Datasheet LTC3874 (Analog Devices) - 10

LTC3874
OPERATION
The start-up of each channel’s output voltage VOUT is
Table 1
controlled by the master controller. After the RUN pins are
PHASMD CHANNEL 0 PHASE CHANNEL 1 PHASE
released, the master controller drives the output based on GND 180° 0° the programmed delay time and rise time. The slave con- 1/3 INTV troller LTC3874 follows the I CC 60° 300° TH voltage set by the master 2/3 INTV to supply the same current to the output during startup. CC or Float 120° 240° INTVCC 90° 270°
Light Load Current Operation (Discontinuous
The SYNC pin is used to synchronize switching frequency
Conduction Mode, Continuous Conduction Mode)
between the master and slave controllers. Input capacitance The LTC3874 can operate either in discontinuous conduc- ESR requirements and efficiency losses are substantially tion mode or forced continuous conduction mode. To reduced because the peak current drawn from the input select forced continuous mode, tie the MODE pin to a DC capacitor is effectively divided by the number of phases voltage above 2V (e.g., INTVCC). To select discontinuous used and power loss is proportional to the RMS current conduction mode, tie the MODE pin to a DC voltage below squared. A two stage, single output voltage implementa- 1.4V (e.g., GND).In forced continuous mode, the induc- tion can reduce input path power loss by 75% and radi- tor current is allowed to reverse at light loads or under cally reduce the required RMS current rating of the input large transient conditions. The peak inductor current is capacitor(s). determined by the voltage on the ITH pin. In this mode, the efficiency at light loads is lower than in discontinuous mode.
Single Output Multiphase Operation
However, continuous mode has the advantages of lower The LTC3874 is configured for single output multiphase output ripple and less interference with audio circuitry. converters with a master controller by making these When the MODE pin is connected to GND, the LTC3874 connections operates in discontinuous mode at light loads. At very light loads, the current comparator I • CMP may remain tripped for Tie all the ITH pins of paralleled channels together for several cycles and force the external top MOSFET to stay current sharing between masters and slaves; off for the same number of cycles (i.e., skipping pulses). • Tie all SYNC or PLLIN pins of paralleled channels to- This mode provides higher light load efficiency than forced gether or tie the master chip’s CLKOUT pin to the slave continuous mode and the inductor current is not allowed chip’s SYNC pin for switching frequency synchronization to reverse. There is a 500k pull-down resistor internally among channels. connected to the MODE pin. If the MODE0/1 pins are left floating, both channels are in discontinuous conduction • Tie all the RUN pins of paralleled channels together for mode by default. startup and shutdown at the same time. • Tie the fault indictor pin of the master controller if avail-
Multichip Operations (PHASMD and SYNC Pins)
able to the FAULT pin of the slave controller for fault The PHASMD pin determines the relative phases between protection. the internal channels as well as the external clock signal on • The LTC3874 MODE pin can be tied to the master chip SYNC pin as shown in Table 1. The phases tabulated are PGOOD pin for start-up control. During soft-start, the relative to zero degree phase being defined as the
falling
LTC3874 operates in DCM mode. When the soft-start edge of the clock on SYNC pin. interval is done, the LTC3874 operates in CCM mode. Examples of single output multiphase converters are shown in Figure 1. 3874fb 10 For more information www.linear.com/LTC3874 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