LMV115MG/NOPB National Semiconductor, LMV115MG/NOPB Datasheet - Page 11

• Manufacturer Part Number: LMV115MG/NOPB
• Description: IC BUFFER OSC GSM 30MHZ SC70-6
• Manufacturer: National Semiconductor

Specifications of LMV115MG/NOPB

• Function: Oscillator Buffer
• Frequency: 30MHz
• Rf Type: GSM
• Secondary Attributes: 2.5V ~ 3.3V supply
• Package / Case: 6-TSSOP, SC-88, SOT-363
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LMV115MG; LMV115MGTR

Application Section
In Figure 3 the input signal is swept between both supply
rails (0V - 2.8V). The linear part of the plot ‘V
covers approximately the voltage range between 1.0V and
2.0V. If a difference of 50mV between output and input is
acceptable, the output range is between 1.05V and 2.15V
(see curve V
swing can be determined by using Figure 4. In the plot ‘Gain
vs. V
from 1.15V till 2.1V. Outside this range the gain differs from
1. This will introduce distortion of the output signal.
Another point is the DC bias voltage necessary to get the
optimum output voltage swing. As discussed above, the
output voltage swing can be 1V
resistors are used, the DC bias will be 1.4V, which is half of
the supply voltage of 2.8V. In this situation the output swing
will exceed the lower limit of 1.15V, so it is necessary to
introduce a small DC offset of 200mV to make use of the full
output swing range of the output stage.
IN
’ it can be seen that the gain is flat for input voltages
OUT
FIGURE 3. V
– V
FIGURE 4. Gain
IN
). Alternatively the output voltage
PP
OUT
, but if the two internal bias
(Continued)
– V
IN
OUT
20075134
vs. V
20075133
IN
’
11
DRIVING RESISTIVE AND CAPACITIVE LOADS
The maximum output current of the LMV115 is about 200µA
which means the output can drive a maximum load of 1V/
200µA = 5kΩ. Using lower load resistances will exceed the
maximum linear output current. The LMV115 can drive a
small capacitive load, but make sure that every capacitor
directly connected to the output becomes part of the loop of
the buffer and will reduce the gain/phase margin, increasing
the instability at higher capacitive values. This will lead to
peaking in the frequency response and in extreme situations
oscillations can occur. A good practice when driving larger
capacitive loads is to include a series resistor to the load
capacitor. A to D converters present complex and varying
capacitive loads to the buffer. The best value for this isolation
resistance is often found by experimentation.
SHUTDOWN MODE
LMV115 offers a shutdown function that can be used to
disable the device and to optimize current consumption.
Switching between the normal mode and the shutdown
mode requires connecting the shutdown pin either to the
negative or the positive supply rail. If directly connected to
one of the supply rails, the part is guaranteed in the correct
mode. But if the shutdown pin is driven by other output
stages, there is a voltage range in which the installed mode
is not certainly set and it is recommended not to drive the
shutdown pin in this voltage range. As can be seen in Figure
5 this hysteresis varies from 1V to 1.6V. Below 1V the
LMV115 is securely ‘ON’ and above 1.6V securely ‘OFF’
while using a supply voltage of 2.8V.
PRINTED CIRCUIT BOARD LAYOUT AND COMPONENT
VALUES SELECTION
For a good high frequency design both the active parts and
the passive ones should be suitable for the purpose they are
used for. Amplifying high frequencies is possible with stan-
dard through-hole components, but for frequencies above
several hundreds of MHz the best choice is using surface
mount devices. Nowadays designs are often assembled with
surface mount devices for the aspect of minimizing space,
but this also greatly improves the performance of designs,
handling high frequencies. Another important issue is the
PCB, which is no longer a simple carrier for all the parts and
a medium to interconnect them. The board becomes a real
FIGURE 5. Hysteresis
20075135
www.national.com