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LM9022N Datasheet(PDF) 4 Page - National Semiconductor (TI) |
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LM9022N Datasheet(HTML) 4 Page - National Semiconductor (TI) |
4 / 7 page 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. www.national.com 4 |
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