Datasheet LTC3608 (Analog Devices) - 10

LTC3608 operation
Main Control Loop
Overvoltage and undervoltage comparators OV and UV The LTC3608 is a high efficiency monolithic synchronous, pull the PGOOD output low if the output feedback volt- step-down DC/DC converter utilizing a constant on-time, age exits a ±10% window around the regulation point. current mode architecture. It operates from an input volt- Furthermore, in an overvoltage condition, M1 is turned age range of 4V to 18V (20V maximum) and provides a off and M2 is turned on and held on until the overvoltage regulated output voltage at up to 8A of output current. The condition clears. internal synchronous power switch increases efficiency Foldback current limiting is provided if the output is and eliminates the need for an external Schottky diode. In shorted to ground. As VFB drops, the buffered current normal operation, the top MOSFET is turned on for a fixed threshold voltage ITHB is pulled down by clamp Q3 to interval determined by a one-shot timer OST. When the a 1V level set by Q4 and Q6. This reduces the inductor top MOSFET is turned off, the bottom MOSFET is turned valley current level to one sixth of its maximum value as on until the current comparator ICMP trips, restarting the VFB approaches 0V. one-shot timer and initiating the next cycle. Inductor current Pulling the RUN/SS pin low forces the controller into its is determined by sensing the voltage between the PGND shutdown state, turning off both M1 and M2. Releasing and SW pins using the bottom MOSFET on-resistance. the pin allows an internal 1.2µA current source to charge The voltage on the ITH pin sets the comparator threshold up an external soft-start capacitor, C corresponding to inductor valley current. The error ampli- SS. When this voltage reaches 1.5V, the controller turns on and begins switching, fier, EA, adjusts this voltage by comparing the feedback but with the I signal V TH voltage clamped at approximately 0.6V FB from the output voltage with an internal 0.6V below the RUN/SS voltage. As C reference. If the load current increases, it causes a drop SS continues to charge, the soft-start current limit is removed. in the feedback voltage relative to the reference. The ITH voltage then rises until the average inductor current again
INTV
matches the load current.
CC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most of At light load, the inductor current can drop to zero and the internal controller circuitry is derived from the INTV become negative. This is detected by current reversal CC pin. The top MOSFET driver is powered from a floating comparator IREV which then shuts off M2 (see Func- bootstrap capacitor, C tional Diagram), resulting in discontinuous operation. Both B. This capacitor is recharged from INTV switches wil remain off with the output capacitor supplying CC through an external Schottky diode, DB, when the top MOSFET is turned off. When the EXTV the load current until the I CC pin is TH voltage rises above the zero grounded, an internal 5V low dropout regulator supplies current level (0.8V) to initiate another cycle. Discontinu- the INTV ous mode operation is disabled by comparator F when CC power from VIN. If EXTVCC rises above 4.7V, the internal regulator is turned off, and an internal switch the FCB pin is brought below 0.6V, forcing continuous connects EXTV synchronous operation. CC to INTVCC. This allows a high efficiency source connected to EXTVCC, such as an external 5V sup- The operating frequency is determined implicitly by the ply or a secondary output from the converter, to provide top MOSFET on-time and the duty cycle required to main- the INTVCC power. Voltages up to 7V can be applied to tain regulation. The one-shot timer generates an on-time EXTVCC for additional gate drive. If the input voltage is that is proportional to the ideal duty cycle, thus holding low and INTVCC drops below 3.5V, undervoltage lockout frequency approximately constant with changes in VIN. circuitry prevents the power switches from turning on. The nominal frequency can be adjusted with an external resistor, RON. 3608fc 0 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts