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LTC1265 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1265 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LTC1265/LTC1265-3.3/LTC1265-5 capacitor discharges past VTH1 until its voltage drops below VTH2. Comparator S then trips and a sleep signal is generated. The circuit now enters into sleep mode with the power MOSFET turned off. In sleep mode, the LTC1265 is in standby and the load current is supplied by the output capacitor. All unused circuitry is shut off, reducing quies- cent current from 2mA to 160 µA. When the output capaci- tor discharges by the amount of the hysteresis of the comparator V, the P-channel switch turns on again and the process repeats itself. During Burst Mode operation the peak inductor current is set at 25mV/RSENSE. To avoid the operation of the current loop interfering with Burst Mode operation, a built-in offset VOS is incorporated in the gain stage. This prevents the current from increas- ing until the output voltage has dropped below a minimum threshold. Using constant off-time architecture, the operating fre- quency is a function of the voltage. To minimize the frequency variation as dropout is approached, the off-time controller increases the discharge current as VIN drops below VOUT + 2V. In dropout the P-channel MOSFET is turned on continuously (100% duty cycle) providing low dropout operation with VOUT ≅ VIN. The basic LTC1265 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 Schottky diode D1 is selected followed by CIN and COUT. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- oped across RSENSE, the threshold of the comparator determines the peak inductor current. Depending on the load current condition, the threshold of the comparator lies between 25mV/RSENSE and 150mV/RSENSE. The maxi- mum output current of the LTC1265 is: IOUT(MAX) = 150mV RSENSE IRIPPLE 2 – (Amps) where IRIPPLE is the peak-to-peak inductor ripple current. At a relatively light load, the LTC1265 is in Burst Mode operation. In this mode the peak inductor current is set at 25mV/RSENSE. To fully benefit from Burst Mode operation, the inductor current should be continuous during burst periods. Hence, the peak-to-peak inductor ripple current must not exceed 25mV/RSENSE. To account for light and heavy load conditions, the IOUT(MAX) is then given by: IOUT(MAX) = 150mV RSENSE 25mV 2 • RSENSE 137.5mV RSENSE – = (Amps) (Amps) Solving for RSENSE and allowing a margin of variations in the LTC1265 and extended component values yields: RSENSE = 100mV IOUT(MAX) ( Ω) The LTC1265 is rated with a capability to supply a maximum of 1.2A of output current. Therefore, the minimum value of RSENSE that can be used is 0.083Ω. A graph for selecting RSENSE versus maximum output is given in Figure 2. Figure 2. Selecting RSENSE (Refer to Functional Diagram) MAXIMUM OUTPUT CURRENT (A) 0 0.3 0.4 0.5 0.8 1265 G10 0.2 0.1 0 0.2 0.4 0.6 1 OPERATIO APPLICATIO S I FOR ATIO |
Similar Part No. - LTC1265_15 |
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Similar Description - LTC1265_15 |
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