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LTC1147-5 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1147-5 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LTC1147-3.3 LTC1147-5/LTC1147L sn1147 1147fds MAXIMUM OUTPUT CURRENT (A) 0 0.15 0.20 4 LTC1147 • F02 0.10 0.05 0 1 2 3 5 Figure 2. Selecting RSENSE L and CT Selection for Operating Frequency The LTC1147 series use a constant off-time architecture with tOFF determined by an external timing capacitor CT. Each time the P-channel 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: CT = 1 (1.3)(104)(f) VIN – VOUT VIN + VD ) ) Where VD is the drop across the Schottky diode. A graph for selecting CT versus frequency including the effects of input voltage is given in Figure 3. As the operating frequency is increased the gate charge losses will reduce efficiency (see Efficiency Consider- ations). The complete expression for operating frequency The LTC1147 series automatically extend 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. The basic LTC1147 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 MOSFET and D1 are selected. Finally, CIN and COUT are selected and the loop is compensated. The circuit shown in Figure 1 can be configured for operation up to an input voltage of 16V. If the application requires higher input voltage, then the synchronous switched LTC1149 should be used. Consult factory for lower minimum input voltage version. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1147 series current comparator has a thresh- old 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 LTC1147 series and external component values yields: RSENSE = 100mV IMAX A graph for selecting RSENSE versus maximum output current is given in Figure 2. The load current below in which Burst Mode operation commences, 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 APPLICATIO S I FOR ATIO |
Similar Part No. - LTC1147-5_15 |
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Similar Description - LTC1147-5_15 |
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