Datasheet LTC3630A (Analog Devices) - 10

LTC3630A
operaTion (Refer to Block Diagram)
supply droop is desired, a capacitor can be placed from which can be greater than 1A. The power dissipation of the SS pin to ground. The 5µA current that is sourced the LTC3630A can increase dramatically during dropout out of this pin will create a smooth voltage ramp on the operation especially at input voltages less than 10V. The capacitor. If this ramp rate is slower than the internal increased power dissipation is due to higher potential 0.8ms soft-start, then the output voltage will be limited output current and increased P-channel MOSFET on- by the ramp rate on the SS pin instead. The internal and resistance. See the Thermal Considerations section of the external soft-start functions are reset on start-up and after Applications Information for a detailed example. an undervoltage event on the input supply.
Input Voltage and Overtemperature Protection
The peak inductor current is not limited by the internal or external soft-start functions; however, placing a capacitor When using the LTC3630A, care must be taken not to from the ISET pin to ground does provide this capability. exceed any of the ratings specified in the Absolute Maxi- mum Ratings section. As an added safeguard, however,
Peak Inductor Current Programming
the LTC3630A incorporates an overtemperature shutdown The peak current comparator nominally limits the peak feature. If the junction temperature reaches approximately inductor current to 1.2A. This peak inductor current can 180°C, the LTC3630A will enter thermal shutdown mode. be adjusted by placing a resistor from the I Both power switches will be turned off and the SW node SET pin to ground. The 5µA current sourced out of this pin through will become high impedance. After the part has cooled the resistor generates a voltage that adjusts the peak cur- below 160°C, it will restart. The overtemperature level is rent comparator threshold. not production tested. During sleep mode, the current sourced out of the I The LTC3630A can provide a programmable undervoltage SET pin is reduced to 1µA. The I lockout which can also serve as a precise input voltage SET current is increased back to 5µA on the first switching cycle after exiting sleep mode. The monitor by using a resistive divider from VIN to GND with I the tap connected to the RUN pin. Switching is enabled SET current reduction in sleep mode, along with adding a filtering capacitor, C when the RUN pin voltage exceeds 1.21V and is disabled ISET, from the ISET pin to ground, provides a method of reducing light load output voltage when dropping below 1.1V. Pulling the RUN pin below ripple at the expense of lower efficiency and slightly de- 700mV forces a low quiescent current shutdown (5µA). graded load step transient response. Furthermore, if the input voltage falls below 3.5V typi- cal (3.7V maximum), an internal undervoltage detector For applications requiring higher output current, the disables switching. LTC3630A provides a feedback comparator output pin (FBO) for combining the output current of multiple When switching is disabled, the LTC3630A can safely LTC3630As. By connecting the FBO pin of a “master” sustain input voltages up to the absolute maximum rating LTC3630A to the V of 80V. Input supply undervoltage events trigger a soft- FB pin of one or more “slave” LTC3630As, the output currents can be combined to source much start reset, which results in a graceful recovery from an more than 500mA. input supply transient.
Dropout Operation High Input Voltage Considerations
When the input supply decreases toward the output sup- When operating with an input voltage to output voltage ply, the duty cycle increases to maintain regulation. The differential of more than 65V, a minimum output load P-channel MOSFET top switch in the LTC3630A allows current of 5mA is required to maintain a well-regulated the duty cycle to increase all the way to 100%. At 100% output voltage under all operating conditions, including duty cycle, the P-channel MOSFET stays on continu- shutdown mode. If this 5mA minimum load is not available, ously, providing output current equal to the peak current, then the minimum output voltage that can be maintained by the LTC3630A is limited to VIN – 65V. 3630afc 10 For more information www.linear.com/LTC3630A 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