LTM4623
11
4623fc
For more information www.linear.com/LTM4623
applicaTions inForMaTion
to SGND. At light loads the internal current comparator
may remain tripped for several cycles and force the top
MOSFET to stay off for several cycles, thus skipping cycles.
The inductor current does not reverse in this mode.
Forced Continuous Current Mode (CCM)
In applications where fixed frequency operation is more
critical than low current efficiency, 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 INTV
CC
. In this mode, inductor current
is allowed to reverse during low output loads, the COMP
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 continuous mode
is disabled and inductor current is prevented from revers
-
ing until the LTM4623’s output voltage is in regulation.
Operating Frequency
The operating frequency of the LTM4623 is optimized to
achieve the compact package size and the minimum out-
put ripple voltage while still keeping high efficiency. The
default operating frequency is internally set to 1MHz. In
most applications, no additional frequency adjusting is
required.
If any operating frequency other than 1MHz is required
by application, the operating frequency can be increased
by adding a resistor, RFSET, between the FREQ pin and
SGND, as shown in Figure 28. The operating frequency
can be calculated as:
f(Hz) =
162k ||R
FSET
(Ω)
To reduce switching current ripple, 2MHz operating fre-
quency is required for 3.3V to 5.5V output with R
FSET
=162k
to SGND.
The operating frequency can also be decreased by adding
a resistor between the FREQ pin and INTV
CC
, calculated as:
f(Hz) = 1MHz −
R
FSET
(Ω)
The programmable operating frequency range is from
800kHz to 4MHz.
Please note a minimum switching frequency is required
for given V
IN
, V
OUT
operating conditions to keep a maxi-
mum peak-to-peak inductor ripple current below 2A for
the LTM4623.
The peak-to-peak inductor ripple current can be calculated
as:
ΔI
P-P
= V
OUT
1−
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
•
1
f
SW
(MHz)
The maximum 2A peak-to-peak inductor ripple current
is enforced due to the nature of the valley current mode
control to maintain output voltage regulation at no load.
Frequency Synchronization and Clock In
The power module has a phase-locked loop comprised
of an internal voltage controlled oscillator and a phase
detector. This allows the internal top MOSFET turn-on to be
locked to the rising edge of the external clock. The external
clock frequency range must be within ±30% around the
set operating frequency. A pulse detection circuit is used
to detect a clock on the CLKIN pin to turn on the phase-
locked loop. The pulse width of the clock has to be at least
100ns. The clock high level must be above 2V and clock
low level below 0.3V. During the start-up of the regulator,
the phase-locked loop function is disabled.
Multiphase Operation
For output loads that demand more than 3A of current,
multiple LTM4623s can be paralleled to run out of phase
to provide more output current without increasing input
and output voltage ripples.
The CLKOUT signal can be connected to the CLKIN pin of
the following LTM4623 stage to line up both the frequency
and the phase of the entire system. Tying the PHMODE pin
to INTVCC, SGND or INTV
CC
/2 generates a phase differ-
ence (between CLKIN and CLKOUT) of 180°, 120°
, or 90°
respectively, which corresponds to 2-phase, 3-phase or
4-phase operation. A total of 12 phases can be cascaded
