Datasheet

LTC3861
15
3861fa
For more information www.linear.com/LTC3861
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’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 (600kHz and 1MHz).
Table 1 highlights these modes.
Table 1. Frequency Selection
CLKIN PIN FREQ PIN FREQUENCY
Clocked R
FREQ
to GND 250kHz to 2.25MHz
High or Float R
FREQ
to GND 250kHz to 2.25MHz
Low Low 600kHz
Low High 1MHz
No external PLL filter is required to synchronize the
LTC3861 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 features CLKOUT and PHSMD pins that al-
low multiple
LTC3861 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 CH-1 to CLKOUT PHASE
Float 180° 90°
Low 180° 60°
High 120° 240°
Using the LTC3861 Error Amplifiers in
Multiphase Applications
Due to the low output impedance of the error amplifiers,
multiphase
applications using the LTC3861 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
would
be with a single comparable output stage, where n
is equal to the number of phases.
The main advantages of PolyPhase operation are ripple
current cancellation in the input and output capacitors, a
faster load step response due to a smaller clock delay and
operaTion
(Refer to Functional Diagram)