Datasheet LTC3835 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Low IQ Synchronous Step-Down Controller |
| Pages / Page | 30 / 10 — OPERATION (Refer to Functional Diagram). Main Control Loop. Shutdown and … |
| Revision | F |
| File Format / Size | PDF / 818 Kb |
| Document Language | English |
OPERATION (Refer to Functional Diagram). Main Control Loop. Shutdown and Start-Up (RUN and TRACK/SS Pins). INTV
Model Line for this Datasheet
Text Version of Document
link to page 13 LTC3835
OPERATION (Refer to Functional Diagram) Main Control Loop
turns off. If the input voltage VIN decreases to a voltage The LTC3835 uses a constant-frequency, current mode close to VOUT, the loop may enter dropout and attempt step-down architecture. During normal operation, the to turn on the top MOSFET continuously. The dropout external top MOSFET is turned on when the clock sets detector detects this and forces the top MOSFET off for the RS latch, and is turned off when the main current about one twelfth of the clock period every tenth cycle to comparator, ICMP, resets the RS latch. The peak inductor allow CB to recharge. current at which ICMP trips and resets the latch is con-
Shutdown and Start-Up (RUN and TRACK/SS Pins)
trolled by the voltage on the ITH pin, which is the output of the error amplifier EA. The error amplifier compares The LTC3835 can be shut down using the RUN pin. Pulling the output voltage feedback signal at the VFB pin, (which this pin below 0.7V shuts down the main control loop is generated with an external resistor divider connected of the controller. A low disables the controller and most across the output voltage, VOUT, to ground) to the inter- internal circuits, including the INTVCC regulator, at which nal 0.800V reference voltage. When the load current time the LTC3835 draws only 10µA of quiescent current. increases, it causes a slight decrease in VFB relative to Releasing the RUN pin allows an internal 0.5µA current the reference, which cause the EA to increase the ITH volt- to pull up the pin and enable that controller. Alternatively, age until the average inductor current matches the new the RUN pin may be externally pulled up or driven directly load current. by logic. Be careful not to exceed the Absolute Maximum After the top MOSFET is turned off each cycle, the bot- rating of 7V on this pin. tom MOSFET is turned on until either the inductor current The start-up of the output voltage V starts to reverse, as indicated by the current comparator OUT is controlled by the voltage on the TRACK/SS pin. When the voltage on IR, or the beginning of the next clock cycle. the TRACK/SS pin is less than the 0.8V internal reference, the LTC3835 regulates the V
INTV
FB voltage to the TRACK/SS
CC/EXTVCC Power
pin voltage instead of the 0.8V reference. This allows Power for the top and bottom MOSFET drivers and most the TRACK/SS pin to be used to program a soft start by other internal circuitry is derived from the INTVCC pin. connecting an external capacitor from the TRACK/SS pin When the EXTVCC pin is left open or tied to a voltage less to SGND. An internal 1µA pull-up current charges this than 4.7V, an internal 5.25V low dropout linear regulator capacitor creating a voltage ramp on the TRACK/SS pin. supplies INTVCC power from VIN. If EXTVCC is taken above As the TRACK/SS voltage rises linearly from 0V to 0.8V 4.7V, the 5.25V regulator is turned off and a 7.5V low (and beyond), the output voltage VOUT rises smoothly dropout linear regulator is enabled that supplies INTVCC from zero to its final value. power from EXTVCC. If EXTVCC is less than 7.5V (but Alternatively the TRACK/SS pin can be used to cause greater than 4.7V), the 7.5V regulator is in dropout and the start-up of V INTV OUT to “track” that of another supply. CC is approximately equal to EXTVCC. When EXTVCC Typically, this requires connecting to the TRACK/SS pin an is greater than 7.5V (up to an absolute maximum rating external resistor divider from the other supply to ground of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC (see Applications Information section). pin allows the INTVCC power to be derived from a high efficiency external source such as one of the LTC3835 When the RUN pin is pulled low to disable the LTC3835, switching regulator outputs. or when VIN drops below its undervoltage lockout thresh- old of 3.5V, the TRACK/SS pin is pulled low by an internal The top MOSFET driver is biased from the floating boot- MOSFET. When in undervoltage lockout, the controller is strap capacitor CB, which normally recharges during each disabled and the external MOSFETs are held off. off cycle through an external diode when the top MOSFET Rev. F 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts
