ltm4619 Linear Technology Corporation, ltm4619 Datasheet - Page 10

• Manufacturer Part Number: ltm4619
• Description: Dual, 26vin, 4a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTM4619
Mode Selections and Phase-Locked Loop
The LTM4619 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 a DC voltage below 0.8V. To select pulse-skipping mode
of operation, tie the MODE/PLLIN pin to INTV
Burst Mode operation, fl oat the MODE/PLLIN pin.
Frequency Synchronization
A phase-lock loop is available on the LTM4619 to synchro-
nize 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/PLLFLTR pin has the required onboard
PLL fi lter components for clock synchronization. The LTM
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.
10
CC
. To select
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 pro-
gram the controller’s operating frequency from 250kHz to
780kHz using a voltage divider to INTV
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
signifi cant amount of cycle skipping can occur with cor-
respondingly larger current and voltage ripple.
Refer to the fi gure of Output Voltage vs Minimum On-Time
to choose a proper frequency.
Figure 2. Switching Frequency vs FREQ/PLLFLTR Pin Voltage
900
800
700
600
500
400
300
200
100
0
0
0.5
FREQ PIN VOLTAGE (V)
1
1.5
2
CC
4619 F02
(see Figure 19).
2.5
4619f