Datasheet LT3581 (Analog Devices) - 10

LT3581
OPERATION OPERATION – OVERVIEW
VIN V The LT3581 uses a constant-frequency, current mode con- IN trol scheme to provide excellent line and load regulation. 1.33V ACTIVE/ – RUVLO1 LOCKOUT The part’s undervoltage lockout (UVLO) function, together SHDN with soft-start and frequency foldback, offers a controlled + 11.6µA means of starting up. Fault features are incorporated in the RUVLO2 AT 1.33V (OPTIONAL) LT3581 to aid in the detection of output shorts, over-volt- GND age, and overtemperature conditions. Refer to the Block Diagram (Figure 1) and the State Diagram (Figure 2) for 3581 F03 the following description of the part’s operation.
Figure 3. Configurable UVLO
The LT3581 also has internal UVLO circuitry that disables
OPERATION – START-UP
the chip when VIN < 2.3V (typical). Several functions are provided to enable a very clean start-up for the LT3581:
Soft-Start of Switch Current
The soft-start circuitry provides for a gradual ramp-up
Precise Turn-On Voltage
of the switch current (refer to Commanded Current Limit The SHDN pin is compared to an internal voltage reference vs SS Voltage in Typical Performance Characteristics). to give a precise turn on voltage level. Taking the SHDN pin When the part is brought out of shutdown, the external above 1.33V (typical) enables the part. Taking the SHDN pin SS capacitor is first discharged which resets the states below 300mV shuts down the chip, resulting in extremely of the logic circuits in the chip. Then an integrated 250k low quiescent current. The SHDN pin has 30mV of hysteresis resistor pulls the SS pin to ~1.8V. The ramp rate of the SS to protect against glitches and slow ramping. pin voltage is set by this 250k resistor and the external capacitor connected to this pin. Once SS gets to 1.8V, the
Undervoltage Lockout (UVLO)
CLKOUT pin is enabled, and an internal regulator pulls The SHDN pin can also be used to create a configurable the pin up quickly to ~2.1V. Typical values for the external UVLO. The UVLO function sets the turn on/off of the LT3581 soft-start capacitor range from 100nF to 1μF. at a desired input voltage (VINUVLO). Figure 3 shows how a resistor divider (or single resistor) from V
Soft-Start of External PMOS (if used)
IN to the SHDN pin can be used to set VINUVLO. RUVLO2 is optional. It may The soft-start circuitry also gradually ramps up the GATE be left out, in which case set it to infinite in the equation pin pull-down current which allows an external PMOS to below. For increased accuracy, set RUVLO2 ≤ 10k. Pick slowly turn on (M1 in Block Diagram). The GATE pin current RUVLO1 as follows: increases linearly with the SS voltage, with a maximum V current of 933µA when the SS voltage gets above 500mV. R INUVLO – 1.33V UVLO1 = Note that if the GATE pin voltage is less than 2V for SS  1.33V  voltages exceeding 500mV, then the GATE pin impedance  R UVLO2  + 11.6µA to ground is 2kΩ. The soft turn on of the external PMOS helps limit inrush current at start-up, making hot-plugs of LT3581s feasible and safe. 3581fb 10 For more information www.linear.com/LT3581 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram State Diagram Operation Applications Information Appendix Typical Applications Typical Application Related Parts