LMV115MG NSC [National Semiconductor], LMV115MG Datasheet - Page 10

• Manufacturer Part Number: LMV115MG
• Description: GSM Baseband 30MHz 2.8V Oscillator Buffer
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Section
GENERAL
The LMV115 is specially designed to minimize the effects of
spurious signals from the base band chip to the oscillator.
Beside this the influence of varying load resistance and
capacitance to the oscillator is minimized, while increasing
the drive capability. The input of the LMV115 is internally
biased with two equal resistors to the power supply rails, and
makes AC coupling possible without external bias resistors
at the input. The LMV115 has excellent gain phase margin.
The LMV115 offers a shutdown pin that can be used to
disable the device in order to optimize current consumption
and also has a feature to control the supply voltage to an
external oscillator. When the shutdown pin is connected to
V
SWITCHED POWER SUPPLY CONNECTION
The LMV115 features an enable/disable function for an ex-
ternal oscillator by controlling its supply voltage (pin 4). See
also the schematic diagram on the front page. During normal
operating mode, pin 4 is connected to the positive supply rail
via an internal switch. The resistance between the positive
supply rail and pin 4, R
characterization table. Oscillators with a supply current up to
several milliamps can easily be powered from pin 4. During
shutdown, pin 4 is switched to the negative supply rail. The
simplified schematic for this part of the device is shown in
Figure 1
INPUT CONFIGURATION
The input of the LMV115 is internally biased at mid-supply by
a divider of two equal resistors. With the LMV115 in shut-
down mode, the internal resistor connected to the V
shortened to the negative power supply rail via a switch. This
makes the power consumption in ‘off’ mode almost zero, but
causes a small difference for the input impedance between
the on and off modes. Both resistors are 110kΩ so the
resulting input impedance will be approximately 55kΩ. The
input configuration allows AC coupling on the input of the
LMV115. A simplified schematic of the input is shown in
Figure 2.
DD
the device is in shutdown mode.
FIGURE 1. Supply For External Oscillator
ON
, is specified in the electrical
20075131
DD
is
10
PSRR
If an AC signal is applied to one of the supply lines, while the
input is floating, the signal at the input pin is half the signal at
the supply line, causing the same signal at the output of the
buffer. This will result in a PSRR of only 6dB (see Figure 2).
In a typical application the input is driven from a low ohmic
source that means the disturbance at the supply lines is
attenuated by the series resistors of 110k and the source
impedance. In case the buffer is connected to a 50Ω source,
the resulting suppression will be 20*log [(R
= 67dB for signals at the supply line. The PSRR can also be
measured correctly for this type of input by shorten the input
to mid-supply. Due to the internal structure it is not recom-
mended to measure with the input connected to ground. To
measure correctly the PSRR, two signals are applied to both
V
In this case, both signals are subtracted and there will be no
signal at the input. The resulting disturbance at the output is
now only caused by the signals at the supply lines.
INPUT AND OUTPUT LEVEL
Due to the internal loop gain of 1, the output will follow the
input. The output voltage cannot swing as close to the supply
rail as the input voltage. For linear operation the input volt-
age swing should not exceed the output voltage swing. The
restrictions for the output voltage can be examined by the
two curves in Figure 3. The curve V
response of the output signal versus the input signal and the
curve V
output and the input signal.
DD
and V
OUT
EE
– V
but with 180˚ phase difference (see Figure 2).
FIGURE 2. Dual Supply Mode
IN
(V) shows the difference between the
OUT
1
+ R
(V) shows the
BIAS
20075132
)/R
BIAS
]