Datasheet LTC3638 (Analog Devices) - 9

LTC3638
OPERATION (Refer to Block Diagram) Start-Up and Shutdown
By connecting the FBO pin of a master LTC3638 to the VFB If the voltage on the RUN pin is less than 0.7V, the LTC3638 pin of one or more slave LTC3638s, the output currents enters a shutdown mode in which all internal circuitry is can be combined to source 250mA times the number of disabled, reducing the DC supply current to 1.4µA. When the LTC3638s. 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 switch in the LTC3638 allows the duty An internal 1ms soft-start function limits the ramp rate of cycle to increase all the way to 100%. At 100% duty cycle, the output voltage on start-up to prevent excessive input the P-channel MOSFET stays on continuously, providing supply droop. If a longer ramp time and consequently less output current equal to the peak current, which is twice supply droop is desired, a capacitor can be placed from the maximum load current when not in dropout. the SS pin to ground. The 5µA current that is sourced out of this pin will create a smooth voltage ramp on the
Input Voltage and Overtemperature Protection
capacitor. If this ramp rate is slower than the internal 1ms soft-start, then the output voltage will be limited by the When using the LTC3638, care must be taken not to ramp rate on the SS pin. The internal and external soft- exceed any of the ratings specified in the Absolute Maxi- start functions are reset on start-up, after an undervoltage mum Ratings section. As an added safeguard, however, or overvoltage event on the input supply, and after an the LTC3638 incorporates an overtemperature shutdown overtemperature shutdown. feature. If the junction temperature reaches approximately 180°C, the LTC3638 will enter thermal shutdown mode.
Peak Inductor Current Programming
The power switch will be turned off and the SW node will become high impedance. After the part has cooled below The peak current comparator nominally limits the peak 160°C, it will restart. The overtemperature level is not inductor current to 575mA. This peak inductor current production tested. can be adjusted by placing a resistor from the ISET pin to ground. The 5µA current sourced out of this pin through The LTC3638 additionally implements protection features the resistor generates a voltage that adjusts the peak cur- which inhibit switching when the input voltage is not within rent comparator threshold. a programmable operating range. By use of a resistive divider from the input supply to ground, the RUN and During sleep mode, the current sourced out of the ISET pin OVLO pins serve as a precise input supply voltage monitor. is reduced to 1µA. The ISET current is increased back to 5µA Switching is disabled when either the RUN pin falls below on the first switching cycle after exiting sleep mode. The 1.1V or the OVLO pin rises above 1.21V, which can be ISET current reduction in sleep mode, along with adding configured to limit switching to a specific range of input a filtering network, RISET and CISET, from the ISET pin to supply voltage. Furthermore, if the input voltage falls below ground, provides a method of reducing light load output 3.5V typical (3.8V maximum), an internal undervoltage voltage ripple at the expense of lower efficiency and slightly detector disables switching. degraded load step transient response. When switching is disabled, the LTC3638 can safely sustain For applications requiring higher output current, the input voltages up to the absolute maximum rating of 140V. LTC3638 provides a feedback comparator output pin (FBO) Input supply undervoltage or overvoltage events trigger a for combining the output current of multiple LTC3638s. soft-start reset, which results in a graceful recovery from an input supply transient. 3638fa For more information www.linear.com/LTC3638 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 Typical Application Related Parts