Datasheet

LTM4627
13
4627fc
For more information www.linear.com/LTM4627
Multiphase Operation
For outputs that demand more than 15A of load current,
multiple LTM4627 devices can be paralleled to provide
more output current without increasing input and output
ripple voltage. The MODE_PLLIN pin allows the LTM4627
to be synchronized to an external clock and the internal
phase-locked loop allows the LTM4627 to lock onto input
clock phase as well. The f
SET
resistor is selected for nor-
mal frequency, then the incoming clock can synchronize
the device over the specified range. See Figure 20 for a
synchronizing example circuit.
A multiphase power supply significantly reduces the
amount of ripple current in both the input and output ca
-
pacitors. 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. See Application Note 77.
The L
TM4627 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 and V
FB
pins of each LTM4627 together to
share the current evenly. Figure 20 shows a schematic of
the parallel design.
Input RMS Ripple Current Cancellation
Application Note 77 provides a detailed explanation of
multiphase operation. The input RMS ripple current can
-
cellation mathematical derivations are presented, and a
graph is displayed representing the RMS ripple current
reduction as a function of the number of interleaved phases
(see Figure 2).
PLL, Frequency Adjustment and Synchronization
The LTM4627 switching frequency is set by a resistor (R
fSET
)
from the f
SET
pin to signal ground. A 10µA current (I
FREQ
)
flowing out of the f
SET
pin through R
fSET
develops a volt-
age on f
SET
. R
fSET
can be calculated as:
R
fSET
=
FREQ
500kHz / V
+ 0.2V






1
10µA
The relationship of f
SET
voltage to switching frequency is
shown in Figure 3. For low output voltages from 0.8V to
1.5V, 400kHz operation is an optimal frequency for the best
power conversion efficiency while maintaining the induc
-
tor ripple current to about 30% to 40% of maximum load
current. For output voltages from 1.8V to 3.0V, 500kHz to
600kHz is optimal. For output voltages from 3.0V to 5.0V
,
750kHz operation is optimal, but due to the higher ripple
current at 5V operation the output current is limited to 10A.
The LTM4627 can be synchronized from 250kHz to 800kHz
with an input clock that has a high level above 2V and a
low level below 0.8V. However, a 400kHz low end operating
frequency is recommended to limit inductor ripple current.
See the Typical Applications section for synchronization
examples. The LTM4627 minimum on-time is limited to
approximately 90ns. Guardband the on-time to 130ns.
The on-time can be calculated as:
t
ON(MIN)
=
1
FREQ
•
V
OUT
V
IN






Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK/SS pin. The output can be tracked up
and down with another regulator. The master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider
to implement coincident tracking. The LTM4627 uses an
accurate 60.4k resistor internally for the top feedback
resistor. Figure 4 shows an example of coincident tracking.
V
OUT(SLAVE)
= 1+
60.4k
R
TA






• V
TRACK
V
TRACK
is the track ramp applied to the slave’s track pin.
V
TRACK
has a control range of 0V to 0.6V, or the internal
reference voltage. When the master’s output is divided
down with the same resistor values used to set the slave’s
output, then the slave will coincident track with the master
until it reaches its final value. The master will continue to
APPLICATIONS INFORMATION