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LT3751EUFD-TR Datasheet(PDF) 11 Page - Linear Technology |
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LT3751EUFD-TR Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LT3751 11 3751f Information section for proper use of LVGATE). With the gate driver output high, the external NMOS turns on, forcing VTRANS – VDS(ON) across the primary winding. Consequently, current in the primary coil rises linearly at a rate (VTRANS – VDS(ON))/LPRI. The input voltage is mir- rored on the secondary winding –N • (VTRANS – VDS(ON)) which reverse-biases the diode and prevents current flow in the secondary winding. Thus, energy is stored in the core of the transformer. 3. Secondary Energy Transfer When current limit is reached, the current limit comparator resets the NMOS switch latch and the device enters the third phase of operation, secondary energy transfer. The energy stored in the transformer core forward-biases the diode and current flows into the output capacitor. During this time, the output voltage (neglecting the diode drop) is reflected back to the primary coil. If the target output voltage is reached, the VOUT comparator resets the master latch and the DONE pin goes low. Otherwise, the device enters the next phase of operation. 4. Discontinuous Mode Detection During secondary energy transfer to the output capacitor, (VOUT + VDIODE)/N will appear across the primary wind- ing. A transformer with no energy cannot support a DC voltage, so the voltage across the primary will decay to zero. In other words, the drain of the NMOS will ring down from VTRANS + (VOUT + VDIODE)/N to VTRANS. When the drain voltage falls to VTRANS + 20μA • RDCM, the DCM OPERATION Figure 3. Start-Up Protection Circuitry Figure 4. DCM Comparator Thresholds comparator sets the NMOS switch latch and a new charge cycle begins. Steps 2-4 continue until the target output voltage is reached. Start-Up Protection The LT3751 at start-up, when the output voltage is very low (or shorted), usually does not have enough VDRAIN node voltage to trip the DCM comparator. The part in start-up mode uses the internal 26kHz clock and an auxiliary current comparator. Figure 3 shows a simplified block diagram of the start-up circuitry. FROM AUXILIARY CURRENT COMPARATOR 3751 F03 FROM CLK FROM GATE DRIVER ON FROM DCM COMPARATOR RESET INCREMENT COUNTER 2 RESET INCREMENT COUNTER 1 SWITCH LATCH START-UP (DCM THRESHOLD = VTH1) BELOW VTH2 (WAIT FOR TIME-OUT) VDRAIN 3751 F04 VOUT DCM 1-SHOT VTH1 VTH2 V BOUNDARY-MODE (DCM THRESHOLD = VTH2) t Toggling the CHARGE pin always generates a start-up one-shot to turn on the external switch, initiating the charg- ing process. After the start-up one-shot, the LT3751 waits for either the DCM comparator to generate a one-shot or the output of the start-up protection circuitry going high, which ever comes first. If the switch drain node, VDRAIN, is below the DCM comparator threshold (see Entering Normal Boundary Mode), the DCM comparator will never fire and the start-up circuitry is dominant. |
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