LM9022MX/NOPB National Semiconductor, LM9022MX/NOPB Datasheet - Page 4

• Manufacturer Part Number: LM9022MX/NOPB
• Description: IC VF DISP FILAMENT DRIVER 8SOIC
• Manufacturer: National Semiconductor

Specifications of LM9022MX/NOPB

• Display Type: Vacuum Fluorescent (VF)
• Current - Supply: 6.5mA
• Voltage - Supply: 2 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Interface: -
• Configuration: -
• Digits Or Characters: -
• Other names: LM9022MX

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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 circuits 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 circuit
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, sinusoidal, triangu-
lar, trapezoidal, clipped sinusoidal or any other shape, de-
pending on the feedback circuit or the signal source used. The
output of this POA1 is connected externally 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 con-
nected 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
plifiers internally.
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 configuration 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 conditions.
POWER DISSIPATION
For the SO package, θ
θ
assuming free air operation. The θ
using some form of heat sinking. The resultant θ
JA
= 107°C/W, and for the MSOP package, θ
Figure
Figure 1
1, the LM9022 has two operational am-
JA
= 140°C/W, for the DIP package,
shows that the output of amplifier
200215 Version 5 Revision 2
JA
can be decreased by
JA
JA
= 210°C/W
will be the
Print Date/Time: 2009/12/02 10:11:32
4
summation of the θ
of the package, θ
and θ
adding additional copper area around the LM9022, the θ
can be reduced from its free air value for the SO package.
Depending on the ambient temperature, T
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 in-
creased, the θ
duced. For the typical application of a 5V power supply, with
an 10Ω load, and no additional heatsinking, the maximum
ambient temperature possible without violating the maximum
junction temperature is approximately 61°C provided that de-
vice operation is around the maximum power dissipation point
and assuming surface mount packaging.
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 by-
pass the supply pin to ground. These capacitors should be as
close to the LM9022 as is physically possible, and are in ad-
dition 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
voltage is available at the bypass pin and is applied directly
to the non-inverting input of the inverting driver. Typical ap-
plications 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 am-
plifier off when a logic high is placed on the shutdown pin. The
trigger point between a logic low and logic high level is typi-
cally half- supply. It is best to switch between ground and
supply to provide maximum device performance. By switch-
ing the shutdown pin to V
will be minimized in idle mode. While the device will be dis-
abled with shutdown pin voltages less then V
current may be greater than the typical value of 0.6µA. In ei-
ther case, the shutdown pin should be tied to a definite voltage
to avoid unwanted state changes.
SA
is the heat sink to ambient thermal resistance. By
JA
CS
decreased, or the ambient temperature re-
JC
is the case to heat sink thermal resistance
, θ
CS
DD
, and θ
, the LM9022 supply current draw
SA
. θ
JC
is the junction to case
A
, and the θ
DD
, the idle
DD
. This
JA
JA
,