LT3957
3
TERMINAL OPTIONS
DC1544A has both
GND
and
SGND
terminals. The sche-
matic shows that the GND and SGND pins of the IC are
not tied together outside the IC. They are internally con-
nected. It is important that power ground currents do not
run on the SGND components. SGND is used to termi-
nate the signal ground components such as compensa-
tion and feedback. Input and output power as well as
input and output caps, catch diodes, etc. should all con-
nect to the GND plane. The SGND terminals are provided
in case the customer would like to make signal ground
measurements, but they do not need to be used. Im-
properly connecting input power to the SGND terminal
results in noisy behavior of the IC.
EN/UVLO
(enable / undervoltage lockout) is provided as
a terminal so that the part can quickly be placed in shut-
down by tying EN/UVLO to GND. The EN/UVLO pin is
already being used as an undervoltage lockout threshold
for the VIN with resistors R1 and R2. Undervoltage lock-
out stops the IC from switching when VIN is too low, but
shutdown further reduces the VIN pin quiescent current
when EN/UVLO is tied directly to GND. It is okay to leave
the EN/UVLO terminal floating.
INTVCC
is provided as a terminal to be able to power
INTVCC externally or to tie INTVCC directly to VIN when
VIN remains below 8V. It is okay to leave the INTVCC
terminal floating.
SYNC
is provided as a terminal to be able to externally
synchronize the internal power switching frequency. If
SYNC is not used, the terminal should be left floating and
the JP1 position should be set at ‘NO SYNC’. If SYNC is
used, the switching frequency of SYNC should be 20%
higher than the programmed switching frequency of the
IC and the JP1 position must be set at ‘SYNC’. Please
see the datasheet for details regarding the use of SYNC
and the switching frequency range.
