Datasheet

LTM4622
11
Rev. G
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APPLICATIONS INFORMATION
always turns on with each oscillator pulse. During start-
up, forced continuous mode is disabled and inductor
current is prevented from reversing until the LTM4622’s
output voltage is in regulation.
Operating Frequency
The operating frequency of the LTM4622 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 appli-
cations, 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, R
FSET
, between the FREQ pin and GND,
as shown in Figure26. The operating frequency can be
calculated as:
f Hz
( )
=
3.2e11
324k ||R
FSET
Ω
( )
To reduce switching current ripple, 1.5MHz to 2.5MHz
operating frequency is required for 2.5V to 5.5V output
with R
FSET
to GND.
V
OUT
0.6V to
1.8V 2.5V 3.3V 5V
f
SW
1MHz 1.5MHz 2MHz 2.5MHz
R
FSET
Open 649kΩ 324kΩ 215kΩ
The operating frequency can also be decreased by adding
a resistor between the FREQ pin and INTV
CC
, calculated
as:
f Hz
( )
= 1MHz –
5.67e11
R
FSET
Ω
( )
The programmable operating frequency range is from
800kHz to 4MHz.
Frequency Synchronization
The power module has a phase-locked loop comprised of
an internal voltage controlled oscillator and a phase detec-
tor. This allows the internal top MOSFET turn-on to be
locked to the rising edge of the external clock. The exter-
nal clock frequency range must be within ±30% around
the set operating frequency. A pulse detection circuit is
used to detect a clock on the SYNC/MODE 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. The presence
of an external clock will place both regulator channels into
forced continuous mode operation. During the start-up of
the regulator, the phase-locked loop function is disabled.
Multiphase Operation
For output loads that demand more than 2.5A of current,
two outputs in the LTM4622 or even multiple LTM4622s
can be paralleled to run out of phase to provide more
output current without increasing input and output volt-
age ripples.
A multiphase power supply significantly 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 out-
put voltage). The output ripple amplitude is also reduced
by the number of phases used when all of the outputs
are tied together to achieve a single high output current
design.
The two switching mode regulator channels inside the
LTM4622 are internally set to operate 180° out of phase.
Multiple LTM4622s could easily operate 90 degrees,
60 degrees or 45 degrees shift which corresponds to
4-phase, 6-phase or 8-phase operation by letting SYNC/
MODE of the LTM4622 synchronize to an external multi-
phase oscillator like LTC
®
6902. Figure2 shows a 4-phase
design example for clock phasing.
Figure2. Example of Clock Phasing for 4-Phase
Operation with LTC6902
4622 F02
V
+
3.3V INTV
CC
PH
SET
LTC6902
33.2k, 1.5MHz
0°
SYNC/MODE V
OUT1
10A
180°
V
OUT2
DIV
GND
90°
SYNC/MODE V
OUT1
270°
0°
90°
V
OUT2
MOD
OUT1
OUT2
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