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LT1615IS5-1 Datasheet(PDF) 4 Page - Linear Integrated Systems |
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LT1615IS5-1 Datasheet(HTML) 4 Page - Linear Integrated Systems |
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4 / 8 page  4 LT1615/LT1615-1 sn16151 16151fas BLOCK DIAGRA Figure 1. LT1615 Block Diagram – + – + 5 400ns ONE-SHOT DRIVER RESET ENABLE 42mV* 0.12 Ω A2 A1 Q3 2 R4 140k R3 30k R6 40k R5 40k Q2 X10 Q1 3 VIN FB 4 SHDN 1 SW GND 1615/-1 BD L1 C2 VOUT VIN D1 R2 (EXTERNAL) R1 (EXTERNAL) VOUT C1 * 12mV FOR LT1615-1 OPERATIO The LT1615 uses a constant off-time control scheme to provide high efficiencies over a wide range of output current. Operation can be best understood by referring to the block diagram in Figure 1. Q1 and Q2 along with R3 and R4 form a bandgap reference used to regulate the output voltage. When the voltage at the FB pin is slightly above 1.23V, comparator A1 disables most of the internal cir- cuitry. Output current is then provided by capacitor C2, which slowly discharges until the voltage at the FB pin drops below the lower hysteresis point of A1 (typical hysteresis at the FB pin is 8mV). A1 then enables the internal circuitry, turns on power switch Q3, and the current in inductor L1 begins ramping up. Once the switch current reaches 350mA, comparator A2 resets the one- shot, which turns off Q3 for 400ns. L1 then delivers current to the output through diode D1 as the inductor current ramps down. Q3 turns on again and the inductor current ramps back up to 350mA, then A2 resets the one- shot, again allowing L1 to deliver current to the output. This switching action continues until the output voltage is charged up (until the FB pin reaches 1.23V), then A1 turns off the internal circuitry and the cycle repeats. The LT1615 contains additional circuitry to provide protection during start-up and under short-circuit conditions. When the FB pin voltage is less than approximately 600mV, the switch off-time is increased to 1.5 µs and the current limit is reduced to around 250mA (70% of its normal value). This reduces the average inductor current and helps minimize the power dissipation in the LT1615 power switch and in the external inductor and diode. The LT1615-1 operates in the same manner, except the switch current is limited to 100mA (the A2 reference voltage is 12mV instead of 42mV). |
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