LTC3870_15 Datasheet, PDF (10/22 Pages) Linear Technology – PolyPhase Step-Down Slave Controller for LTC3880/LTC3883 with Digital Power System Management

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