Datasheet LTC3874-1 (Analog Devices) - 9

LTC3874-1
operaTion 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 LTC3874-1 can operate either in discontinuous con- ESR requirements and efficiency losses are substantially duction mode or forced continuous conduction mode. reduced because the peak current drawn from the input To select forced continuous mode, tie the MODE pin to a capacitor is effectively divided by the number of phases DC voltage above 2V (e.g., INTV used and power loss is proportional to the RMS current CC). To select discontinu- ous conduction mode, tie the MODE pin to a DC voltage squared. A two stage, single output voltage implementa- below 1.4V (e.g., GND).In forced continuous mode, tion can reduce input path power loss by 75% and radi- the inductor current is allowed to reverse at light loads cally reduce the required RMS current rating of the input or under large transient conditions. The peak inductor capacitor(s). current is determined by the voltage on the ITH pin. In
Single Output Multiphase Operation
this mode, the efficiency at light loads is lower than in discontinuous mode. However, continuous mode has the The LTC3874-1 is configured for single output multiphase advantages of lower output ripple and less interference converters with a master controller by making these con- with audio circuitry. When the MODE pin is connected to nections GND, the LTC3874-1 operates in discontinuous mode at • light loads. At very light loads, the current comparator Tie all the ITH pins of paralleled channels together for current sharing between masters and slaves; ICMP may remain tripped for several cycles and force the external top MOSFET to stay off for the same number of • Tie all SYNC or PLLIN pins of paralleled channels to- cycles (i.e., skipping pulses). This mode provides higher gether or tie the master chip’s CLKOUT pin to the slave light load efficiency than forced continuous mode and the chip’s SYNC pin for switching frequency synchronization inductor current is not allowed to reverse. There is a 500k among channels. pull-down resistor internally connected to the MODE pin. • Tie all the RUN pins of paralleled channels together for If the MODE0/1 pins are left floating, both channels are in startup and shutdown at the same time. discontinuous conduction mode by default. • 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 protection. The PHASMD pin determines the relative phases between the internal channels as well as the external clock signal on • The LTC3874-1 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
LTC3874-1 operates in DCM mode. When the soft-start edge of the clock on SYNC pin. interval is done, the LTC3874-1 operates in CCM mode.
Table 1
Examples of single output multiphase converters are shown in Figure 1.
PHASMD CHANNEL 0 PHASE CHANNEL 1 PHASE
GND 180° 0° The Typical Application on the first page of this data sheet 1/3 INTVCC 60° 300° is a basic LTC3874-1 application circuit configured as a 2/3 INTV slave control er. In paral eled operation, the current sensing CC or Float 120° 240° INTV scheme and circuit parameters in the LTC3874-1 have to CC 90° 270° be the same as the master controller to achieve balanced current sharing between masters and slaves. Input and output capacitors are selected based on RMS current rating, ripple and transient specs. 38741f For more information www.linear/LTC3874-1 9 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