Datasheet

LTM4619
10
4619fb
For more information www.linear.com/LTM4619
Mode Selections and Phase-Locked Loop
The LTM4619 can be enabled to enter high efficiency
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 a DC voltage below 0.8V. To select pulse-skipping mode
of operation, tie the MODE/PLLIN pin to INTV
CC
. To select
Burst Mode operation, float the MODE/PLLIN pin.
Frequency Synchronization
A phase-lock loop is available on the LTM4619 to syn
-
chronize the internal clock to an external clock source
connected on the MODE/PLLIN pin. The clock high level
needs to be higher than 1.6V and the clock low level needs
to be lower than 1V. The frequency programming voltage
and or the programming voltage divider must be removed
from the FREQ/PLLFLTR pin when synchronizing to an
external clock. The FREQ/PLLFL
TR pin has the required
onboard PLL filter components for clock synchronization.
The LTM4619 will default to forced continuous mode while
being clock synchronized. Channel 1 is synchronized to
the rising edge on the external clock, and channel 2 is 180
degrees out-of-phase with the external clock.
applicaTions inForMaTion
Frequency Selection
The switching frequency of the LTM4619’s controllers
can be selected using the FREQ/PLLFLTR pin. If the
MODE/PLLIN pin is not being driven by an external clock
source, the FREQ/PLLFLTR pin can be set from 0V to 2.4V to
program the controller’s operating frequency from 250kHz
to 780kHz using a voltage divider to INTV
CC
(see Figure
19). The typical frequency is 780kHz. If the output is too
low or the minimum on-time is reached, the frequency
needs to decrease to enlarge the turn-on time. Otherwise,
a significant amount of cycle skipping can occur with cor
-
respondingly larger current and voltage ripple.
Figure 2. Switching Frequency vs FREQ/PLLFLTR Pin Voltage
FREQ/PLLFLTR PIN VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
0.5 1 1.5 2
4619 F02
2.5
0
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600