Datasheet LT1944-1 (Analog Devices) - 4
| Manufacturer | Analog Devices |
| Description | Dual Micropower Step-Up DC/DC Converter |
| Pages / Page | 8 / 4 — BLOCK DIAGRA. Figure 1. LT1944-1 Block Diagram. OPERATIO |
| File Format / Size | PDF / 151 Kb |
| Document Language | English |
BLOCK DIAGRA. Figure 1. LT1944-1 Block Diagram. OPERATIO
Model Line for this Datasheet
Text Version of Document
LT1944-1
W BLOCK 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 1.5µs 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 – 12mV 21mV – A2 A2B SWITCHER 1 SWITCHER 2 GND PGND PGND 3 9 7 1944-1 BD
Figure 1. LT1944-1 Block Diagram U OPERATIO
The LT1944-1 uses a constant off-time control scheme to current ramps back up to 100mA, then A2 resets the one- provide high efficiencies over a wide range of output shot, again allowing L1 to deliver current to the output. current. Operation can be best understood by referring to This switching action continues until the output voltage is the block diagram in Figure 1. Q1 and Q2 along with R3 and charged up (until the FB1 pin reaches 1.23V), then A1 R4 form a bandgap reference used to regulate the output turns off the internal circuitry and the cycle repeats. The voltage. When the voltage at the FB1 pin is slightly above LT1944-1 contains additional circuitry to provide protec- 1.23V, comparator A1 disables most of the internal cir- tion during start-up and under short-circuit conditions. cuitry. Output current is then provided by capacitor C2, When the FB1 pin voltage is less than approximately which slowly discharges until the voltage at the FB1 pin 600mV, the switch off-time is increased to 1.5µs and the drops below the lower hysteresis point of A1 (typical current limit is reduced to around 70mA (70% of its hysteresis at the FB pin is 8mV). A1 then enables the normal value). This reduces the average inductor current internal circuitry, turns on power switch Q3, and the and helps minimize the power dissipation in the power current in inductor L1 begins ramping up. Once the switch switch and in the external inductor and diode. current reaches 100mA, comparator A2 resets the one- The second switching regulator operates in the same shot, which turns off Q3 for 400ns. L1 then delivers manner, but with a 175mA current limit and an off-time of current to the output through diode D1 as the inductor 1.5µs. With this longer off-time, switcher 2 is ideal for very current ramps down. Q3 turns on again and the inductor low duty cycle applications (i.e. Li-Ion to 5V boost converters). 4
W BLOCK 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 1.5µs 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 – 12mV 21mV – A2 A2B SWITCHER 1 SWITCHER 2 GND PGND PGND 3 9 7 1944-1 BD
Figure 1. LT1944-1 Block Diagram U OPERATIO
The LT1944-1 uses a constant off-time control scheme to current ramps back up to 100mA, then A2 resets the one- provide high efficiencies over a wide range of output shot, again allowing L1 to deliver current to the output. current. Operation can be best understood by referring to This switching action continues until the output voltage is the block diagram in Figure 1. Q1 and Q2 along with R3 and charged up (until the FB1 pin reaches 1.23V), then A1 R4 form a bandgap reference used to regulate the output turns off the internal circuitry and the cycle repeats. The voltage. When the voltage at the FB1 pin is slightly above LT1944-1 contains additional circuitry to provide protec- 1.23V, comparator A1 disables most of the internal cir- tion during start-up and under short-circuit conditions. cuitry. Output current is then provided by capacitor C2, When the FB1 pin voltage is less than approximately which slowly discharges until the voltage at the FB1 pin 600mV, the switch off-time is increased to 1.5µs and the drops below the lower hysteresis point of A1 (typical current limit is reduced to around 70mA (70% of its hysteresis at the FB pin is 8mV). A1 then enables the normal value). This reduces the average inductor current internal circuitry, turns on power switch Q3, and the and helps minimize the power dissipation in the power current in inductor L1 begins ramping up. Once the switch switch and in the external inductor and diode. current reaches 100mA, comparator A2 resets the one- The second switching regulator operates in the same shot, which turns off Q3 for 400ns. L1 then delivers manner, but with a 175mA current limit and an off-time of current to the output through diode D1 as the inductor 1.5µs. With this longer off-time, switcher 2 is ideal for very current ramps down. Q3 turns on again and the inductor low duty cycle applications (i.e. Li-Ion to 5V boost converters). 4
