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CD43-100 Datasheet(PDF) 4 Page - Linear Technology |
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CD43-100 Datasheet(HTML) 4 Page - Linear Technology |
4 / 8 page 4 LT1945 1945f BLOCK DIAGRA Figure 1. LT1945 Block Diagram OPERATIO The LT1945 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 NFB1 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 NFB1 pin goes above the hysteresis point of A1 (typical hysteresis at the NFB1 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 to 350mA, then A2 again resets the one-shot. This switching action continues until the output voltage is charged up (until the NFB1 pin reaches –1.23V), then A1 turns off the internal circuitry and the cycle repeats. The second switching regulator is a step-up converter (which generates a positive output) but the basic opera- tion is the same.The LT1945 contains additional circuitry to provide protection during start-up and under short- circuit conditions. When the FB2 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 power switch and in the external inductor and diode. – + – + 8 400ns ONE-SHOT DRIVER RESET RESET ENABLE 42mV 0.12 Ω A2 A1 Q3 9 3 R4 280k R3 60k R6 80k R5 80k Q2 X10 Q1 1 VIN NFB1 2 SHDN1 10 SW1 PGND GND 1945 BD L1 C3 C2 VOUT1 VIN D1 R2 (EXTERNAL) R1 (EXTERNAL) VOUT1 C1 – + – + 400ns ONE-SHOT ENABLE 42mV 0.12 Ω A2B A1B Q3B 7 R4B 140k R3B 30k R6B 40k R5B 40k Q2B X10 Q1B 5 VIN FB2 4 SHDN2 6 SW2 PGND L3 L2 C4 VOUT2 VIN D2 R2B (EXTERNAL) R1B (EXTERNAL) VOUT2 SWITCHER 1 SWITCHER 2 DRIVER |
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