feature

Fast transient response Low dropout load regulation up to 3A: 0.05% trim current limit typical On-chip thermal limit Standard TO-220 , TO-263, TO-263 heart cut, and TO-252 packages

application

Pentium Grade GTL+ Bus Power Low Voltage Logic Power Switching Power Supply Post Regulator 12V to 5V Linear Regulator 3.3V to 1.5V Linear Regulator

illustrate

The FAN1587A , FAN1587A-1.5, and FAN1587A-3.3 are three-terminal regulators with 3A output current capability. These units have been optimized for low voltage applications including VTT bus termination where response and minimum input voltage are critical. The FAN1587A is ideal for low voltage microprocessor applications requiring a regulated output from 1.5V to 3.6V with an input supply of 12V or lower. FAN1587A-1.5 provides fixed 1.5V with 3A current capability for GTL+ bus VTT termination. The FAN1587A-3.3 provides a fixed 3.3V current capability for logic IC operation. The current limit is trimmed to ensure the specified output current and to control the short circuit current. On-chip thermal limiting protects against overload and ambient temperatures that can cause over-connection. FAN1587A, FAN1587A-1.5 and FAN1587A-3.3 are industry standard TO-220, TO-263, TO-263 heart cut and TO-252 (DPAK) power packs.

Application Information: General FAN1587A, FAN1587A-1.5 and FAN1587A-3.3 are three terminal regulators optimized for GTL+VTT terminal applications. These units are short-circuit protected and provide thermal shutdown for connection temperatures exceeding approximately 150°C. The 1587A series of fans provide low voltage drop and fast transient response. Frequency compensation is used low while remaining stable. This addresses the need for low-voltage, high-speed microprocessor buses such as GTL+. Stability The FAN1587A series requires an output capacitor as part of the frequency compensation. A 22µF solid tantalum or 100µF aluminum electrolytic output is recommended for stability. Frequency Compensation These devices optimize ESR capacitors for lower frequency response. A bypass capacitor such as 22µf tantalum or 100µf aluminum is recommended for low ripple and fast transient response to the FAN1587A pins. When these bypass capacitors are not used for regulation pins, larger values of output capacitors provide equally good results. output capacitor esr and load current. The FAN1587A series does not require any protection diodes for normal operation. For the fan 1587A, 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 keep the device safe under short circuit conditions.

Protection diodes are usually not required between the input and output pins. The internal diodes between the input and output pins of the FAN1587A series can handle microsecond inrush currents of 50A to 100A. Even with large output capacitors it is difficult to obtain these inrush current 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 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 to output voltage difference causes the internal transistors to fail without any protection circuit functionality.

Ripple Suppression In applications where improved ripple suppression is required, the bypass capacitor from the fan 1587A's regulation pin to ground reduces output ripple by a ratio of vout/1.25v. At the ripple frequency, the impedance of the trim pin capacitor should be less than the value of r1 (typically 100Ω to 120Ω in the feedback divider network) range. Therefore, the value of the trim pin capacitor required 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 FAN1587A regulator generates a 1.25V reference voltage between the output pin and the adjust pin (see figure). 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 very small, and the need to set the voltage is only a consideration for very accurate output.

Load regulation cannot provide true remote sensing load because the FAN1587A 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 × (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 is equivalent 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 FAN1587A 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 fan1587at to generate 3A@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 FAN1587A series is connected to the output of the device.