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SI4700DY Scheda tecnica(PDF) 7 Page - Vishay Siliconix |
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SI4700DY Scheda tecnica(HTML) 7 Page - Vishay Siliconix |
7 / 8 page Si4700DY Vishay Siliconix New Product Document Number: 71110 S-00025—Rev. A, 24-Jan-00 www.vishay.com S FaxBack 408-970-5600 2-7 TYPICAL APPLICATIONS The Si4700 is designed to be used to select one of two power sources for a circuit, such as needed to implement ACPI in desktop computers. In this application, parts of the circuit must run off an always-on power supply when the computer is in suspend mode. When in normal mode, these circuits run off the main power supply. The Si4700DY contains an n-channel MOSFET and a p-channel MOSFET switch connected together to make a single-pole, double-throw switch. An additional on-board small signal MOSFET provides a ground referenced logic input. When the control input is high, the power MOSFET gates are pulled to ground, and the p-channel MOSFET is on. When the input is low, the gates are pulled above the supply rail, and the n-channel MOSFET is on (pulling the gate of the p-channel above the source potential has no effect). The gate drive for the n-channel device, Q1, uses an external 12-V supply via an external resistor. A typical value for this resistor is around 20 k W, but the value is not critical as long as the current in Q3 is kept below 0.05 A. A higher value of resistance reduces the current while in suspend mode, but also introduces a longer delay when turning on Q1. The Si4700DY switch is a break-before-make configuration, therefore sufficient capacitance must be present on the isolated load to ensure hold up during switching. Due to fast switching times this should not be significant and is preferred over a make-before-break that would connect the two power supplies directly for a short period. Note that the n-channel MOSFET is oriented to ensure that the internal diode does not conduct while the sub-circuit is isolated. In this direction it also provides a fail-safe path for the circuit’s power through the diode. The forward drop of the p-channel MOSFET’s diode will block any current back-feeding the secondary supply, assuming the two supplies are very close in voltage. The Si4700DY has a maximum rDS(on) of 30 mW for the n-channel MOSFET used during normal operation and 70 m W for the p-channel used when the computer is in suspend, making it ideal for loads up to 3 A or higher depending on voltage drop requirements. It can be used on any rail voltage between 2.5 V and 8 V (based on an absolute max of 12 V), with a logic input between 2.5-V and 5-V nominal. FIGURE 1. ACPI Power Switching Application SPDT switch required to isolate cir- cuit and connect to VSTANDBY Always-On Circuit Main Circuit Main power supply shutdown control in ATX power supply Seconday supply (always on) Main Supply ACPI Control |
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