Datasheet

10
LTC1682/LTC1682-3.3/LTC1682-5
APPLICATIONS INFORMATION
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General Layout Considerations
Due to the high switching frequency and high transient
currents produced by the device, careful board layout is a
must. A clean board layout using a ground plane and short
connections to all capacitors will improve noise perfor-
mance and ensure proper regulation (Figure 5).
The FILT pin on the LTC1682-3.3/LTC1682-5 is a high
impedance node. Leakage currents at this pin must be
minimized.
Measuring Output Noise
Measuring the LTC1682 low noise levels requires care.
Figure 6 shows a test setup for taking the measurement.
Good connection and signal handling technique should
yield about 500µV
P-P
over a 2.5MHz bandwidth. The noise
measurement involves AC coupling the LTC1682 output
into the test setup’s input and terminating this connection
with 50Ω. Coaxial connections must be maintained to
preserve measurement integrity.
Power-On Reset
Upon initial power-up, a power-on reset circuit ensures
that the internal functions are correctly initialized when
power is applied. Once V
IN
reaches approximately 1V, the
power-on reset circuit will enable the part as long as the
SHDN pin is held high.
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
device power dissipation P
D
= I
OUT
(2V
IN
– V
OUT
) +
V
IN
(4mA). The device dissipates the majority of its heat
through its pins, especially GND (Pin 4). Thermal resis-
tance to ambient can be optimized by connecting GND to
a large copper region on the PCB, which serves as a heat
sink. Applications which operate the LTC1682 near maxi-
mum power levels should maximize the copper area at all
pins except C
+
, C
–
and FILT/FB and ensure that there is
some airflow over the part to carry away excess heat.
1
2
3
4
LTC1682-3.3/5
8
7
6
5
C1
V
OUT
V
IN
1682 F05
C3
C2
GND
SHDN
C4
C
FILT
LTC1682
DEMO
BOARD
OSCILLOSCOPE
BANDWIDTH
FILTER
CONNECT BNC AND
R
LOAD
GROUND TO THE
OUTPUT CAPACITOR
GROUND TERMINAL E5
PLACE COUPLING
CAPACITOR IN SHIELDED
BOX WITH COAXIAL
CONNECTOR
COUPLING
CAPACITOR
BNC CABLES
OR COUPLERS
BATTERY OR
LOW NOISE DC
POWER SUPPLY
V
OUT
R
LOAD
R*
R*
*50Ω TERMINATIONS
HP-11048C OR
EQUIVALENT
R*
NOTE: KEEP BNC CONNECTIONS
AS SHORT AS POSSIBLE
PLACE BANDWIDTH FILTER
COMPONENTS IN SHIELDED BOX
WITH COAXIAL CONNECTORS
PREAMPLIFIER
1822
INPUT
1682 F06
–
+
20dB
Figure 6. LTC1682 Noise Measurement Test Setup
Figure 5