LT1944 WBLOCK DIAGRA D1 D2 L1 L2 V V IN OUT1 VOUT2 VIN C1 C2 C3 VIN SHDN1 SW1 SW2 SHDN2 8 2 10 6 4 VIN R5 R6 R6B R5B 40k 40k 40k 40k A1 A1B + + ENABLE ENABLE VOUT1 VOUT2 – – R1 Q1B R1B (EXTERNAL) FB1 Q1 (EXTERNAL) Q2 400ns FB2 1 400ns Q2B Q3 Q3B 5 X10 ONE-SHOT ONE-SHOT X10 R2 DRIVER DRIVER R2B (EXTERNAL) (EXTERNAL) R3 RESET RESET R3B 30k 30k + + R4 R4B 0.12Ω 0.12Ω 140k 140k – 42mV 42mV – A2 A2B GND PGND PGND 3 9 7 1944 BD Figure 1. LT1944 Block DiagramUOPERATIO The LT1944 uses a constant off-time control scheme to current ramps down. Q3 turns on again and the inductor provide high efficiencies over a wide range of output current ramps back up to 350mA, then A2 resets the one- current. Operation can be best understood by referring to shot, again allowing L1 to deliver current to the output. the block diagram in Figure 1. Q1 and Q2 along with R3 and This switching action continues until the output voltage is R4 form a bandgap reference used to regulate the output charged up (until the FB1 pin reaches 1.23V), then A1 voltage. When the voltage at the FB1 pin is slightly above turns off the internal circuitry and the cycle repeats. The 1.23V, comparator A1 disables most of the internal cir- LT1944 contains additional circuitry to provide protection cuitry. Output current is then provided by capacitor C2, during start-up and under short-circuit conditions. When which slowly discharges until the voltage at the FB1 pin the FB1 pin voltage is less than approximately 600mV, the drops below the lower hysteresis point of A1 (typical switch off-time is increased to 1.5µs and the current limit hysteresis at the FB pin is 8mV). A1 then enables the is reduced to around 250mA (70% of its normal value). internal circuitry, turns on power switch Q3, and the This reduces the average inductor current and helps current in inductor L1 begins ramping up. Once the switch minimize the power dissipation in the power switch and in current reaches 350mA, comparator A2 resets the one- the external inductor and diode. The second switching shot, which turns off Q3 for 400ns. L1 then delivers regulator operates in the same manner. current to the output through diode D1 as the inductor 4
