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LTC1143L 数据表(PDF) 9 Page - Linear Technology |
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LTC1143L 数据表(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC1143/LTC1143L LTC1143L-ADJ APPLICATIONS INFORMATION MAXIMUM OUTPUT CURRENT (A) 0 0.20 0.15 0.10 0.05 0 4 1143 F02 1 2 3 5 Figure 2. Selecting RSENSE The load current below 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 The LTC1143 series automatically extends tOFF during a short circuit to allow sufficient time for the inductor current to decay between switch cycles. The resulting ISC(AVG) to be reduced to approximately IMAX. L and CT Selection for Operating Frequency Each regulator section of the LTC1143 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 that 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: C f VV VV T IN OUT IN D = − + 1 13 10 4 . where VD is the drop across the 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 be higher, reducing efficiency (see Efficiency Considerations). The complete expression for operating frequency of the circuit in Figure 1 is given by: f t V V OFF OUT IN ≈− 1 1 where: tC V V OFF T REG OUT = 13 104 . VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT is the measured output voltage. Thus VREG/VOUT = 1 in regulation. Note that as VIN decreases, the frequency decreases. When the input-to-output voltage differential drops below 1.5V for a particular section, the LTC1143 series reduces tOFF in that section by increasing the discharge current in CT. This prevents audible operation prior to dropout. FREQUENCY (kHz) 0 50 100 150 200 LTC1143 F03 250 1000 800 600 400 200 0 300 VIN = 12V VIN = 10V VIN = 7V VSENSE = VOUT = 5V Figure 3. Timing Capacitor Value |
类似零件编号 - LTC1143L |
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类似说明 - LTC1143L |
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