Datasheet LT3580 (Analog Devices) - 7

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