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LTC1266-3.3 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1266-3.3 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC1266 LTC1266-3.3/LTC1266-5 RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1266 series current comparator has a threshold range which extends from a minimum of 25mV/RSENSE (when Burst Mode operation is enabled) to a maximum of 155mV/RSENSE. The current comparator threshold sets the peak of the inductor ripple current, yielding a maxi- mum 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 LTC1266 series and external component values yields: RSENSE = 100mV IMAX A graph for selecting RSENSE vs maximum output current is given in Figure 2. IBURST ≈ 15mV RSENSE ISC(PK) = 155mV RSENSE The LTC1266 series 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. L and CT Selection for Operating Frequency The LTC1266 series uses a constant off-time architecture with tOFF determined by an external timing capacitor CT. Each time the topside MOSFET switch turns on, the voltage on CT is reset to approximately 3.3V. During the off-time, CT is discharged by a current which is propor- tional to VOUT. The voltage on CT is analogous to the current in inductor L, which likewise decays at a rate proportional to VOUT. Thus the inductor value must track the timing capacitor value. The value of CT is calculated from the desired continuous mode operating frequency, f: CT = 1 2.6 • 104 • f assumes VIN = 2VOUT, (Figure 1 circuit). A graph for selecting CT vs frequency including the effects of input voltage is given in Figure 3. Figure 2. Selecting RSENSE The load current, below which Burst Mode operation commences, (IBURST), and the peak short-circuit cur- rent, (ISC(PK)), both track IMAX. Once RSENSE has been chosen, IBURST and ISC(PK) can be predicted from the following: MAXIMUM OUTPUT CURRENT (A) 0 75 100 1266 F02 50 25 0 4 8 2 6 10 Figure 3. Timing Capacitor Value FREQUENCY (kHz) 0 0 200 400 100 200 300 400 1266 F03 600 800 500 VIN = 12V VIN = 5V VOUT = 3.3V APPLICATIO S I FOR ATIO |
Similar Part No. - LTC1266-3.3_15 |
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Similar Description - LTC1266-3.3_15 |
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