Datasheet

LTM4649
12
4649f
For more information www.linear.com/LTM4649
APPLICATIONS INFORMATION
PLL and Frequency Synchronization
The LTM4649 device operates over a range of frequen-
cies to improve power conversion efficiency. The nominal
switching frequency is 450kHz. It can also be synchronized
from 400kHz to 750kHz with an input clock that has a high
level above 2V and a low level below 0.8V at the CLKIN pin.
Once the LTM4649 is synchronizing to an external clock
frequency, it will always be running in Forced Continuous
Operation. The 400kHz low end operation frequency limit
is put in place to limit inductor ripple current.
Multiphase Operation
For outputs that demand more than 10A of load current,
multiple LTM4649 devices can be paralleled to provide more
output current and reduced input and output voltage ripple.
The CLKOUT signal together with CLKIN pin can be used
to cascade additional power stages to achieve the multi-
phase power supply solution. Tying the PHMODE pin to
INTV
CC
, GND, or (floating) generates a phase difference
(between CLKIN and CLKOUT) of 180°, 120°, or 90°
respectively as shown in Table 2. A total of 4 phases can
be cascaded to run simultaneously with respect to each
other by programming the PHMODE pin of each LTM4649
channel to different levels. Figure 3 shows a 3-phase de-
sign and 4-phase design example for clock phasing with
the PHASMD table.
Table 2. PHASEMD and CLKOUT Signal Relationship
PHASEMD GND FLOAT INTV
CC
CLKOUT 120° 90° 180°
The LTM4649 device is an inherently current mode con-
trolled device, so parallel modules will have good current
sharing. This will balance the thermals in the design. Tie
the COMP, V
FB
, TRACK/SS and RUN pins of each LTM4649
together to share the current evenly. Figures 19 and 20
show a schematic of the parallel design.
A multiphase power supply could significantly reduce
the amount of ripple current in both the input and output
capacitors. The RMS input ripple current is reduced by,
and the effective ripple frequency is multiplied by, the
number of phases used (assuming that the input voltage
is greater than the number of phases used times the output
voltage). The output ripple amplitude is also reduced by
the number of phases used.
4649 F03
4-PHASE DESIGN
3-PHASE DESIGN
90 DEGREE90 DEGREE 90 DEGREE
0 PHASE
FLOAT
FLOAT FLOAT FLOAT
CLKIN
V
OUT
PHASMD
CLKOUT
90 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
180 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
270 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
120 DEGREE
120 DEGREE
0 PHASE
GND GND GND
CLKIN
V
OUT
PHASMD
CLKOUT
120 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
240 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
Figure 3. Examples of 3-Phase, 4-Phase Operation with PHASMD Table
FREQ PIN VOLTAGE (V)
0
FREQUENCY (kHz)
900
800
600
400
100
200
700
500
300
0
2
4649 F02
2.51 1.50.5
Figure 2. Operating Frequency vs FREQ Pin Voltage