feature

Nearly Transient Response Low Voltage Drop Up to 5A Load Regulation: 0.05% Trim Typical Current Limiting On-Chip Thermal Limiting Packages: TO-220 , TO-252, TO-263 and TO-263 Heart Cut

application

AGTL+ Bus Power for FC-PGA Low Voltage Logic Power Switching Power Supply Post Regulator 3.3V to 1.5V Linear Regulator 5V to 1.8V Linear Regulator

illustrate

The Fan1585A, Fan1585A-1.5, and Fan1585A-1.8 are output current low attenuation three terminal regulator capability. These devices have been optimized for low voltage applications, including VTT bus termination transient response and where the minimum input voltage is critical. The FAN1585A-1.5 Offers fixed 1.5V with 5A bus VTT termination current capability PGA FAN1585A of the present invention Ideal for low voltage microprocessor applications requiring regulated output 1.5v to 3.6v with an input supply of 5v or less, or for fc- with significant trace resistance The pga application current limit is compressed to the specific output current and controls the short circuit current. The thermal confinement of the chip provides protection against any combination of overloaded ambient temperatures that will cause the superjunction temperature. The FAN1585A series regulators are available in the industrystandard to-220, to-252, to-263 and to-263 centercut power packs

Application Information General The FAN1585A and FAN1585A-1.5 are three termination regulators optimized for GTL+VTT termination and logic applications. These units are short-circuit protected and provide thermal shutdown when the regulator temperature exceeds about 150°C. The FAN1585A series offers low voltage drop and fast transient response. Frequency compensation uses low ESR capacitors while remaining stable. This needs to address low voltage high speed microprocessor bus like GTL+. Stability The FAN1585A series requires an output capacitor as part of the frequency compensation. It is recommended to use 22μF solid tantalum or 100μF aluminum electrolytic to ensure stable output. Frequency Compensation These devices optimize ESR capacitors for lower frequency response. In general, it is recommended to use capacitors with ESR less than 300 mA. Bypass capacitors such as 22µF tantalum or 100µF aluminum on the regulation pin of the fan 1585A are also recommended for low ripple and fast transient response. When these bypass capacitors are not used on the adjust pins, larger values of output capacitors provide equally good results. Refer to the Typical Performance Output Capacitor Settling graph for characteristic ESR vs. load current. The FAN1585A series does not require any protection diodes for normal operation. For the fan 1585A, internal resistors limit the internal current path on the adjustment 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 FAN1585A 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 output value capacitors, such as 1000µF to 5000µF pins, can be damaged by a momentary short circuit to ground. A crowbar circuit at the input can generate these levels of current; then it is recommended to use diodes from the output to the input as shown. Normally, normal power cycling or the system "hot swapping" will not draw enough current to cause any damage. 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 fail, and all protection circuits fail to function properly.

Ripple Suppression The bypass capacitor from the fan 1585A's regulation pin to ground reduces output ripple by the ratio of VOUT/1.25V in applications where improved ripple suppression is required. The impedance frequency of the adjustment pin capacitor at the ripple should be less than the value of R1 (usually 100Ω to 120Ω in the feedback divider network) in the range shown in the figure. Therefore, the value of the required adjustment 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 0.22 microF is required. The output voltage FAN1585A regulator generates a 1.25V reference voltage between the output pin and the adjust pin. Placing resistor R1 between these two terminals causes a constant current to flow through R1 and down through R2 to set the total output voltage. Typically, this current is specified as a minimum load current of 10mA. The current from the regulation pin plus the current from R1 is typically 35µA. Its output voltage contribution is small and only needs to be considered when a very precise output voltage setting is required.

Load regulation cannot provide true remote sensing load because the FAN1585A series are three terminal devices. Load regulation is subject to a connected load regulator. 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.

For adjustable voltage devices, negative side sensing is the correct Kelvin connection at the bottom of the output divider to return 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. The figure 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 foot of 16 gauge wire. This equates to 4mV per foot at 1A load current. At higher load currents, this drop accounts for a large percentage of the overall regulation. It is as short as possible on the regulator and load, using large wire or PC board traces.

Thermal Factor The FAN1585A series overload protects the internal power and thermal limiting circuit conditions. 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. For example, using FAN1585AT to generate 5A@3.3V power supply (3.2V to 3.6V) voltage is 1.5V±2%.

The IC specifies a thermal resistance connected to the case connected 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 radiator. The chassis-to-heatsink interface is recommended for use with thermally conductive materials. Use thermally conductive spacers if the equipment enclosure must be electrically isolated and include resistance to its contribution to total heat. The case of the FAN1585A series is connected to the output of the device.