Datasheet LTC3617 (Analog Devices) - 9

LTC3617
operation Main Control Loop
reference voltage of the error amplifier which controls the output voltage. Therefore, large transients on this pin will The LTC3617 is a monolithic, constant frequency, current impact the behavior of the output. mode step-down DC/DC converter. During normal opera- tion, the internal top power switch (P-channel MOSFET) is
VIN Overvoltage Protection
turned on at the beginning of each clock cycle. Current in the inductor increases until the current comparator trips In order to protect the internal power MOSFET devices against transient voltage spikes, the LTC3617 constantly and turns off the top power switch. The peak inductor monitors the V current when the current comparator trips is controlled IN pin for an overvoltage condition. When V by the voltage on the ITH pin. The error amplifier adjusts IN rises above 6.5V, the regulator suspends operation by shutting off both MOSFETS. The regulator executes its the voltage on the ITH pin by comparing the feedback soft-start function when exiting an overvoltage condition. signal from a resistor divider on the VFB pin with a refer- ence voltage on the VTTR pin. VTTR is the output of an
Low Supply Operation
op amp buffer that expresses one-half the voltage on the VDDQIN pin. When the load current increases, it causes a The LTC3617 is designed to operate down to an input reduction in the feedback voltage relative to the reference. supply voltage of 2.25V. An important consideration at low The error amplifier raises the ITH voltage until the average input supply voltages is that the RDS(ON) of the P-channel and N-channel power switches increases. The user should inductor current matches the new load current. Typical calculate the power dissipation when the LTC3617 is used voltage range for the ITH pin is from 0.2V to 1.05V with at 100% duty cycle with low input voltages to ensure that 0.575V corresponding to zero current. thermal limits are not exceeded. See the Typical Perfor- When the top power switch shuts off, the synchronous mance Characteristics graphs. bottom power switch (N-channel MOSFET) turns on until either the bottom current limit is reached or the next clock
Short-Circuit Protection
cycle begins. The bottom current limit is typically set at The peak inductor current when the current comparator –10A. shuts off the top power switch is controlled by the voltage The operating frequency defaults to 2.25MHz when RT is on the ITH pin. connected to SVIN, or can be set by an external resistor If the output current increases, the error amplifier raises the connected between the RT pin and ground, or by a clock ITH pin voltage until the average inductor current matches signal applied to the RT pin. The switching frequency can the new load current. In normal operation the LTC3617 be set from 300kHz to 4MHz. clamps the maximum ITH pin voltage at approximately 1.05V which corresponds typically to 10A peak inductor current. Overvoltage and undervoltage comparators pull the PGOOD output low if the output voltage varies more than When the output is shorted to ground, the inductor cur- ±8% (typical) from the set point. rent decays very slowly during a single switching cycle. To prevent current runaway from occurring, a secondary
VTTR Voltage Buffer Output
current limit is imposed on the inductor current. If the inductor current measured through the bottom MOSFET An internal high accuracy op amp buffer generates a VTTR increases beyond 12A typical, the top power MOSFET will pin voltage that is equal to VDDQIN • 0.5. VTTR can source be held off and switching cycles will be skipped until the and sink up to 10mA and is stable with a maximum bypass inductor current decreases below this limit. capacitor of 0.1µF. Short-circuit current limit is set around 20mA to prevent damage to the op amp. VTTR is also the 3617fa 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts