Datasheet LT1617, LT1617-1 (Analog Devices) - 4

LT1617/LT1617-1
W BLOCK DIAGRA
C3 L1 L2 V V IN OUT D1 C1 V C2 IN SHDN SW 5 4 1 R5 R6 80k 80k A1 + ENABLE – Q1 Q2 400ns Q3 X10 ONE-SHOT DRIVER R3 RESET V 60k OUT + R4 R1 0.12Ω 280k (EXTERNAL) NFB 3 – 42mV* A2 R2 (EXTERNAL) GND 2 1617/-1 BD * 12mV FOR LT1617-1
Figure 1. LT1617 Block Diagram U OPERATIO
The LT1617 uses a constant off-time control scheme to inductors L1 and L2 begins ramping up. Once the switch provide high efficiencies over a wide range of output current reaches 350mA, comparator A2 resets the one- current. Operation can be best understood by referring to shot, which turns off Q3 for 400ns. L2 continues to deliver the block diagram in Figure 1. Q1 and Q2 along with R3 and current to the output while Q3 is off. Q3 turns on again and R4 form a bandgap reference used to regulate the output the inductor currents ramp back up until the switch voltage. When the voltage at the NFB pin is slightly below current reaches 350mA, then A2 again resets the one- –1.23V, comparator A1 disables most of the internal shot. This switching action continues until the output circuitry. Output current is then provided by capacitor C2, voltage is charged up (until the NFB pin reaches –1.23V), which slowly discharges until the voltage at the NFB pin then A1 turns off the internal circuitry and the cycle goes above the hysteresis point of A1 (typical hysteresis repeats. The LT1617-1 operates in the same manner, at the NFB pin is 8mV). A1 then enables the internal except the switch current is limited to 100mA (the A2 circuitry, turns on power switch Q3, and the current in reference voltage is 12mV instead of 42mV). 4