Datasheet LTC3630 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | High Efficiency, 65V 500mA Synchronous Step-Down Converter |
| Pages / Page | 26 / 10 — operaTion (Refer to Block Diagram). Dropout Operation. Peak Inductor … |
| File Format / Size | PDF / 291 Kb |
| Document Language | English |
operaTion (Refer to Block Diagram). Dropout Operation. Peak Inductor Current Programming
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LTC3630
operaTion (Refer to Block Diagram)
An internal 0.8ms soft-start function limits the ramp rate
Dropout Operation
of the output voltage on start-up to prevent excessive input When the input supply decreases toward the output sup- supply droop. If a longer ramp time and consequently less ply, the duty cycle increases to maintain regulation. The supply droop is desired, a capacitor can be placed from P-channel MOSFET top switch in the LTC3630 allows the the SS pin to ground. The 5µA current that is sourced duty cycle to increase all the way to 100%. At 100% duty out of this pin will create a smooth voltage ramp on the cycle, the P-channel MOSFET stays on continuously, pro- capacitor. If this ramp rate is slower than the internal viding output current equal to the peak current, which can 0.8ms soft-start, then the output voltage will be limited be greater than 1A. The power dissipation of the LTC3630 by the ramp rate on the SS pin instead. The internal and can increase dramatically during dropout operation espe- external soft-start functions are reset on start-up and after cially at input voltages less than 10V. The increased power an undervoltage event on the input supply. dissipation is due to higher potential output current and The peak inductor current is not limited by the internal or increased P-channel MOSFET on-resistance. See the Ther- external soft-start functions; however, placing a capacitor mal Considerations section of the Applications Information from the ISET pin to ground does provide this capability. for a detailed example.
Peak Inductor Current Programming Input Voltage and Overtemperature Protection
The peak current comparator nominally limits the peak When using the LTC3630, care must be taken not to inductor current to 1.2A. This peak inductor current can exceed any of the ratings specified in the Absolute Maxi- be adjusted by placing a resistor from the ISET pin to mum Ratings section. As an added safeguard, however, ground. The 5µA current sourced out of this pin through the LTC3630 incorporates an overtemperature shutdown the resistor generates a voltage that adjusts the peak cur- feature. If the junction temperature reaches approximately rent comparator threshold. 180°C, the LTC3630 will enter thermal shutdown mode. During sleep mode, the current sourced out of the I Both power switches will be turned off and the SW node SET pin is reduced to 1µA. The I will become high impedance. After the part has cooled SET current is increased back to 5µA on the first switching cycle after exiting sleep mode. The below 160°C, it will restart. The overtemperature level is I not production tested. SET current reduction in sleep mode, along with adding a filtering capacitor, CISET, from the ISET pin to ground, The LTC3630 can provide a programmable undervoltage provides a method of reducing light load output voltage lockout which can also serve as a precise input voltage ripple at the expense of lower efficiency and slightly de- monitor by using a resistive divider from VIN to GND with graded load step transient response. the tap connected to the RUN pin. Switching is enabled For applications requiring higher output current, the when the RUN pin voltage exceeds 1.21V and is disabled LTC3630 provides a feedback comparator output pin (FBO) when dropping below 1.1V. Pulling the RUN pin below for combining the output current of multiple LTC3630s. By 700mV forces a low quiescent current shutdown (5µA). connecting the FBO pin of a “master” LTC3630 to the V Furthermore, if the input voltage falls below 3.5V typi- FB pin of one or more “slave” LTC3630s, the output currents cal (3.7V maximum), an internal undervoltage detector can be combined to source much more than 500mA. disables switching. When switching is disabled, the LTC3630 can safely sus- tain input voltages up to the absolute maximum rating of 70V. Input supply undervoltage events trigger a soft-start reset, which results in a graceful recovery from an input supply transient. 3630fd 10 For more information www.linear.com/LTC3630 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
operaTion (Refer to Block Diagram)
An internal 0.8ms soft-start function limits the ramp rate
Dropout Operation
of the output voltage on start-up to prevent excessive input When the input supply decreases toward the output sup- supply droop. If a longer ramp time and consequently less ply, the duty cycle increases to maintain regulation. The supply droop is desired, a capacitor can be placed from P-channel MOSFET top switch in the LTC3630 allows the the SS pin to ground. The 5µA current that is sourced duty cycle to increase all the way to 100%. At 100% duty out of this pin will create a smooth voltage ramp on the cycle, the P-channel MOSFET stays on continuously, pro- capacitor. If this ramp rate is slower than the internal viding output current equal to the peak current, which can 0.8ms soft-start, then the output voltage will be limited be greater than 1A. The power dissipation of the LTC3630 by the ramp rate on the SS pin instead. The internal and can increase dramatically during dropout operation espe- external soft-start functions are reset on start-up and after cially at input voltages less than 10V. The increased power an undervoltage event on the input supply. dissipation is due to higher potential output current and The peak inductor current is not limited by the internal or increased P-channel MOSFET on-resistance. See the Ther- external soft-start functions; however, placing a capacitor mal Considerations section of the Applications Information from the ISET pin to ground does provide this capability. for a detailed example.
Peak Inductor Current Programming Input Voltage and Overtemperature Protection
The peak current comparator nominally limits the peak When using the LTC3630, care must be taken not to inductor current to 1.2A. This peak inductor current can exceed any of the ratings specified in the Absolute Maxi- be adjusted by placing a resistor from the ISET pin to mum Ratings section. As an added safeguard, however, ground. The 5µA current sourced out of this pin through the LTC3630 incorporates an overtemperature shutdown the resistor generates a voltage that adjusts the peak cur- feature. If the junction temperature reaches approximately rent comparator threshold. 180°C, the LTC3630 will enter thermal shutdown mode. During sleep mode, the current sourced out of the I Both power switches will be turned off and the SW node SET pin is reduced to 1µA. The I will become high impedance. After the part has cooled SET current is increased back to 5µA on the first switching cycle after exiting sleep mode. The below 160°C, it will restart. The overtemperature level is I not production tested. SET current reduction in sleep mode, along with adding a filtering capacitor, CISET, from the ISET pin to ground, The LTC3630 can provide a programmable undervoltage provides a method of reducing light load output voltage lockout which can also serve as a precise input voltage ripple at the expense of lower efficiency and slightly de- monitor by using a resistive divider from VIN to GND with graded load step transient response. the tap connected to the RUN pin. Switching is enabled For applications requiring higher output current, the when the RUN pin voltage exceeds 1.21V and is disabled LTC3630 provides a feedback comparator output pin (FBO) when dropping below 1.1V. Pulling the RUN pin below for combining the output current of multiple LTC3630s. By 700mV forces a low quiescent current shutdown (5µA). connecting the FBO pin of a “master” LTC3630 to the V Furthermore, if the input voltage falls below 3.5V typi- FB pin of one or more “slave” LTC3630s, the output currents cal (3.7V maximum), an internal undervoltage detector can be combined to source much more than 500mA. disables switching. When switching is disabled, the LTC3630 can safely sus- tain input voltages up to the absolute maximum rating of 70V. Input supply undervoltage events trigger a soft-start reset, which results in a graceful recovery from an input supply transient. 3630fd 10 For more information www.linear.com/LTC3630 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
