LTM4625Y Просмотр технического описания (PDF) - Analog Devices
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LTM4625Y Datasheet PDF : 26 Pages
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LTM4625
APPLICATIONS INFORMATION
Discontinuous Current Mode (DCM)
In applications where low output ripple and high efficiency
at intermediate current are desired, discontinuous current
mode (DCM) should be used by connecting the MODE pin
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 INTVCC. 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 LTM4625’s output voltage is in regulation.
Operating Frequency
The operating frequency of the LTM4625 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 1MHz. In most applications,
no additional frequency adjustment is required.
If an operating frequency other than 1MHz is required by
the application, the operating frequency can be increased
by adding a resistor, RFSET, between the FREQ pin and
SGND, as shown in Figure 22. The operating frequency
can be calculated as:
f
(Hz
)
=
1
1.6e11
62k ||RFSET
(Ω
)
The operating frequency can also be decreased by adding
a resistor between the FREQ pin and INTVCC, calculated as:
f
(Hz
)
=
1
M
Hz
–
2.8e11
RFSET (Ω
)
The programmable operating frequency range is from
800kHz to 4MHz.
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
resistor 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 5A of current,
multiple LTM4625s 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 LTM4625 stage to line up both the frequency
and the phase of the entire system. Tying the PHMODE pin
to INTVCC, SGND or INTVCC/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
to run simultaneously out of phase with respect to each
other by programming the PHMODE pin of each LTM4625
to different levels. Figure 2 shows a 4-phase design and
a 6-phase design example for clock phasing.
Table 2. PHMODE Pin Status and Corresponding Phase
Relationship (Relative to CLKIN)
PHMODE
INTVCC
SGND
INTVCC/2
CLKOUT
180°
120°
90°
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
Rev D
10
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