feature

Fast Transient Response Low Voltage Drop Load Regulation up to 4.5A: 0.5% Typical On-Chip Thermal Limit Standard TO-220 , TO-263 Center Cut and TO-252 Packages

application

Desktop PC, RISC and Embedded Processor Supply GTL, SSTL Logic Reference Bus Power Low Voltage VCC Logic Power Battery Power Supply Circuit Switching Power Supply Post Regulator Cable and ADSL Modem DSP Core Power Supply Set Top Box and Network Box Module Supply

describe

FAN1084 , FAN1084-1.5 and FAN1084-3.3 are three-terminal regulators capable of low output current of 4.5A. These units have been optimized for low voltage applications including VTT bus termination where response and minimum input voltage are critical. The FAN1084 is ideal for low voltage microprocessor applications requiring a regulated output of 1.5V to 3.6A with an input supply of 5V or less. FAN1084-1.5 provides 1.5V and 4.5A current capability termination of GTL+ bus VTT. The FAN1084-3.3 provides a fixed 3.3V output at 4.5A. On-chip thermal limiting provides that any combination of overload and ambient temperature will result in excessive junction temperature. The FAN1084 series regulators are available in industry standard TO-220, TO-263 mid-cut and TO-252 (DPAK). Power pack.

application information

Generally FAN1084, FAN1084-1.5 and FAN1084-3.3 are three-terminal regulators optimized for GTL+VTT termination and logic applications. These devices are short-circuit protected and provide thermal shutdown to shut down the regulator when the junction temperature exceeds approximately 150 °C. The FAN1084 series offers low voltage and fast transient response. Frequency compensation uses capacitors to lower ESR while maintaining stability. This is critical in order to meet the needs of a low voltage high speed microprocessor bus like GTL+. Stability The FAN1084 series requires an output capacitor for frequency compensation. A 22µF solid tantalum or 100µF aluminum electrolytic is recommended for stability. Frequency compensation for these devices utilizes low esr capacitors to optimize frequency response. In general, an ESR of <0.2 is recommended. Bypass capacitors such as 22µF tantalum or 100µF aluminum on adjust pins are also recommended for low ripple and fast transient response of the FAN1084. When these bypass capacitors are not used at the regulation pins, smaller values of output capacitance provide equally good results. Refer to the Typical Performance Characteristics Output Capacitor esr Stability Chart with Load Current Protection Diodes In normal operation, the FAN1084 series does not require any protection diodes. For the FAN1084, internal resistors limit the internal current path on the adjust pins. Therefore, even with bypass capacitors on the regulation pins, no protection diodes are required to ensure device safety in short-circuit conditions. Protection diodes between input and output pins are usually not required. The internal diodes between the input and the output pins of the FAN1084 series can withstand microsecond surge currents of 50A to 100A, even with large value output capacitors it is difficult to obtain those surge values during normal operation. Only large value output capacitors, such as 1000µF to 5000µF, and the momentary short circuit of the input pin to ground will damage it. A crowbar circuit at the input can generate current levels; diodes from output to input are recommended as shown. Normally, normal power cycling or system "hot swapping" will not draw enough current to cause any harm. Regarding the output, without any device or any IC regulator, exceeding the maximum input-output voltage difference causes the internal transistor to turn off and then all the protection circuits cannot work properly. The regulation pin can be driven on a transient basis of ±7V on the output without any device degradation. As with any IC regulator, exceeding the maximum input-output voltage difference causes the internal transistors to turn off and then all protection circuits fail to function properly.

Ripple suppression

In applications requiring improved ripple rejection, bypassing the capacitor from the fan 1084 regulation pin to ground reduces the output ripple by a ratio of Vout/1.25V. The impedance of the trim pin capacitor at the ripple frequency should be less than the value of r1 (usually 100Ω to 120Ω in the feedback divider network in the figure). Therefore, the value of the required trim pin capacitor is a function of the input ripple frequency. For example, if r1 is equal to 100Ω and the ripple frequency is equal to 120Hz, the trim pin capacitor should be 22µF. At 10kHz, only 0.22µF is needed. The output voltage FAN1084 regulator generates a 1.25V reference voltage between the output pin and the adjust pin. Placing a resistor r1 between these two terminals produces a constant current flowing through r1 and down through r2 to set the total output voltage. Typically, this current is a minimum load current of 10mA. The current from the adjustment pin is summed with the current from r1. It has a small output voltage contribution and should only be considered if a very precise output voltage setting is required.

load regulation

A true remote sensing load cannot be provided because the FAN1084 series are three terminal devices. Load regulation is limited by the resistance of the wires connecting the regulator to the load. Adjusting the load specification according to the datasheet is measured on the bottom of the package. For fixed voltage devices, the negative side sensing is the correct Kelvin connection to the device ground pin back to the negative side of the load. as the picture shows.

The Kelvin connection at the bottom of the output divider returns to the negative side of the load. The best load regulation is obtained when the top of the resistor divider r1 is connected directly to the regulator output instead of the load. illustrates this. If r1 is connected to the load, the effective resistance is between the regulator and the load: Rp x (1+r2/r1), Rp = parasitic wire resistance The connections shown in the diagram do not multiply rp by the division ratio. For example, Rp is about 4 milliohms per 16 gauge pin. That means 4mv load current per foot at 1A. At higher load currents, this drop represents a large percentage of the overall regulation. It's important to keep the positive lead between the regulator and the load as short as possible, and use large wire or PC board traces.

thermal conditions

The FAN1084 series protects itself from overload conditions with an internal power supply and thermal limiting circuit. However, for normal continuous load conditions, do not exceed the maximum junction temperature rating. It is important to consider all sources of thermal resistance from junction to surroundings. These sources include connection-to-case resistance, case-to-heatsink interface resistance, and heatsink resistance. Thermal resistance specifications more accurately reflect device temperature and ensure safe operating temperatures. The Power Characteristics section provides individual thermal resistances and maximum junction temperatures for control circuits and power transistors. Calculate the maximum joining temperature of the two sections to ensure that the limit is reached. For example, use fan1084t to generate 4.5A@3.3V power supply (3.2V to 3.6V) 1.5V. Assuming VIN = 3.4V worst case Vout = 1.475V worst case IOUT = 4.5A continuous Ta = 60°C θ ambient temperature = 5°C/W (assuming heat sink and thermally conductive material) The power dissipation in this application is : Pd=(VIN-Voucher)*(input)=(3.6–1.475)*(4.5)=9.6W From spec sheet: Tj=Ta+(Pd)*(theta case to ambient+thetajc)=60+ (9.6)*(5+3=137°C junction temperature is below the maximum thermal limit. The IC specifies a thermal resistance connected to the case to the bottom of the case just below the die. This is the path of least resistance for heat flow. Proper installation ensures that From this area to the heat sink. Using thermally conductive material is recommended to use the chassis to heat sink interface. Use thermally conductive spacers if the device enclosure must be electrically isolated and include resistance to its contribution to total heat. The FAN1084 series of enclosures are directly connected to the output of the device.