LTC3861-1 Datasheet, PDF (15/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Operation (Refer to Functional Diagram)
is disabled for that channel. The TRACK/SS pins should
also be connected together so that the slave phases can
detect when soft-start is complete and to synchronize the
nonsynchronous mode of operation.
Frequency Selection and the Phase-Locked Loop (PLL)
The selection of the switching frequency is a trade-off
between efficiency, transient response and component
size. High frequency operation reduces the size of the
inductor and output capacitor as well as increasing the
maximum practical control loop bandwidth. However,
efficiency is generally lower due to increased transition
and switching losses.
The LTC3861-1’s switching frequency can be set in three
ways: using an external resistor to linearly program the
frequency, synchronizing to an external clock, or simply
selecting one of two fixed frequencies (400kHz and
600kHz). Table 1 highlights these modes.
Table 1. Frequency Selection
CLKIN PIN
FREQ PIN
Clocked
High or Float
Low
R_FREQ to GND
R_FREQ to GND
Low
Low
High
FREQUENCY
250kHz to 2.25MHz
250kHz to 2.25MHz
400kHz
600kHz
No external PLL filter is required to synchronize the
LTC3861-1 to an external clock. Applying an external clock
signal to the CLKIN pin will automatically enable the PLL
with internal filter.
Constant-frequency operation brings with it a number
of benefits: inductor and capacitor values can be chosen
for a precise operating frequency and the feedback loop
can be similarly tightly specified. Noise generated by the
circuit will always be at known frequencies.
Using the CLKOUT and PHSMD Pins in
Multiphase Applications
The LTC3861-1 features CLKOUT and PHSMD pins that
allow multiple LTC3861-1 ICs to be daisychained together
in multiphase applications. The clock output signal on the
CLKOUT pin can be used to synchronize additional ICs in
a 3-, 4-, 6- or 12-phase power supply solution feeding a
single high current output, or even several outputs from
the same input supply.
The PHSMD pin is used to adjust the phase relationship
between channel 1 and channel 2, as well as the phase
relationship between channel 1 and CLKOUT, as sum-
marized in Table 2. The phases are calculated relative to
zero degrees, defined as the rising edge of PWM1. Refer
to Applications Information for more details on how to
create multiphase applications.
Table 2. Phase Selection
PHSMD PIN CH-1 to CH-2 PHASE
Float
180°
Low
180°
High
120°
CH-1 to CLKOUT PHASE
90°
60°
240°
Using the LTC3861-1 Error Amplifiers in
Multiphase Applications
Due to the low output impedance of the error amplifiers,
multiphase applications using the LTC3861-1 use one
error amplifier as the master with all of the slaves’
error amplifiers disabled. The channel 1 error amplifier
(phase = 0°) may be used as the master with phases 2
through n (up to 12) serving as slaves. To disable the
slave error amplifiers connect the FB pins of the slaves
to V_CC. This three-states the output stages of the ampli-
fiers. All COMP pins should then be connected together
to create PWM outputs for all phases. As noted in the
section on soft-start, all TRACK/SS pins should also be
shorted together. Refer to the Multiphase Operation sec-
tion in Applications Information for schematics of various
multiphase configurations.
Theory and Benefits of Multiphase Operation
Multiphase operation provides several benefits over tra-
ditional single phase power supplies:
n _{Greater output current capability}
n _{Improved transient response}
n _{Reduction in component size}
n _{Increased real world operating efficiency}
Because multiphase operation parallels power stages,
the amount of output current available is n times what it
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