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LM9022 Datasheet(PDF) 4 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM9022
Description  Vacuum Fluorescent Display Filament Driver
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LM9022 Datasheet(HTML) 4 Page - National Semiconductor (TI)

   
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Application Information
LM9022 FUNCTIONALITY
Typically a VFD filament requires a low voltage AC power
source in order to create a constant brightness across its
length. Such a power source is not readily available in a
most systems. Existing implementations show different cir-
cuits for supplying an AC power for a VFD filament but they
require an AC power input, or do not have a standby control,
or generate high EMI. The LM9022 can solve all these
problems in one compact circuit.
A DC power supply is employed to power two power opera-
tional amplifiers: POA1 and POA2. The first power opera-
tional amplifier (POA1) can utilize an external feedback cir-
cuit that will cause it self-oscillate. In a second version,
POA1 is driven from an external signal source. The shape of
the output wave delivered by POA1 can be square, sinusoi-
dal, triangular, trapezoidal, clipped sinusoidal or any other
shape, depending on the feedback circuit or the signal
source used. The output of this POA1 is connected exter-
nally to one end of the VFD filament, and internally to the
input of a second power operational amplifier: POA2. POA2
is internally configured as an inverting unity gain circuit. The
output of the POA2 is connected to the second end of the
VFD filament. This provides a differential and symmetrical
AC signal to the fila
An external standby control signal applied to the Shutdown
pin can be used to turn of both power operational amplifiers.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM9022 has two operational
amplifiers internally. Figure 1 shows that the output of am-
plifier one serves as the input to amplifier two, which results
in both amplifiers producing signals identical in magnitude,
but 180˚ out of phase.
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions.
POWER DISSIPATION
For the SO package,
θ
JA = 140˚C/W, for the DIP package,
θ
JA = 107˚C/W, and for the MSOP package,
θ
JA = 210˚C/W
assuming free air operation. The
θ
JA can be decreased by
using some form of heat sinking. The resultant
θ
JA will be the
summation of the
θ
JC,
θ
CS, and
θ
SA.
θ
JC is the junction to
case of the package,
θ
CS is the case to heat sink thermal
resistance and
θ
SA is the heat sink to ambient thermal
resistance. By adding additional copper area around the
LM9022, the
θ
JA can be reduced from its free air value for
the SO package. Depending on the ambient temperature,
T
A, and the
θ
JA, Equation 2 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 1 is greater than that of Equation 2,
then either the supply voltage must be decreased, the load
impedance increased, the
θ
JA decreased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 10
Ω load, and no additional heatsink-
ing, the maximum ambient temperature possible without
violating the maximum junction temperature is approximately
61˚C provided that device operation is around the maximum
power dissipation point and assuming surface mount pack-
aging.
POWER SUPPLY PIN
As with any power device, proper supply bypassing is critical
for low noise performance. Typical applications will require
both a 22µf electrolyte and a 0.1µF ceramic capacitor to
bypass the supply pin to ground. These capacitors should be
as close to the LM9022 as is physically possible, and are in
addition to any capacitors that may be needed for regulator
stability.
BYPASS PIN
The internal bias circuit (Fig 1) generates an internal refer-
ence voltage that is typically equal to one half of V
DD. This
voltage is available at the bypass pin and is applied directly
to the non-inverting input of the inverting driver. Typical
applications will require a bypass capacitor in the range of
0.1µF to 1µF to bypass the supply pin to ground. This
capacitor should be as close to the LM9022 as is physically
possible.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM9022 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half- supply. It is best to switch between ground and
supply to provide maximum device performance. By switch-
ing the shutdown pin to V
DD, the LM9022 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less then V
DD, the idle
current may be greater than the typical value of 0.6µA. In
either case, the shutdown pin should be tied to a definite
voltage to avoid unwanted state changes.
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