Datasheet LTC3637 (Analog Devices) - 10

LTC3637
OPERATION (Refer to Block Diagram)
the SS pin to ground. The 5µA current that is sourced
Input Voltage and Overtemperature Protection
out of this pin will create a smooth voltage ramp on the When using the LTC3637, care must be taken not to capacitor. If this ramp rate is slower than the internal exceed any of the ratings specified in the Absolute Maxi- 0.8ms soft-start, then the output voltage will be limited mum Ratings section. As an added safeguard, however, by the ramp rate on the SS pin instead. The internal and the LTC3637 incorporates an overtemperature shutdown external soft-start functions are reset on start-up and after feature. If the junction temperature reaches approximately an undervoltage or overvoltage event on the input supply. 180°C, the LTC3637 will enter thermal shutdown mode. The peak inductor current is not limited by the internal or Both power switches will be turned off and the SW node external soft-start functions; however, placing a capacitor will become high impedance. After the part has cooled from the ISET pin to ground does provide this capability. below 160°C, it will restart. The overtemperature level is not production tested.
Peak Inductor Current Programming
The LTC3637 additionally implements protection features The peak current comparator nominally limits the peak which inhibit switching when the input voltage is not within inductor current to 2.4A. This peak inductor current can a programmed operating range. By using a resistive di- be adjusted by placing a resistor from the ISET pin to vider from the input supply to ground, the RUN and OVLO ground. The 5µA current sourced out of this pin through pins can serve as a precise input supply voltage monitor. the resistor generates a voltage that adjusts the peak cur- Switching is disabled when either the RUN pin falls below rent comparator threshold. 1.1V or the OVLO pin rises above 1.21V, which can be During sleep mode, the current sourced out of the I configured to limit switching to a specific range of input SET pin is reduced to 1µA. The I supply voltage. Pulling the RUN pin below 700mV forces SET current is increased back to 5µA on the first switching cycle after exiting sleep mode. The a low quiescent current shutdown (3µA). Furthermore, if I the input voltage falls below 3.5V typical (3.7V maximum), SET current reduction in sleep mode, along with adding a filtering capacitor, C an internal undervoltage detector disables switching. ISET, from the ISET pin to ground, provides a method of reducing light load output voltage When switching is disabled, the LTC3637 can safely sustain ripple at the expense of lower efficiency and slightly de- input voltages up to the absolute maximum rating of 80V. graded load step transient response. Input supply undervoltage or overvoltage events trigger a soft-start reset, which results in a graceful recovery from
Dropout Operation
an input supply transient. When the input supply decreases toward the output sup- ply, the duty cycle increases to maintain regulation. The
High Input Voltage Considerations
P-channel MOSFET top switch in the LTC3637 allows When operating with an input voltage to output voltage dif- the duty cycle to increase all the way to 100%. At 100% ferential of more than 65V, a minimum output load current duty cycle, the P-channel MOSFET stays on continuously, of 10mA is required to maintain a well-regulated output providing output current equal to the peak current, which voltage under all operating conditions, including shutdown can be greater than 2A. The power dissipation of the mode. If this 10mA minimum load is not available, then LTC3637 can increase dramatically during dropout opera- the minimum output voltage that can be maintained by tion especially at input voltages less than 10V. The increased the LTC3637 is limited to VIN – 65V. power dissipation is due to higher potential output current and increased P-channel MOSFET on-resistance. See the Thermal Considerations section of the Applications Information for a detailed example. 3637fa 10 For more information www.linear.com/LTC3637 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