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LTC1267CG-ADJ Datasheet(PDF) 8 Page - Linear Technology |
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LTC1267CG-ADJ Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LTC1267 LTC1267-ADJ/LTC1267-ADJ5 The LTC1267 Compared to the LTC1159, LTC1149 and LTC1142 Family The LTC1267 family is a dual LTC1159. Identical to the LTC1159, the LTC1267 can reduce the quiescent and shutdown currents by making use of an internal switch which allows the driver and control sections to be powered from an external source to improve efficiency. The basic LTC1267 application circuit shown in Figure 1 is limited to a maximum input voltage of 30V due to external MOSFET breakdown. If the application does not require greater than 18V operation the LTC1142HV should be used. Component Selection The basic LTC1267 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of RSENSE. Once RSENSE is known, CT and L can be chosen. Next, the power MOSFETs and diode are selected. Finally, CIN and COUT are selected and the loop is compensated. Since the adjustable, 3.3V and 5V sections in the LTC1267 are identical, the process of component selection is the same for both sections. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1267 current comparators have a threshold range which extends from a minimum of 25mV/RSENSE to a maximum of 150mV/RSENSE. The current comparator threshold sets the peak of the inductor ripple current, yielding a maximum output current IMAX equal to the peak value less half the peak-to-peak ripple current. For proper Burst Mode operation, IRIPPLE(P-P) must be less than or equal to the minimum current comparator threshold. Since efficiency generally increases with ripple current, the maximum allowable ripple current is assumed, i.e., IRIPPLE(P-P) = 25mV/RSENSE (see CT and L Selection for Operating Frequency). Solving for RSENSE and allowing a margin for variations in the LTC1267 and external compo- nent values yields: RSENSE = 100mV IMAX APPLICATIO S I FOR ATIO The LTC1267 works well with values of RSENSE from 0.02 Ω to 0.2Ω. Figure 2 shows the selection of RSENSE vs maximum output current. MAXIMUM OUTPUT CURRENT (A) 0 0.20 0.15 0.10 0.05 0 4 LTC1267 • F02 1 2 3 5 Figure 2. Selecting RSENSE The load current below which Burst Mode operation com- mences, IBURST and the peak short-circuit current ISC(PK) both track IMAX. Once RSENSE has been chosen, IBURST and ISC(PK) can be predicted from the following: IBURST ≈ 15mV RSENSE ISC(PK) = 150mV RSENSE The LTC1267 automatically extends tOFF during a short circuit to allow sufficient time for the inductor current to decay between switch cycles. The resulting ripple current causes the average short-circuit current ISC(AVG) to be reduced to approximately IMAX. CT and L Selection for Operating Frequency Each regulator section of the LTC1267 uses a constant off- time architecture with tOFF determined by an external timing capacitor CT. The value of CT is calculated from the desired continuous mode operating frequency (fO): CT = 1 – 7.8 × 10–5 fO VOUT VIN ) ) A graph for selecting CT vs frequency including the effects of input voltage is given in Figure 3. |
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