Datasheet LTC3874-1 (Analog Devices) - 8

LTC3874-1
operaTion Main Control Loop
current mode architectures, the current limit threshold is The LTC3874-1 is a constant frequency, LTC proprietary still a function of the inductor peak current and the DCR current mode step-down slave control er for paral el opera- value, and can be accurately set with the ILIM and ITH pins. tion with master controllers. During normal operation, each
INTV
top MOSFET is turned on when the clock for that channel
CC/EXTVCC Power
sets the RS latch, and turned off when the main current Power for most internal circuitry is derived from the INTVCC comparator, ICMP, resets the RS latch. The peak inductor pin. When the EXTVCC pin is left open or tied to a voltage current at which ICMP resets the RS latch is controlled by less than 4.7V, an internal 5.5V linear regulator supplies the voltage on the ITH pin, which is the output of the master INTVCC power from VIN. If EXTVCC is taken above 4.7V controller. When the load current increases, the master and VIN is higher than 7V, the 5.5V regulator is turned off controller increases the ITH voltage, which in turn causes and an internal switch is turned on connecting EXTVCC. the peak current in the corresponding slave channels to EXTVCC can be applied before VIN. Using the EXTVCC allows increase, until the average inductor current matches the the INTVCC power to be drawn from an external source. new load current. After the top MOSFET has turned off, the bottom MOSFET is turned on until the beginning of
Start-Up and Shutdown (RUN0, RUN1)
the next cycle in Continuous Conduction Mode (CCM) or The two channels of the LTC3874-1 can be independently until the inductor current starts to reverse, as indicated shut down using the RUN0 and RUN1 pins. Pulling either by the reverse current comparator IREV, in Discontinuous of these pins below 1.4V shuts down the main control Conduction Mode (DCM). The LTC3874-1 slave controllers circuits for that channel. During shutdown, the PWM pin is DO NOT regulate the output voltage but regulate the cur- in three-state mode. Pulling either of these pins above 2V rent in each channel for current sharing with the master enables the controller. The RUN0/1 pins are actively pulled controllers. Output voltage regulation is achieved through down until the INTVCC voltage passes the undervoltage the voltage feedback control loop in the master controllers. lockout threshold of 3.8V. For multiphase operation, the RUN0/1 pins must be connected together and driven by
Sub-Milliohm DCR Current Sensing
the RUN pins on the master controller. Because a large The LTC3874-1 employs a unique architecture to enhance RC filter in the LTC3874-1 needs to settle during initializa- the signal-to-noise ratio that enables it to operate with tion, the RUN pins can only be pulled up 4ms after VIN a small sense signal of a sub-milliohm value inductor is ready. Do not exceed the Absolute Maximum Rating of DCR to improve power efficiency and reduce jitter due to 6V on these pins. switching noise. The start-up of each channel’s output voltage VOUT is Floating or pulling the LOWDCR pin high will enable sub- controlled by the master controller. After the RUN pins are milliohm DCR current sensing. The LTC3874-1 can sense released, the master controller drives the output based on a DCR value as low as 0.2mΩ with careful PCB layout. the programmed delay time and rise time. The slave con- The proprietary signal processing circuit provides a 14dB troller LTC3874-1 follows the ITH voltage set by the master signal-to-noise ratio improvement. As with conventional to supply the same current to the output during startup. 38741f 8 For more information www.linear.com/LTC3874-1 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Operation Applications Information Typical Application Related Parts