SI4700DY Datasheet, PDF (7/8 Pages) Vishay Siliconix – Power Selector Switch

New Product
Si4700DY
Vishay Siliconix
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 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.
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).
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 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 kW, 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 has a maximum r_DS(on) of 30 mW for the
n-channel MOSFET used during normal operation and 70 mW
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.
Seconday supply
(always on)
SPDT switch required to isolate cir-
cuit and connect to V_STANDBY
Main Supply
Main power supply shutdown
control in ATX power supply
ACPI Control
Main
Circuit
Always-On
Circuit
Document Number: 71110
S-00025—Rev. A, 24-Jan-00
FIGURE 1. ACPI Power Switching Application
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