Datasheet LTC3618 (Analog Devices) - 10

LTC3618
operaTion Main Control Loop
a clock signal applied to the MODE/SYNC pin. The switch- The LTC3618 is a dual monolithic step-down DC/DC con- ing frequency can be set from 400kHz to 4MHz (see the verter featuring current-mode, constant-frequency opera- Applications Information section). tion. The regulated output voltage of the second step-down Overvoltage and undervoltage comparators pull the PGOOD converter is equal to VDDQIN • 0.5. An additional internal output low if the output voltage varies more than ±8% amplifier provides a VTTR output equal to VDDQIN • 0.5, (typical) from the set point. which is capable of driving a ±10mA load.
V
During normal operation, the internal top power switch
IN Overvoltage Protection
(P-channel MOSFET) of each channel is turned on at In order to protect the internal power MOSFET devices the beginning of its clock cycle. Current in the inductor against transient voltage spikes, the LTC3618 constantly increases until the current comparator trips and turns monitors the VIN pin for an overvoltage condition. When off the top power MOSFET. The peak inductor current at VIN rises above 6.5V, the regulator suspends operation which the current comparator shuts off is controlled by by shutting off the MOSFETs. The regulator executes its the voltage on the ITH pin. The error amplifier adjusts soft-start when exiting an overvoltage condition. the voltage on the ITH pin by comparing the feedback signals VFBX (derived from an external resistor divider on
MODE SELECTION
the VFB1 pin) with a reference (0.6V for VDDQ, VDDQIN • 0.5 for V The MODE/SYNC pin is used to select one of two different TT). When the load current increases, it causes a reduction in the feedback voltage relative to the reference. operating modes for VDDQ. When the MODE/SYNC pin The error amplifier raises the ITH voltage until the average is tied to SVIN, pulse-skipping mode is selected, when inductor current matches the new load current. Typical it is tied to ground, forced continuous mode is selected voltage range for the ITH pin is from 0.55V to 1.05V with (Figure 1). VTT is always in forced continuous mode. 0.55V corresponding to zero current.
VTTR Voltage Buffer Output
When the top power MOSFET shuts off, the synchronous An internal high accuracy op amp buffer generates power switch (N-channel MOSFET) turns on until either a VTTR pin voltage that is equal to VDDQIN • 0.5. the current limit is reached or the next clock cycle begins. VTTR can source and sink up to 10mA and is stable The bottom current limit is typically set at –4A for forced with a 0.1µF capacitor. Short circuit current limit is continuous mode and 0A for pulse-skipping mode. set around 20mA to prevent damage to the op amp. The operating frequency defaults to 2.25MHz when The VTTR output is also the reference voltage for VTT. RT is connected to SVIN, or can be set by an external Therefore, large transients on this pin will impact the resistor connected between the RT pin and ground, or by behavior at the VTT output. LTC3618 LTC3618 V SV IN IN V SV IN IN MODE MODE 0V SGND 0V SGND 3618 F01
1a. Pulse-Skipping Mode 1b. Forced Continuous Mode Figure 1. VDDQ Modes of Operation
3618fc 10 For more information www.linear.com/LTC3618 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