Datasheet LTC3870 (Analog Devices) - 10

LTC3870
operaTion
SGND, the LTC3870 operates in discontinuous mode at
Single Output Multiphase Operation
light loads. At very light loads, the current comparator The LTC3870 is designed for multiphase converters with ICMP may remain tripped for several cycles and force the the master controller 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 the current sharing between masters and slaves. Note that inductor current is not allowed to reverse. There are 500k ILIM setup on slaves has to match MFR_PWM_MODE pull down resistors internally connected to the MODE0/ current range setup in masters. MODE1 pins. If MODE0/MODE1 pins are floating, both Tie all SYNC pins together between master and slaves for channels default to discontinuous conduction mode. same switching frequency synchronization; one and only one of the master controllers has to be programmed
Multichip Operation (PHASMD and SYNC Pins)
as master to generate clock signal on the SYNC pin. The PHASMD pin determines the relative phases between Tie all the RUN pins of paralleled channels together between the internal channels as well as the external clock signal on master and slaves for startup and shutdown sequences. the SYNC pin, as shown in Table 1. The phases tabulated are relative to zero degree phase being defined as the Tie the GPIO pin of the master controller to the FAULT pin
falling edge
of the clock on SYNC. of slave controllers and program the master GPIO as
Table 1.
fault sharing for fault protections.
PHASMD Channel 0 Phase Channel 1 Phase
Examples of single output multiphase converters are GND 180° 0° shown in Figure 1. 1/3 INTVCC 60° 300° 2/3 INTV
Inductor Current Sensing
CC or Float 120° 240° INTVCC 90° 270° Like the LTC3880/LTC3883, LTC3870 can use either induc- The SYNC pin is used to synchronize switching frequency tor DCR or RSENSE to sense the inductor current. Inductor between master and slave controllers. Input capacitance DCR current sensing provides a lossless method of sens- ESR requirements and efficiency losses are substantially ing the instantaneous current. Therefore, it can provide reduced because the peak current drawn from the input higher efficiency for applications with high output currents. capacitor is effectively divided by the number of phases However, the DCR of a copper inductor typically has 10% used and power loss is proportional to the RMS current tolerance. For precise current sensing, a precision sensing squared. A two-phase, single output voltage implementa- resistor RSENSE can be used to sense the inductor current. tion can reduce input path power loss by 75% and radi- It is important to match the current sensing circuit between cally reduce the required RMS current rating of the input master controllers and slave controllers to guarantee bal- capacitor(s). anced load sharing and overcurrent protection. 3870fb 10 For more information www.linear.com/LTC3870