Datasheet LTC3874 (Analog Devices) - 9

LTC3874
OPERATION Main Control Loop INTVCC/EXTVCC Power
The LTC3874 is a constant frequency, LTC proprietary cur- Power for the top and bottom MOSFET drivers and most rent mode step-down slave controller for parallel operation other internal circuitry is derived from the INTVCC pin. with master controllers. During normal operation, each When the EXTVCC pin is left open or tied to a voltage less top MOSFET is turned on when the clock for that channel than 4.7V, an internal 5.5V linear regulator supplies INTVCC sets the RS latch, and turned off when the main current power from VIN. If EXTVCC is taken above 4.7V and VIN comparator, ICMP, resets the RS latch. The peak inductor is higher than 7V, the 5.5V regulator is turned off and an current at which ICMP resets the RS latch is controlled by internal switch is turned on connecting EXTVCC. EXTVCC the voltage on the ITH pin, which is the output of the master can be applied before VIN. Using the EXTVCC allows the controller. When the load current increases, the master INTVCC power to be drawn from an external source. controller increases the ITH voltage, which in turn causes Each top MOSFET driver is biased from the floating boot- the peak current in the corresponding slave channels to strap capacitor C increase, until the average inductor current matches the B, which normally recharges during each off cycle through an external diode when the top MOSFET new load current. After the top MOSFET has turned off, turns off. If the input voltage V the bottom MOSFET is turned on until the beginning of IN decreases to a voltage close to V the next cycle in Continuous Conduction Mode (CCM) or OUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. A dropout detector until the inductor current starts to reverse, as indicated detects this and forces the top MOSFET off for about one- by the reverse current comparator IREV, in Discontinuous twelfth of the clock period plus 100ns every three cycles Conduction Mode (DCM). The LTC3874 slave controllers to allow C DO NOT regulate the output voltage but regulate the cur- B to recharge. rent in each channel for current sharing with the master
Start-Up and Shutdown (RUN0, RUN1)
controllers. Output voltage regulation is achieved through the voltage feedback control loop in the master controllers. The two channels of the LTC3874 can be independently shut down using the RUN0 and RUN1 pins. Pulling either
Sub-Milliohm DCR Current Sensing
of these pins below 1.4V shuts down the main control circuits for that channel. During shutdown, both TG The LTC3874 employs a unique architecture to enhance and BG are pulled down to turn off the external power the signal-to-noise ratio that enables it to operate with MOSFETs. Pulling either of these pins above 2V enables a small sense signal of a sub-milliohm value inductor the controller. The RUN0/1 pins are actively pulled down DCR to improve power efficiency and reduce jitter due to until the INTV switching noise. CC voltage passes the undervoltage lockout threshold of 3.8V. For multiphase operation, the RUN0/1 Floating or pulling the LOWDCR pin high will enable sub- pins must be connected together and driven by the RUN milliohm DCR current sensing. The LTC3874 can sense pins on the master controller. Because a large RC filter in a DCR value as low as 0.2mΩ with careful PCB layout. the LTC3874 needs to settle during initialization, the RUN The proprietary signal processing circuit provides a 14dB pins can only be pulled up 4ms after VIN is ready. Do not signal-to-noise ratio improvement. As with conventional exceed the Absolute Maximum Rating of 6V on these pins. current mode architectures, the current limit threshold is still a function of the inductor peak current and the DCR value, and can be accurately set with the ILIM and ITH pins. 3874fb For more information www.linear.com/LTC3874 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 Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts