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LT1945 Datasheet(PDF) 4 Page - Linear Technology |
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LT1945 Datasheet(HTML) 4 Page - Linear Technology |
4 / 8 page LT1945 4 1945fa PIN FUNCTIONS NFB1 (Pin 1): Feedback Pin for Switcher 1. Set the output voltage by selecting values for R1 and R2. SHDN1 (Pin 2): Shutdown Pin for Switcher 1. Tie this pin to 0.9V or higher to enable device. Tie below 0.25V to turn it off. GND (Pin 3): Ground. Tie this pin directly to the local ground plane. SHDN2 (Pin 4): Shutdown Pin for Switcher 2. Tie this pin to 0.9V or higher to enable device. Tie below 0.25V to turn it off. FB2 (Pin 5): Feedback Pin for Switcher 2. Set the output voltage by selecting values for R1B and R2B. SW2 (Pin 6): Switch Pin for Switcher 2. This is the collector of the internal NPN power switch. Minimize the metal trace area connected to the pin to minimize EMI. PGND (Pins 7, 9): Power Ground. Tie these pins directly to the local ground plane. Both pins must be tied. VIN (Pin 8): Input Supply Pin. Bypass this pin with a capacitor as close to the device as possible. SW1 (Pin 10): Switch Pin for Switcher 1. This is the collector of the internal NPN power switch. Minimize the metal trace area connected to the pin to minimize EMI. BLOCK DIAGRAM – + – + 8 400ns ONE-SHOT DRIVER RESET RESET ENABLE 42mV 0.12Ω 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 0.12Ω 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 Figure 1. LT1945 Block Diagram OPERATION 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 |
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