OPERATION (Refer to Functional Diagram) Main Control Loop
turns off. If the input voltage VIN decreases to a voltage The LTC3835 uses a constant-frequency, current mode close to VOUT, the loop may enter dropout and attempt step-down architecture. During normal operation, the to turn on the top MOSFET continuously. The dropout external top MOSFET is turned on when the clock sets detector detects this and forces the top MOSFET off for the RS latch, and is turned off when the main current about one twelfth of the clock period every tenth cycle to comparator, ICMP, resets the RS latch. The peak inductor allow CB to recharge. current at which ICMP trips and resets the latch is con-
Shutdown and Start-Up (RUN and TRACK/SS Pins)
trolled by the voltage on the ITH pin, which is the output of the error amplifier EA. The error amplifier compares The LTC3835 can be shut down using the RUN pin. Pulling the output voltage feedback signal at the VFB pin, (which this pin below 0.7V shuts down the main control loop is generated with an external resistor divider connected of the controller. A low disables the controller and most across the output voltage, VOUT, to ground) to the inter- internal circuits, including the INTVCC regulator, at which nal 0.800V reference voltage. When the load current time the LTC3835 draws only 10µA of quiescent current. increases, it causes a slight decrease in VFB relative to Releasing the RUN pin allows an internal 0.5µA current the reference, which cause the EA to increase the ITH volt- to pull up the pin and enable that controller. Alternatively, age until the average inductor current matches the new the RUN pin may be externally pulled up or driven directly load current. by logic. Be careful not to exceed the Absolute Maximum After the top MOSFET is turned off each cycle, the bot- rating of 7V on this pin. tom MOSFET is turned on until either the inductor current The start-up of the output voltage V starts to reverse, as indicated by the current comparator OUT is controlled by the voltage on the TRACK/SS pin. When the voltage on IR, or the beginning of the next clock cycle. the TRACK/SS pin is less than the 0.8V internal reference, the LTC3835 regulates the V
INTV
FB voltage to the TRACK/SS
CC/EXTVCC Power
pin voltage instead of the 0.8V reference. This allows Power for the top and bottom MOSFET drivers and most the TRACK/SS pin to be used to program a soft start by other internal circuitry is derived from the INTVCC pin. connecting an external capacitor from the TRACK/SS pin When the EXTVCC pin is left open or tied to a voltage less to SGND. An internal 1µA pull-up current charges this than 4.7V, an internal 5.25V low dropout linear regulator capacitor creating a voltage ramp on the TRACK/SS pin. supplies INTVCC power from VIN. If EXTVCC is taken above As the TRACK/SS voltage rises linearly from 0V to 0.8V 4.7V, the 5.25V regulator is turned off and a 7.5V low (and beyond), the output voltage VOUT rises smoothly dropout linear regulator is enabled that supplies INTVCC from zero to its final value. power from EXTVCC. If EXTVCC is less than 7.5V (but Alternatively the TRACK/SS pin can be used to cause greater than 4.7V), the 7.5V regulator is in dropout and the start-up of V INTV OUT to “track” that of another supply. CC is approximately equal to EXTVCC. When EXTVCC Typically, this requires connecting to the TRACK/SS pin an is greater than 7.5V (up to an absolute maximum rating external resistor divider from the other supply to ground of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC (see Applications Information section). pin allows the INTVCC power to be derived from a high efficiency external source such as one of the LTC3835 When the RUN pin is pulled low to disable the LTC3835, switching regulator outputs. or when VIN drops below its undervoltage lockout thresh- old of 3.5V, the TRACK/SS pin is pulled low by an internal The top MOSFET driver is biased from the floating boot- MOSFET. When in undervoltage lockout, the controller is strap capacitor CB, which normally recharges during each disabled and the external MOSFETs are held off. off cycle through an external diode when the top MOSFET Rev. F 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts
