Datasheet LTC3870-1 (Analog Devices) - 8

LTC3870-1
operaTion Main Control Loop Start-Up and Shutdown (RUN0, RUN1)
The LTC3870-1 is a constant frequency, current mode The two channels of the LTC3870-1 can independently start step-down slave controller for parallel operation with the up and shut down using the RUN0 and RUN1 pins. Pulling LTC3887-1. During normal operation, each top MOSFET either of these pins below 1.4V shuts down the control is turned on when the clock for that channel sets the RS circuits for that channel. During shutdown, the PWM pin is latch, and turned off when the main current comparator, in three-state mode. Pulling either of these pins above 2V ICMP, resets the RS latch. The peak inductor current at enables the corresponding channel and internal circuits. which ICMP resets the RS latch is controlled by the voltage During startup, the RUN0/RUN1 pins are actively pulled on the ITH pin, which is tied directly to the corresponding down until the INTVCC voltage passes the undervoltage ITH pin of the master controllers (LTC3887-1). When the lockout threshold of 4V. For multiphase parallel operation, load current increases, LTC3887-1 master controllers the RUN0/RUN1 pins have to be connected and driven by drive and increase the ITH voltage, which in turn causes the RUN pins of the master controller. Do not exceed the the peak current in the corresponding slave channels to Absolute Maximum Rating of 6V on these pins. increase, until the average inductor current matches the The start-up of each channel’s output voltage V new load current. After the top MOSFET has been turned OUT is control ed and programmed by the master control er. off, the bottom MOSFET is turned on until the beginning After the RUN pins are released, the master control er of the next cycle in Continuous Conduction Mode (CCM) drives the output based on the programmed delay time or until the inductor current starts to reverse, as indicated and rise time, and the slave control er LTC3870-1 just by the reverse current comparator IREV, in Discontinuous fol ows the master to supply equivalent current to the Conduction Mode (DCM). LTC3870-1 slave controllers DO output during start-up. NOT regulate the output voltage but regulate the current in each channel for current sharing with master controllers.
Light Load Current Operation (Discontinuous
Output voltage regulation is achieved through the voltage
Conduction Mode, Continuous Conduction Mode)
feedback loops in the master controller. The LTC3870-1 can be set to operate either in Discontinu-
INTVCC/EXTVCC Power
ous Conduction Mode (DCM) or forced Continuous Con- duction Mode (CCM). To select forced Continuous Mode Power for most internal circuitry is derived from the INTVCC of operation, tie the MODE pin to a DC voltage above 2V pin. Normally an internal 5.0V linear regulator supplies (e.g., INTV INTV CC). To select Discontinuous Conduction Mode CC power from VIN. In high VIN applications, if a high of operation, tie the MODE pin to a DC voltage below 1.4V efficiency external voltage source is available for the EXTVCC (e.g., SGND). In forced continuous operation, the induc- pin, another internal 5.0V linear regulator is enabled and tor current is allowed to reverse at light loads or under supplies INTVCC power from EXTVCC. To enable the linear large transient conditions. The peak inductor current is regulator driven by the EXTVCC pin, VIN needs to be higher determined by the voltage on the I than 6.5V and EXTV TH pin. In this mode, CC pin voltage has to be higher than the efficiency at light loads is lower than in discontinu- 4.8V. Do not exceed 14V on the EXTVCC pin. ous mode operation. However, continuous mode has the advantages of lower output ripple and less interference with audio circuitry. When the MODE pin is connected to 38701f 8 For more information www.linear.com/LTC3870-1 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts