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LT3467 Datasheet(PDF) 7 Page - Linear Technology |
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LT3467 Datasheet(HTML) 7 Page - Linear Technology |
7 / 28 page LT3580 7 3580fg OPERATION Figure 1. SEPIC Topology Allows for the Input to Span the Output Voltage. Coupled or Uncoupled Inductors Can Be Used. Follow Noted Phasing if Coupled Figure 2. Dual Inductor Inverting Topology Results in Low Output Ripple. Coupled or Uncoupled Inductors Can Be Used. Follow Noted Phasing if Coupled D1 SHUTDOWN L2 C3 L1 R1 VIN > VOUT OR VIN = VOUT OR VIN < VOUT VOUT VIN SW 3580 F01 LT3580 RT RC C2 SHDN GND FB VC SYNC SS RT CC CSS C1 + + • • D1 SHUTDOWN C3 L1 R1 VIN VOUT VIN SW 3580 F02 LT3580 RT RC C2 SHDN GND FB VC SYNC SS RT CC CSS C1 L2 + + •• SEPIC Topology The LT3580 can be configured as a SEPIC (single-ended primary inductance converter). This topology allows for the input to be higher, equal, or lower then the desired output voltage. Output disconnect is inherently built into the SEPIC topology, meaning no DC path exists between the input and output. This is useful for applications requiring the output to be disconnected from the input source when the circuit is in shutdown. Inverting Topology The LT3580 can also work in a dual inductor inverting topology. The part’s unique feedback pin allows for the inverting topology to be built by simply changing the connection of external components. This solution results in very low output voltage ripple due to inductor L2 in series with the output. Abrupt changes in output capacitor current are eliminated because the output inductor deliv- ers current to the output during both the off-time and the on-time of the LT3580 switch. Start-Up Operation Several functions are provided to enable a very clean start-up for the LT3580. • First, the SHDN pin voltage is monitored by an internal voltage reference to give a precise turn-on voltage level. An external resistor (or resistor divider) can be connected from the input power supply to the SHDN pin to provide a user-programmable undervoltage lockout function. • Second, the soft-start circuitry provides for a gradual ramp-up of the switch current. When the part is brought out of shutdown, the external SS capacitor is first discharged (providing protection against SHDN pin glitches and slow ramping), then an integrated 275k resistor pulls the SS pin up to ~2.2V. By connecting an external capacitor to the SS pin, the voltage ramp rate on the pin can be set. Typical values for the soft-start capacitor range from 100nF to 1μF. • Finally, the frequency foldback circuit reduces the switching frequency when the FB pin is in a nominal range of 350mV to 900mV. This feature reduces the minimum duty cycle that the part can achieve thus allowing better control of the switch current during start-up. When the FB voltage is pulled outside of this range, the switching frequency returns to normal. Current Limit and Thermal Shutdown Operation The LT3580 has a current limit circuit not shown in the Block Diagram. The switch current is consistently moni- tored and not allowed to exceed the maximum switch current at a given duty cycle (see the Electrical Charac- teristics table). If the switch current reaches this value, the SR latch (SR1) is reset regardless of the state of the comparator (A1/A2). Also not shown in the Block Diagram is the thermal shutdown circuit. If the temperature of the part exceeds approximately 165°C, the SR2 latch is set regardless of the state of the comparator (A1/A2). A full soft-start cycle will then be initiated. The current limit and thermal shutdown circuits protect the power switch as well as the external components connected to the LT3580. |
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