LTC1928-5
7
19285fa
For more information www.linear.com/LTC 1928-5
APPLICATIONS INFORMATION
Shutdown
When CN/SHDN = 0V, the part will be in shutdown, the
supply current will be <8µA and V
OUT
will be shorted
to ground through a 160Ω switch. In addition, CPO
will be high impedance and disconnected from V
IN
and
CN/SHDN.
Shutdown is achieved by internally sampling the
CN/SHDN pin for a low voltage. Time between shutdown
samples is about 30µs. During the sample time the charge
pump switches are disabled and CN/SHDN must be pulled
to ground within 400ns. A resistor value between 100Ω
and 1k is recommended. Parasitic lead capacitance should
be minimized on the CN/SHDN pin.
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
Figure 3. LTC1928-5 Noise Measurement Test Setup
Figure 4. LTC1928-5, External Load on CPO, No Shutdown State
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
performance 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 connec
-
tion with 50Ω. Coaxial connections must be maintained
to preserve measurement integrity.
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
OR COUPLERS
BATTERY OR
LOW NOISE DC
V
OUT
R
LOAD
R*
R*
*50Ω TERMINATIONS
HP-11048C OR
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×
4
5
6
3
2
1
V
OUT
GND
V
IN
CPO
CP
CN/SHDN
IN
GND
OUT
C
OUT
4.7µF
C
IN
4.7µF
C
FLY
0.47µF
IN
3V
C
CPO
4.7µF
10µF
19285 F04
ADDITIONAL
LDO
V
RIPPLE
< 800µV
P-P
