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

θ

θ

assuming free air operation. The

θ

using some form of heat sinking. The resultant

θ

summation of the

θ

θ

θ

θ

case of the package,

θ

resistance and

θ

resistance. By adding additional copper area around the

LM9022, the

θ

the SO package. Depending on the ambient temperature,

T

θ

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

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

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

draw will be minimized in idle mode. While the device will be

disabled with shutdown pin voltages less then V

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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