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ADM1051A Scheda tecnica(PDF) 9 Page - Analog Devices |
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ADM1051A Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 10 page REV. 0 ADM1051/ADM1051A –9– SUPPLY DECOUPLING The supply to the drain of an external MOSFET should be decoupled as close as possible to the drain pin of the device, with at least 100 µF to ground. The output from the source of the MOSFET should be decoupled as close as possible to the source pin of the device. Decoupling capacitors should be chosen to have a low Equivalent Series Resistance (ESR), typically 50 m Ω or lower. With the MOSFETs specified, and two 100 µF capacitors in parallel, the circuit will be stable for load currents up to 2 A. The VCC pin of the ADM1051/ADM1051A should be decoupled with at least 1 µF to ground, connected as close as possible to the VCC and GND pins. In practice, the amount of decoupling required will depend on the application. PC motherboards are notoriously noisy envi- ronments, and it may be necessary to employ distributed decoupling to achieve acceptable noise levels on the supply rails. SHDN1 VCC ADM1051 SHDN2 1 F LEAVE OPEN OR CONNECT TO LOGIC SIGNALS IF SHUTDOWN REQUIRED PHD55N03LT FORCE 2 SENSE 2 100 F 2 100 F VOUT1 FORCE 1 SENSE 1 VIN 3.3V 12V MTD3055VL 100 F 2 100 F VOUT2 VIN 3.3V Figure 6. Typical ADM1051 Application Circuit CHOICE OF MOSFET As previously discussed, the load current at which an output goes into Hiccup Mode depends on the on resistance of the external MOSFET. If the on resistance is too low, this current may be very high; if the on resistance is high, the trip current may be lower than the maximum required load current. For the primary application of AGP and ICH power supplies and bus termina- tion on personal computer motherboards, devices with very low on resistance, such as the PHD55N03LT from Philips, or the SUB60N06-18 from Siliconix, are suitable. For Channel 2, suitable devices are the MTD3055VL from Motorola and the PHB11N06LT from Philips. POWERING SUPPLY SEQUENCING Some I/O control hub chipsets have power-supply sequencing requirements, which dictate that the 1.818 V supply must never be more than 2 V below the 3.3 V supply. This requirement can be met using the ADM1051A, as shown in Figure 7. In this circuit, VCC is supplied from the 5 V standby rail (5 VSB) and from the 12 V rail via Schottky diodes. 5 VSB is always present when ac power is supplied to the system, so the ADM1051A is powered up, but VCC is below the POR threshold. When the main power supplies are turned on, the Channel 2 output will rise at the same rate as the 3.3 V rail until it regulates at 1.818 V. The 12 V supply will take over from 5 VSB when it exceeds the 5 VSB rail, and Channel 1 will then be subject to the POR delay. This ensures that Channel 2 can never be more than 2 V below Channel 1. SHDN1 VCC ADM1051A SHDN2 0.1 F PHD55N03LT FORCE 2 SENSE 2 100 F 2 100 F VOUT1 FORCE 1 SENSE 1 VIN 3.3V 12V MTD3055VL 100 F 2 100 F VOUT2 VIN 3.3V 3.3V 10k BAT45C BAT45C 5VSB Figure 7. Typical ADM1051A Application Circuit THERMAL CONSIDERATIONS Heat generated in the external MOSFET must be dissipated and the junction temperature of the device kept within accept- able limits. The power dissipated in the device is, of course, the drain-source voltage multiplied by the load current. The required thermal resistance to ambient is given by JA = TJ(MAX) – TAMB(MAX)/(VDS(MAX) × IOUT(MAX)) Surface-mount MOSFETs, such as those specified, must rely on heat conduction through the device leads and the PCB. One square inch of copper (645 sq. mm) gives a thermal resistance of around 60 °C/W for an SOT-223 surface-mount package and 80 °C/W for an SO-8 surface-mount package. For high power dissipation that can be accommodated by a surface-mount package, D 2PAK or TO-220 devices are rec- ommended. These should be mounted on a heat sink with a thermal resistance low enough to maintain the required maxi- mum junction temperature. |
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