharge. The main control loop is shut down by pulling the RUN/ SS1 (RUN/SS2) pin low. Releasing RUN/SS1 (RUN/SS2) allows an internal 3 µA current source to charge soft start capacitor CSS. When CSS reaches 1.3V, the main control loop is enabled with the ITH1 (ITH2) voltage clamped at approximately 30% of its maximum value. As CSS contin- ues to charge, ITH1 (ITH2) is gradually released allowing normal operation to resume. When both RUN/SS1 and RUN/SS2 are low, all LTC1438/LTC1439 functions are shut down. Refer to the LTC1538-AUX/LTC1539 data sheet for 5V keep-alive applications. Comparator OV guards against transient overshoots > 7.5% by turning off the top MOSFET and keeping it off until the fault is removed. Low Current Operation Adaptive Power mode allows the LTC1439 to automati- cally change between two output stages sized for different load currents. The TGL1 (TGL2) and BG1 (BG2) pins drive large synchronous N-channel MOSFETs for operation at high currents, while the TGS1 (TGS2) pin drives a much smaller N-channel MOSFET used in conjunction with a Schottky diode for operation at low currents. This allows the loop to continue to operate at normal operating fre- quency as the load current decreases without incurring the large MOSFET gate charge losses. If the TGS1 (TGS2) pin is left open, the loop defaults to Burst Mode operation in which the large MOSFETs operate intermittently based on load demand. Adaptive Power mode provides constant frequency opera- tion down to approximately 1% of rated load current. This results in an order of magnitude reduction of load current before Burst Mode operation commences. Without the small MOSFET (i.e., no Adaptive Power mode) the transi- tion to Burst Mode operation is approximately 10% of rated load current. The transition to low current operation begins when com- parator I2 detects current reversal and turns off the bottom MOSFET. If the voltage across RSENSE does not exceed the hysteresis of I2 (approximately 20mV) for one full cycle, then on following cycles the top drive is routed to the small MOSFET at the TGS1 (TGS2) pin and the BG1 (BG2) pin is disabled. This continues until an inductor current peak exceeds 20mV/RSENSE or the ITH1 (ITH2) voltage exceeds 0.6V, either of which causes drive to be returned to the TGL1 (TGL2) pin on the next cycle. Two conditions can force continuous synchronous opera- tion, even when the load current would otherwise dictate low current operation. One is when the common mode voltage of the SENSE+ 1 (SENSE+ 2) and SENSE– 1 (SENSE – 2) pins are below 1.4V, and the other is when the SFB1 pin is below 1.19V. The latter condition is used to assist in secondary winding regulation, as described in the Applications Information section. (Refer to Functional Diagram) |
Similar Part No. - LTC1439 |
|
Similar Description - LTC1439 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List 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 |