Datasheet
LTM4618
11
4618fa
APPLICATIONS INFORMATION
Output Capacitors
The LTM4618 is designed for low output voltage ripple
noise. The bulk output capacitors defi ned as C
OUT
are
chosen with low enough effective series resistance (ESR) to
meet the output voltage ripple and transient requirements.
C
OUT
can be a low ESR tantalum capacitor, a low ESR
polymer capacitor or ceramic capacitor. The typical output
capacitance range is from 100µF to 300µF. Additional output
fi ltering may be required by the system designer if further
reduction of output ripple or dynamic transient spikes is
required. Table 4 shows a matrix of different output voltages
and output capacitors to minimize the voltage droop and
overshoot during a 3A/µs transient. The table optimizes the
total equivalent ESR and total bulk capacitance to optimize
the transient performance. Stability criteria are considered
in the Table 4 matrix, and the Linear Technology µModule
Power Design Tool is available for stability analysis. Mul-
tiphase operation will reduce effective output ripple as a
function of the number of phases. Application Note 77
discusses this noise reduction versus output ripple cur-
rent cancellation, but the output capacitance should be
considered carefully as a function of stability and transient
response. The Linear Technology µModule Power Design
Tool can calculate the output ripple reduction as the number
of implemented phases increases by N times.
Mode Selections and Phase-Locked Loop
The LTM4618 can be enabled to enter high effi ciency
Burst Mode operation, constant-frequency, pulse-skipping
mode, or forced continuous conduction mode. To select
the forced continuous operation, tie the MODE/PLLIN pin
to ground. To select pulse-skipping mode of operation,
tie the MODE/PLLIN pin to INTV
CC
. To select Burst Mode
operation, fl oat the pin.
A phase-locked loop (PLL) is available on the LTM4618
to synchronize the internal oscillator to an external clock
source that is connected to the MODE/PLLIN pin. The
incoming clock should be applied before the regulator’s
RUN pin is enabled.
Frequency Selection
The switching frequency of the LTM4618’s controller
can be selected using a DC voltage. If the MODE/PLLIN
pin is not being driven by an external clock source,
the FREQ pin can program the controller’s operating
frequency from 250kHz to 780kHz by connecting a re-
sistor divider as shown in Figure 21. When operating
below 400kHz, ensure the inductor ripple current is at
a reasonable level. The typical frequency is 750kHz.
But if the minimum on-time is reached, a lower frequency
needs to be set to increase the turn-on time. Otherwise, a
signifi cant amount of cycle skipping can occur with cor-
respondingly larger ripple current and voltage ripple.
FREQ PIN VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
0.5 1 1.5 2
4618 F03
2.5
0
100
300
400
500
900
800
700
200
600
Figure 3. Relationship Between Switching
Frequency and Voltage at the FREQ Pin
Frequency Synchronization
The MODE/PLLIN pin allows the LTM4618 to be syn-
chronized to an external clock (between 250kHz to
780kHz) and the internal phase-locked loop allows the
LTM4618 to lock onto input clock phase as well. When
operating below 400kHz, ensure the inductor ripple cur-
rent is at a reasonable level. No more than 50% of the
load current is recommended. The FREQ pin has the
onboard loop fi lter for the PLL. The incoming clock must
be applied before the RUN pin is enabled. For applica-
tions powering the clock source from the LTM4618’s
INTV
CC
, the RUN pin has to be enabled in order to ac-
tivate INTV
CC
for the clock source. In this situation (see
Figure 22) the TK/SS pin can be used to soft-start the
regulator for 100ms using a ≈ 0.22µF capacitor. This will
allow the regulator to synchronize to the right frequency
before the regulator’s inductor ripple current peaks.