Datasheet LT8582 (Analog Devices) - 10

LT8582
OPERATION OPERATION – OVERVIEW
The LT8582 uses a constant frequency, current mode control – scheme to provide excellent line and load regulation. Each VIN VIN 1.31V ACTIVE/ LOCKOUT channel’s undervoltage lockout (UVLO) function, together RUVLO1 SHDN + with soft-start and frequency foldback, offer a controlled means of starting up. Fault features are incorporated into 12.3μA RUVLO2 AT 1.31V each channel of the LT8582 to facilitate the detection of (OPTIONAL) GND output shorts, overvoltage and overtemperature condi- tions. Please refer to the Block Diagram (Figure 1) and 8582 F03 the State Diagram (Figure 2) for the following description
Figure 3. Configurable UVLO
of the part’s operation.
Internal Undervoltage Lockout (UVLO) OPERATION – START-UP
Regardless of where external circuitry sets VINUVLO, the Several functions are provided to enable a very clean LT8582 also has internal UVLO circuitry that disables the start-up of both channels of the LT8582. chip when VIN < 2.3V (typical).
Precise Turn-On Voltage Soft-Start of Switch Current
The SHDN pin on each channel is compared to an internal The soft-start circuitry provides for a gradual ramp-up of voltage reference to give a precise turn on voltage level. Tak- the switch current in each channel (refer to Commanded ing each SHDN pin above 1.31V enables the corresponding Current Limit vs SS Voltage in Typical Performance channel. Taking each SHDN pin below 300mV shuts down Characteristics). When the channel is taken out of shut- the channel, resulting in extremely low quiescent current down, the external SS capacitor is first discharged. This for that channel. The SHDN pin has 35mV of hysteresis resets the state of the logic circuits in the channel. Then to protect against glitches and slow ramping. an integrated 250k resistor pulls the channel’s SS pin to ~1.84V. The ramp rate of the SS pin voltage is set by this
Configurable Undervoltage Lockout (UVLO)
250k resistor and the external capacitor connected to this The SHDN pin can also be used to create a configurable pin. Once SS gets to ~1.84V, the CLKOUT pin is enabled UVLO for each channel. This function sets the turn on/ and an internal regulator pulls the pin up quickly to ~2.1V. off of each of LT8582’s channels at a desired voltage Typical values for the external soft-start capacitor range (VINUVLO). Figure 3 shows how a resistor divider (or a from 100nF to 1μF. single resistor) from VIN to the SHDN pin can be used to program VIN
Soft-Start of External PMOS (if used)
UVLO. RUVLO2 is optional. If left out, set it to infinite in the equation below. For increased accuracy, set The soft-start circuitry also gradually ramps up the GATE RUVLO2 ≤ 10k. Pick RUVLO1 as follows: pin pull-down current for the corresponding channel. This allows an external PMOS to slowly turn on (M1 in Block VINUVLO R = – 1.31V Diagram). The GATE pin current increases linearly with UVLO1 ⎛ 1.31V ⎞ SS voltage, with a maximum current of 1mA when the ⎝⎜ R ⎠⎟ + 12.3μA UVLO2 SS voltage gets above 550mV. Note that if the GATE pin voltage is less than 2V for SS voltages exceeding 550mV, then the GATE pin impedance to ground is 2kΩ. The soft turn on of the external PMOS helps limit inrush current at start up, making hot plugs of LT8582s feasible. 8582f 10 Document Outline FEATURES DESCRIPTION APPLICATIONS 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 PACKAGE DESCRIPTION TYPICAL APPLICATION RELATED PARTS