Datasheet LT3579, LT3579-1 (Analog Devices) - 10

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OPERATION OPERATION – OVERVIEW
VIN VIN The LT3579 uses a constant-frequency, current mode ACTIVE/ control scheme to provide excellent line and load regula- 1.33V – RUVLO1 LOCKOUT tion. The part’s undervoltage lockout (UVLO) function, SHDN + together with soft-start and frequency foldback, offers a 11.6µA controlled means of starting up. Fault features are incor- RUVLO2 AT 1.33V (OPTIONAL) porated in the LT3579 to aid in the detection of output GND shorts, over-voltage, and overtemperature conditions. Refer to the Block Diagram (Figure  1) and the State 35791 F03 Diagram (Figure 2) for the following description of the
Figure 3. Configurable UVLO
part’s operation. The LT3579 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
Soft-Start of Switch Current
start-up for the LT3579. The soft-start circuitry provides for a gradual ramp-up of
Precise Turn-On Voltage
the switch current (refer to Commanded Switch Current The SHDN pin compares to an internal voltage refer- vs. SS in Typical Performance Characteristics). When the ence to give a precise turn on voltage level. Taking the part is brought out of shutdown, the external SS capaci- SHDN pin above 1.33V (typical) enables the part. Taking tor is first discharged which resets the states of the logic the SHDN pin below 0.3V shuts down the chip, resulting circuits in the chip. Then an integrated 250k resistor pulls in extremely low quiescent current. The SHDN pin has the SS pin to ~1.8V at a ramp rate set by the external 30mV of hysteresis to protect against glitches and slow capacitor connected to the pin. Once SS gets to 1.8V, the ramping. CLKOUT pin is enabled, and an internal regulator pul s the pin up quickly to ~2.1V. Typical values for the external
Undervoltage-Lockout (UVLO)
soft-start capacitor range from 100nF to 1μF. The SHDN pin can also be used to create a configurable
Soft-Start of External PMOS (if used)
UVLO. The UVLO function sets the turn on/off of the LT3579 at a desired input voltage (VIN The soft-start circuitry also gradually ramps up the GATE UVLO). Figure 3 shows how a resistor divider (or single resistor) from pin pull-down current which allows an external PMOS to V slowly turn on (M1 in Block Diagram). The GATE pin cur- IN to the SHDN pin can be used to set VINUVLO. RUVLO2 is optional. It may be left out, in which case set it to infinite rent increases linearly with SS voltage, with a maximum in the equation below. For increased accuracy, set R current of 933µA when the SS voltage gets above 500mV. UVLO2 ≤ 10kΩ. Pick R Note that if the GATE pin voltage is less than 2V for SS UVLO1 as follows: voltages exceeding 500mV, then the GATE pin impedance VINUVLO – 1.33V to ground is 2kΩ. The soft turn on of the external PMOS R UVLO1 = helps limit inrush current at start-up, making hot plugs ⎛ 1.33V ⎞ of the LT3579 feasible and safe. ⎜ ⎟ + 11.6µA ⎜ R ⎟ ⎝ UVLO2 ⎠ Rev. B 10 For more information www.analog.com 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 Application Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram State Diagram Operation Applications Information Appendix Typical Application Package Description Revision History Typical Application Related Parts