Datasheet LTC3639 (Analog Devices) - 9

LTC3639
OPERATION (Refer to Block Diagram) Start-Up and Shutdown
a filtering capacitor, CISET, from the ISET pin to ground, If the voltage on the RUN pin is less than 0.7V, the LTC3639 provides a method of reducing light load output voltage enters a shutdown mode in which all internal circuitry is ripple at the expense of lower efficiency and slightly de- disabled, reducing the DC supply current to 1.4µA. When the graded load step transient response. voltage on the RUN pin exceeds 1.21V, normal operation of
Dropout Operation
the main control loop is enabled. The RUN pin comparator has 110mV of internal hysteresis, and therefore must fall When the input supply decreases toward the output sup- below 1.1V to disable the main control loop. ply, the duty cycle increases to maintain regulation. The P-channel MOSFET top switch in the LTC3639 allows An internal 1ms soft-start function limits the ramp rate of the duty cycle to increase all the way to 100%. At 100% the output voltage on start-up to prevent excessive input duty cycle, the P-channel MOSFET stays on continuously, supply droop. If a longer ramp time and consequently less providing output current equal to the peak current, which supply droop is desired, a capacitor can be placed from the is twice the maximum load current when not in dropout. SS pin to ground. The 5µA current that is sourced out of this pin will create a smooth voltage ramp on the capacitor.
Input Undervoltage and Overvoltage Lockout
If this ramp rate is slower than the internal 1ms soft-start, then the output voltage will be limited by the ramp rate The LTC3639 additionally implements protection features on the SS pin instead. The internal and external soft-start which inhibit switching when the input voltage is not within functions are reset on start-up and after an undervoltage a programmable operating range. By use of a resistive or overvoltage event on the input supply. divider from the input supply to ground, the RUN and OVLO pins serve as a precise input supply voltage monitor.
Peak Inductor Current Programming
Switching is disabled when either the RUN pin falls below 1.1V or the OVLO pin rises above 1.21V, which can be The peak current comparator nominally limits the peak configured to limit switching to a specific range of input inductor current to 230mA. This peak inductor current supply voltage. Furthermore, if the input voltage falls below can be adjusted by placing a resistor from the ISET pin to 3.5V typical (3.8V maximum), an internal undervoltage ground. The 5µA current sourced out of this pin through detector disables switching. the resistor generates a voltage that adjusts the peak cur- rent comparator threshold. When switching is disabled, the LTC3639 can safely sustain input voltages up to the absolute maximum rating of 150V. During sleep mode, the current sourced out of the ISET pin Input supply undervoltage or overvoltage events trigger a is reduced to 1µA. The ISET current is increased back to 5µA soft-start reset, which results in a graceful recovery from on the first switching cycle after exiting sleep mode. The an input supply transient. ISET current reduction in sleep mode, along with adding 3639fc For more information www.linear.com/LTC3639 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts