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LT1944-1 Datasheet(PDF) 4 Page - Linear Technology |
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LT1944-1 Datasheet(HTML) 4 Page - Linear Technology |
4 / 8 page 4 LT1944-1 BLOCK DIAGRA Figure 1. LT1944-1 Block Diagram OPERATIO The LT1944-1 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 FB1 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 FB1 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 100mA, 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 100mA, 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 FB1 pin reaches 1.23V), then A1 turns off the internal circuitry and the cycle repeats. The LT1944-1 contains additional circuitry to provide protec- tion during start-up and under short-circuit conditions. When the FB1 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 70mA (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. The second switching regulator operates in the same manner, but with a 175mA current limit and an off-time of 1.5 µs. With this longer off-time, switcher 2 is ideal for very low duty cycle applications (i.e. Li-Ion to 5V boost converters). – + – + 8 400ns ONE-SHOT DRIVER RESET DRIVER RESET ENABLE 12mV 0.12 Ω A2 A1 Q3 9 3 R4 140k R3 30k R6 40k R5 40k Q2 X10 Q1 1 VIN FB1 2 SHDN1 10 SW1 PGND GND 1944-1 BD L1 C2 VOUT1 VIN D1 R2 (EXTERNAL) R1 (EXTERNAL) VOUT1 C1 – + – + 1.5 µs ONE-SHOT ENABLE 21mV 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 L2 C3 VOUT2 VIN D2 R2B (EXTERNAL) R1B (EXTERNAL) VOUT2 SWITCHER 2 SWITCHER 1 |
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