Datasheet LTC3870 (Analog Devices) - 9

LTC3870
operaTion Main Control Loop
off for about one-twelfth of the clock period plus 100ns The LTC3870 is a constant frequency, current mode every three cycles to allow CB to recharge. However, it is step-down slave controller for parallel operation with the recommended that a load be present or the IC operates LTC3880 family master controllers. During normal opera- at low frequency during the drop-out transition to ensure tion, each top MOSFET is turned on when the clock for CB is recharged. that channel sets the RS latch, and turned off when the
Start-Up and Shutdown (RUN0, RUN1)
main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP resets the RS latch The two channels of the LTC3870 can independently is controlled by the voltage on the ITH pin, which is tied start up and shut down using the RUN0 and RUN1 pins. directly to the corresponding ITH pin of the master control- Pulling either of these pins below 1.4V shuts down the lers. When the load current increases, master controllers control circuits for that channel. During shutdown, both drive and increase the ITH voltage, which in turn cause TG and BG are pulled down to turn off the external power the peak current in the corresponding slave channels to MOSFETs. Pulling either of these pins above 2V enables increase, until the average inductor current matches the the corresponding channel and internal circuits. During new load current. After the top MOSFET has been turned startup, the RUN0/RUN1 pins are actively pulled down off, the bottom MOSFET is turned on until the beginning until the INTVCC voltage passes the under-voltage lockout of the next cycle in Continuous Conduction Mode (CCM) threshold of 4V. For multiphase parallel operation, the or until the inductor current starts to reverse, as indicated RUN0/RUN1 pins have to be connected and driven by by the reverse current comparator IREV, in Discontinuous the RUN pins of the master controller. Do not exceed the Conduction Mode (DCM). The LTC3870 slave controllers Absolute Maximum Rating of 6V on these pins. DO NOT regulate the output voltage but regulate the current The start-up of each channel’s output voltage V in each channel for current sharing with master controllers. OUT is controlled and programmed by the master controller. After Output voltage regulation is achieved through the voltage the RUN pins are released, the master controller drives the feedback loops in master controllers. output based on the programmed delay time and rise time, and the slave controller LTC3870 just follows the master
INTVCC/EXTVCC Power
to supply equivalent current to the output during startup. Power for the top and bottom MOSFET drivers and most other internal circuitry is derived from the INTVCC pin.
Light Load Current Operation (Discontinuous
Normally an internal 5.0V linear regulator supplies INTVCC
Conduction Mode, Continuous Conduction Mode)
power from VIN. In high VIN applications, if a high effi- The LTC3870 can be set to operate either in Discontinuous ciency external voltage source is available for the EXTVCC Conduction Mode (DCM) or forced Continuous Conduc- pin, another internal 5.0V linear regulator is enabled and tion Mode (CCM). To select forced Continuous Mode of supplies INTVCC power from EXTVCC. To enable the linear operation, tie the MODE pin to a DC voltage above 2V regulator driven by the EXTVCC pin, VIN has to be higher (e.g., INTV than 6.5V and EXTV CC). To select discontinuous conduction mode CC pin voltage has to be higher than of operation, tie the MODE pin to a DC voltage below 1.4V 4.8V. Do not exceed 14V on the EXTVCC pin. (e.g., SGND). In forced continuous operation, the induc- Each top MOSFET driver is biased from the floating tor current is allowed to reverse at light loads or under bootstrap capacitor CB, which normally recharges during large transient conditions. The peak inductor current is each off cycle through an external diode when the top determined by the voltage on the ITH pin. In this mode, MOSFET turns off. If the input voltage VIN decreases to the efficiency at light loads is lower than in discontinu- a voltage close to VOUT, the loop may enter dropout and ous Mode operation. However, continuous mode has the attempt to turn on the top MOSFET continuously. The advantages of lower output ripple and less interference dropout detector detects this and forces the top MOSFET with audio circuitry. When the MODE pin is connected to 3870fb For more information www.linear.com/LTC3870 9