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LTC1148HVCN Datasheet(PDF) 8 Page - Linear Technology |
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LTC1148HVCN Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC1148 LTC1148-3.3/LTC1148-5 114835fd APPLICATIO S I FOR ATIO The basic LTC1148 series application circuit (fixed output versions) is shown in Figure 1. External compo- nent 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 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 20V. If the application requires higher input voltage, then the LTC1149 or LTC1159 should be used. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1148 series current comparator has 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 LTC1148 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 which Burst Mode operation com- mences (IBURST)andthepeakshort-circuitcurrent(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 Figure 2. Selecting RSENSE MAXIMUM OUTPUT CURRENT (A) 0 0.15 0.20 4 LTC1148 • F02 0.10 0.05 0 1 2 3 5 The LTC1148 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 LTC1148 series uses 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 proportional 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 versus frequency including the effects of input voltage is given in Figure 3. As the operating frequency is increased the gate charge losses will be higher, reducing efficiency (see Efficiency Considerations). The complete expression for operating frequency of the circuit in Figure 1 is given by: |
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