Datasheet

7
LTC1928-5
APPLICATIONS INFORMATION
WUU
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Power-On Reset
Upon initial power-up, a power-on reset circuit ensures
that the internal functions are correctly initialized. Once
V
IN
reaches about 1V, the power-on reset circuit will
enable the part as long as the CN/SHDN pin is not pulled
low.
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
device dissipation P
D
= I
OUT
(2V
IN
– V
OUT
) + V
IN
(2mA). The
device dissipates the majority of its heat through its pins,
especially GND (Pin 2). Thermal resistance to ambient can
be optimized by connecting GND to a large copper region
on the PCB, which serves as a heat sink. Applications that
operate the LTC1928-5 near maximum power levels should
maximize the copper area at all pins except CP and
CN/SHDN and ensure that there is some airflow over the
part to carry away excess heat.
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.
Measuring Output Noise
Measuring the LTC1928 low noise levels requires care.
Figure 3 shows a test setup for taking the measurement.
Good connection and signal handling technique should
yield about 800µV
P-P
over a 2.5MHz bandwidth. The noise
measurement involves AC coupling the LTC1928 output
into the test setup’s input and terminating this connection
with 50Ω. Coaxial connections must be maintained to
preserve measurement integrity.
Figure 3. LTC1928-5 Noise Measurement Test Setup
Figure 4. LTC1928-5, External Load on CPO, No Shutdown State
4
5
6
3
2
1
V
OUT
GND
V
IN
CPO
CP
CN/SHDN
3.3V
IN
GND
OUT
C
OUT
4.7µF
C
IN
4.7µF
C
FLY
0.47µF
5V
V
IN
3V
C
CPO
4.7µF
10µF
19285 F04
LTC1928-5
ADDITIONAL
LDO
V
RIPPLE
< 800µV
P-P
LTC1928
DEMO
BOARD
OSCILLOSCOPE
BANDWIDTH
FILTER
CONNECT BNC AND
R
LOAD
GROUND TO THE
OUTPUT CAPACITOR
GROUND TERMINAL
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
PREAMP
1822
INPUT
19285 F03
–
+
10×
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.