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LTC1628I-PG Datasheet(PDF) 10 Page - Linear Technology |
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LTC1628I-PG Datasheet(HTML) 10 Page - Linear Technology |
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10 / 32 page  10 LTC1628/LTC1628-PG (Refer to Functional Diagram) Main Control Loop The LTC1628 uses a constant frequency, current mode step-down architecture with the two controller channels operating 180 degrees out of phase. During normal opera- tion, each top MOSFET is turned on when the clock for that channel 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 con- trolled by the voltage on the ITH pin, which is the output of each error amplifier EA. The VOSENSE pin receives the voltage feedback signal, which is compared to the internal reference voltage by the EA. When the load current in- creases, it causes a slight decrease in VOSENSE relative to the 0.8V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. After the top MOSFET has turned off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by current compara- tor 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 through an external diode when the top MOSFET turns off. As VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detector de- tects this and forces the top MOSFET off for about 500ns every tenth cycle to allow CB to recharge. The main control loop is shut down by pulling the RUN/SS pin low. Releasing RUN/SS allows an internal 1.2 µA current source to charge soft-start capacitor CSS. When CSS reaches 1.5V, the main control loop is enabled with the ITH voltage clamped at approximately 30% of its maximum value. As CSS continues to charge, the ITH pin voltage is gradually released allowing normal, full-current opera- tion. When both RUN/SS1 and RUN/SS2 are low, all LTC1628 controller functions are shut down, and the STBYMD pin determines if the standby 5V and 3.3V regulators are kept alive. Low Current Operation The FCB pin is a multifunction pin providing two func- tions: 1) to provide regulation for a secondary winding by temporarily forcing continuous PWM operation on controller 1 (or both controllers depending upon the FLTCPL pin); and 2) select between two modes of low current operation. When the FCB pin voltage is below 0.800V, the controller forces continuous PWM current mode operation. In this mode, the top and bottom MOSFETs are alternately turned on to maintain the output voltage independent of direction of inductor current. When the FCB pin is below VINTVCC – 2V but greater than 0.80V, the controller enters Burst Mode operation. Burst Mode operation sets a minimum output current level before inhibiting the top switch and turns off the synchro- nous MOSFET(s) when the inductor current goes nega- tive. This combination of requirements will, at low cur- rents, force the ITH pin below a voltage threshold that will temporarily inhibit turn-on of both output MOSFETs until the output voltage drops. There is 60mV of hysteresis in the burst comparator B tied to the ITH pin. This hysteresis produces output signals to the MOSFETs that turn them on for several cycles, followed by a variable “sleep” interval depending upon the load current. The resultant output voltage ripple is held to a very small value by having the hysteretic comparator after the error amplifier gain block. Constant Frequency Operation When the FCB pin is tied to INTVCC, Burst Mode operation is disabled and the forced minimum output current re- quirement is removed. This provides constant frequency, discontinuous (preventing reverse inductor current) cur- rent operation over the widest possible output current range. This constant frequency operation is not as efficient as Burst Mode operation, but does provide a lower noise, constant frequency operating mode down to approxi- mately 1% of designed maximum output current. Continuous Current (PWM) Operation Tying the FCB pin to ground will force continuous current operation. This is the least efficient operating mode, but may be desirable in certain applications. The output can source or sink current in this mode. When sinking current while in forced continuous operation, current will be forced back into the main power supply potentially boost- ing the input supply to dangerous voltage levels— BEWARE! OPERATIO |
Similar Part No. - LTC1628I-PG |
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Similar Description - LTC1628I-PG |
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