LTM4622A
11
Rev B
APPLICATIONS INFORMATION
on with each oscillator pulse. During start-up, forced
continuous mode is disabled and inductor current is
prevented from reversing until the LTM4622A’s output
voltage is in regulation.
Operating Frequency
The operating frequency of the LTM4622A is optimized
to achieve the compact package size and the minimum
output ripple voltage while still keeping high efficiency.
The default operating frequency is internally set to 1MHz.
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 29. The operating frequency can be
calculated as:
f Hz
( )
=
3.2e11
324k||R
FSET
Ω
( )
Please note a minimum switching frequency is required for
given V
IN
, V
OUT
operating conditions to keep a maximum
peak-to-peak inductor ripple current below 1.2A for the
LTM4622A.
The peak-to-peak inductor ripple current can be calculated as:
Δ
I
P-P
=
V
OUT
2.2
1
−
V
OUT
V
IN
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
•
1
f
SW
(MHz)
The maximum 1.2A 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.
To reduce switching current ripple, 1.5MHz to 2MHz op-
erating frequency is suggested for 5V and above output
with R
FSET
to GND.
V
OUT
1.5V to 3.3V
5V, 8V
12V
f
SW
1MHz
1.5MHz
1.5MHz to 2MHz
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
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 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 2A of current, two
outputs in the LTM4622A or even multiple LTM4622As can
be paralleled to run out of phase to provide more output
current without increasing input and output voltage ripples.
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 when all of the outputs are tied together
to achieve a single high output current design.
The two switching mode regulator channels inside the
LTM4622A are internally set to operate 180° out of phase.
Multiple LTM4622As 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 LTM4622A synchronize to an external multiphase
. Figure 2 shows a 4-phase design
example for clock phasing.
