LM9022
Vacuum Fluorescent Display Filament Driver
General Description
The LM9022 is a bridged power amplifier capable of deliv-
ering typically 2W of continuous average power into a 10
filament load when powered by a 5V power supply.
To conserve power in portable applications, the LM9022's
micropower shutdown mode (I
Q
= 0.6µA, typ) is activated
when V
DD
is applied to the SHUTDOWN pin.
Additional LM9022 features include thermal shutdown pro-
tection, unity-gain stability, and external gain set.
Key Specifications
n
I
DD
during shutdown
0.6µA (typ)
n
Thermal Shutdown Protection
Features
n
No transformers required
n
SO or DIP packaging
Applications
n
VCR/DVD Displays
n
RADIO/TUNER Displays
Typical Application
T
A
= 25°C, V
DD
= 5V, unless otherwise specified.
Connection Diagram
MSOP, Small Outline, and DIP Package
20021502
Top View
Order Number LM9022M or LM9022N
See NS Package Number M08A or N08E
20021501
FIGURE 1. Typical Application Circuit
July 2002
LM9022
V
acuum
Fluorescent
Display
Filament
Driver
© 2002 National Semiconductor Corporation
DS200215
www.national.com
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
-0.3V to +6.0V
Supply Temperature
-65°C to +150°C
Input Voltage
-0.3V to V
DD
+0.3V
Power Dissipation (Note 3)
Internally Limited
ESD Susceptibility (Note 4)
5000V
ESD Susceptibility (Note 5)
250V
Junction Temperature
150°C
Soldering Information
Small Outline Package
Vapor Phase (60 sec.)
215°C
Infrared (15 sec.)
220°C
See AN-450 'Surface Mounting and their Effects on
Product Reliability' for other methods of
soldering surface mount devices.
JC
(typ) -- M08A
35°C/W
JA
(typ) -- M08A
140°C/W
JC
(typ) -- N08E
37°C/W
JA
(typ) -- N08E
107°C/W
Operating Ratings
Temperature Range
T
MIN
T
A
T
MAX
-40°C
T
A
85°C
Supply Voltage
2.0V
V
DD
5.5V
Electrical Characteristics
(Notes 1, 2)
The following specifications apply for V
DD
= 5V, V
PIN3
= V
PIN2
, V
SHUTDOWN
= 0V, and R
L
= 10
unless otherwise specified.
Limits apply for T
A
= 25°C.
Symbol
Parameter
Conditions
LM9022
Min
(Note 7)
Typical
(Note 6)
Max
(Note 7)
Units
I
DD
Quiescent Power Supply
Current
V
IN
= 0V, I
o
= 0A, V
SHUTDOWN
= 0V
6.5
10.0
mA
Power Supply Current during
shutdown
V
SHUTDOWN
= V
DD
(Note 8)
0.6
2
µA
V
BP
Bypass Pin Voltage
V
IN
= 0V
2.4
2.5
2.6
V
V
OUT
Output Voltage Across R
L
R
L
= 10
3.6
4.3
V
R
L
= 20
4.2
4.6
V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 2: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T
JMAX
,
JA
, and the ambient temperature T
A
. The maximum
allowable power dissipation is P
DMAX
= (T
JMAX
T
A
)/
JA
or the number given in Absolute Maximum Ratings, whichever is lower. For the LM9022, T
JMAX
= 150°C.
For the
JA
's for different packages, please see the Application Information section or the Absolute Maximum Ratings section.
Note 4: Human body model, 100pF discharged through a 1.5k
resistor.
Note 5: Machine Model, 220pF240pF discharged through all pins.
Note 6: Typicals are specified at 25°C and represent the parametric norm.
Note 7: Limits are guaranteed to National's AOQL (Average Outgoing Quality Level) by design, test, or statistical analysis.
Note 8: Both outputs are high impedance when in shutdown mode.
LM9022
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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,
JA
= 140°C/W, for the DIP package,
JA
= 107°C/W, and for the MSOP package,
JA
= 210°C/W
assuming free air operation. The
JA
can be decreased by
using some form of heat sinking. The resultant
JA
will be the
summation of the
JC
,
CS
, and
SA
.
JC
is the junction to
case of the package,
CS
is the case to heat sink thermal
resistance and
SA
is the heat sink to ambient thermal
resistance. By adding additional copper area around the
LM9022, the
JA
can be reduced from its free air value for
the SO package. Depending on the ambient temperature,
T
A
, and the
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
JA
decreased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 10
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.
LM9022
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