ltm4616 Linear Technology Corporation, ltm4616 Datasheet - Page 12

• Manufacturer Part Number: ltm4616
• Description: Dual 8a Per Channel Low Vin Dc/dc ?module
• Manufacturer: Linear Technology Corporation

LTM4616
APPLICATIONS INFORMATION
of the maximum peak current value in normal operation
even though the voltage at the I
value. The voltage at the I
average current is greater than the load requirement. As
the I
trips, causing the internal sleep line to go high and turn
off both power MOSFETs.
In Burst Mode operation, the internal circuitry is partially
turned off, reducing the quiescent current to about 450μA
for each output. The load current is now being supplied
from the output capacitors. When the output voltage drops,
causing I
low, and the LTM4616 resumes normal operation. The next
oscillator cycle will turn on the top power MOSFET and the
switching cycle repeats. Each regulator can be confi gured
for Burst Mode operation.
Pulse-Skipping Mode Operation
In applications where low output ripple and high effi ciency
at intermediate currents are desired, pulse-skipping mode
should be used. Pulse-skipping operation allows the
LTM4616 to skip cycles at low output loads, thus increasing
effi ciency by reducing switching loss. Floating the MODE
pin or tying it to V
This allows discontinuous conduction mode (DCM) opera-
tion down to near the limit defi ned by the chip’s minimum
on-time (about 100ns). Below this output current level,
the converter will begin to skip cycles in order to main-
tain output regulation. Increasing the output load current
slightly, above the minimum required for discontinuous
conduction mode, allows constant frequency PWM. Each
regulator can be confi gured for Pulse-Skipping mode.
Forced Continuous Operation
In applications where fi xed frequency operation is more
critical than low current effi ciency, and where the lowest
output ripple is desired, forced continuous operation should
be used. Forced continuous operation can be enabled by
tying the MODE pin to GND. In this mode, inductor cur-
rent is allowed to reverse during low output loads, the I
voltage is in control of the current comparator threshold
throughout, and the top MOSFET always turns on with
each oscillator pulse. During start-up, forced continu-
12
TH
voltage drops below 0.2V, the BURST comparator
TH
to rise above 0.25V, the internal sleep line goes
IN
/2 enables pulse-skipping operation.
TH
pin drops when the inductor’s
TH
pin indicates a lower
TH
ous mode is disabled and inductor current is prevented
from reversing until the LTM4616’s output voltage is in
regulation. Each regulator can be confi gured for Force
Continuous mode.
Multiphase Operation
For output loads that demand more than 8A of current,
two outputs in LTM4616 or even multiple LTM4616s can
be cascaded to run out of phase to provide more output
current without increasing input and output voltage ripples.
The CLKIN pin allows the LTC4616 to synchronize to an
external clock (between 0.75MHz and 2.25MHz) and the
internal phase locked loop allows the LTM4616 to lock
onto CLKIN’s phase as well. The CLKOUT signal can be
connected to the CLKIN pin of the following LTM4616 stage
to line up both the frequency and the phase of the entire
system. Tying the PHMODE pin to SV
(fl oating) generates a phase difference (between CLKIN
and CLKOUT) of 180°, 120° or 90° respectively, which
corresponds to a 2-phase, 3-phase or 4-phase operation. A
total of 12 phases can be cascaded to run simultaneously
with respect to each other by programming the PHMODE
pin of each LTM4616 to different levels. For a 6-phase
example in Figure 2, the 2nd stage that is 120° out of
phase from the 1st stage can generate a 240° (PHMODE
= 0) CLKOUT signal for the 3rd stage, which then can
generate a CLKOUT signal that’s 420°, or 60° (PHMODE
= SV
next two stages can shift 120° (PHMODE = 0) for each
to generate a 300° signal for the 6th stage. Finally, the
signal with a 60° phase shift on the 6th stage (PHMODE
is fl oating) goes back to the 1st stage. Figure 3 shows the
confi guration for 12-phase operation.
A multiphase power supply signifi cantly reduces 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.
The LTM4616 device is an inherently current mode con-
trolled device, so parallel modules will have very good
IN
) for the 4th stage. With the 60° CLKIN input, the
IN
, SGND or SV
IN
4616f
/2