LTM4649_15 Datasheet, PDF (13/30 Pages) Linear Technology – 10A Step-Down DC/DC Module Regulator

LTM4649
Applications Information
900
800
700
600
500
400
300
200
100
0
0
0.5
1
1.5
2
FREQ PIN VOLTAGE (V)
2.5
4649 F02
Figure 2. Operating Frequency vs FREQ Pin Voltage
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
CLKOUT
GND
FLOAT
INTV_CC
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.
CLKOUT
CLKIN
0 PHASE
GND ^VOUT
PHASMD
3-PHASE DESIGN
120 DEGREE
CLKOUT
CLKIN
120 PHASE
GND ^VOUT
PHASMD
120 DEGREE
CLKOUT
CLKIN
240 PHASE
GND ^VOUT
PHASMD
CLKOUT
CLKIN
0 PHASE
FLOAT ^VOUT
PHASMD
4-PHASE DESIGN
90 DEGREE
CLKOUT
90 DEGREE
CLKOUT
90 DEGREE
CLKOUT
CLKIN
90 PHASE
FLOAT ^VOUT
PHASMD
CLKIN
180 PHASE
FLOAT ^VOUT
PHASMD
CLKIN
270 PHASE
FLOAT ^VOUT
PHASMD
Figure 3. Examples of 3-Phase, 4-Phase Operation with PHASMD Table
4649 F03
4649fa
For more information www.linear.com/LTM4649
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