Datasheet LTC3870-1 (Analog Devices) - 9

LTC3870-1
operaTion
GND, the LTC3870-1 operates in discontinuous mode at
Single Output Multiphase Operation
light loads. At very light loads, the current comparator The LTC3870-1 is designed for multiphase converters with ICMP may remain tripped for several cycles and force the the LTC3887-1 by making these connections: external top MOSFET to stay off for the same number of cycles (i.e., skipping pulses). This mode provides higher • Tie all the ITH pins of paralleled channels together for light load efficiency than forced continuous mode and current sharing between masters and slaves. Note that the inductor current is not allowed to reverse. There are ILIM setup on slaves has to match MFR_PWM_MODE 500kΩ pull-down resistors internally connected to the current range setup in masters. MODE0/MODE1 pins. If MODE0/MODE1 pins are floating, • Tie all SYNC pins together between master and slaves both channels default to Discontinuous Conduction Mode. for same switching frequency synchronization; one and only one of the LTC3887-1 controllers has to be
Multichip Operation (PHASMD and SYNC Pins)
programmed as master to generate clock signal on The PHASMD pin determines the relative phases between the SYNC pin. the internal channels as well as the external clock signal on • Tie all the RUN pins of paralleled channels together the SYNC pin, as shown in Table 1. The phases tabulated between master and slaves for startup and shutdown are relative to zero degree phase being defined as the sequences.
falling edge
of the clock on SYNC.
Table 1.
• Tie the GPIO pin of the master controller to the FAULT pin of slave controller and program the master GPIO
PHASMD Channel 0 Phase Channel 1 Phase
as fault sharing for fault protection. GND 180° 0° 1/3 INTVCC 60° 300° Examples of single output multiphase converters are 2/3 INTV shown in Figure 1. CC or Float 120° 240° INTVCC 90° 270°
Inductor Current Sensing
The SYNC pin is used to synchronize switching frequency between master and slave controllers. Input capacitance Like the LTC3887-1, the LTC3870-1 can use either induc- ESR requirements and efficiency losses are substantially tor DCR or RSENSE to sense the inductor current. Inductor reduced because the peak current drawn from the input DCR current sensing provides a lossless method of sens- capacitor is effectively divided by the number of phases ing the instantaneous current. Therefore, it can provide used and power loss is proportional to the RMS current higher efficiency for applications with high output currents. squared. A two-phase, single output voltage implementa- However, the DCR of a copper inductor typically has 10% tion can reduce input path power loss by 75% and radi- tolerance. For precise current sensing, a precision sensing cally reduce the required RMS current rating of the input resistor RSENSE can be used to sense the inductor current. capacitor(s). It is important to match the current sensing circuit between master controllers and slave controllers to guarantee bal- anced load sharing and overcurrent protection. 38701f For more information www.linear.com/LTC3870-1 9 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