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LT1617 Datasheet(PDF) 4 Page - Linear Technology |
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LT1617 Datasheet(HTML) 4 Page - Linear Technology |
4 / 8 page 4 LT1617/LT1617-1 BLOCK DIAGRA Figure 1. LT1617 Block Diagram – + – + 5 400ns ONE-SHOT DRIVER RESET ENABLE 42mV* 0.12 Ω A2 A1 Q3 2 R4 280k R3 60k R6 80k R5 80k Q2 X10 Q1 3 VIN NFB 4 SHDN 1 SW GND 1617/-1 BD L1 C2 VOUT VIN D1 R2 (EXTERNAL) R1 (EXTERNAL) VOUT C1 L2 C3 * 12mV FOR LT1617-1 OPERATIO The LT1617 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 NFB pin is slightly below –1.23V, comparator A1 disables most of the internal circuitry. Output current is then provided by capacitor C2, which slowly discharges until the voltage at the NFB pin goes above the hysteresis point of A1 (typical hysteresis at the NFB pin is 8mV). A1 then enables the internal circuitry, turns on power switch Q3, and the current in inductors L1 and L2 begins ramping up. Once the switch current reaches 350mA, comparator A2 resets the one- shot, which turns off Q3 for 400ns. L2 continues to deliver current to the output while Q3 is off. Q3 turns on again and the inductor currents ramp back up until the switch current reaches 350mA, then A2 again resets the one- shot. This switching action continues until the output voltage is charged up (until the NFB pin reaches –1.23V), then A1 turns off the internal circuitry and the cycle repeats. The LT1617-1 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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