Installing a P-Channel MOSFET on VCC Power Supply to S25FL-S Device to Reduce Power Consumption - KBA219126

Version: **

 

Question:

Can I use a P-Channel MOSFET device to turn OFF the VCC power supply to the S25FL-S flash device to reduce power consumption?

 

Answer:

Yes, you can use such a circuit to manage the power supply to the S25FL-S flash device. Ensure that the time period between the OFF and ON signals to the switched power supply is long enough to complete a full Power-On Reset (POR) of the flash device as defined in the datasheet. See the following figures and table taken from the S25FL128S and S25FL256S datasheet (Document Number: 001-98283, Rev. *J):

 

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When the voltage drops below VCC (cut-off), the flash device is considered to be powered OFF and the device then must go through a full POR cycle to properly reinitialize. To do this, the voltage must drop below VCC (low) for a period of tPD before VCC can ramp up through VCC (min) to nominal operating voltage. If, during a voltage drop, VCC stays above VCC (cut-off), the flash device stays initialized and will work correctly when VCC returns to the nominal range.

 

For board-level power control that affects all components on the board, ensure that the CS# signal trails VCC during the power-down and power-up phases of POR. For chip-level power control that affects only the flash device, ensure that CS# remains HIGH (deselected) during VCC excursions from the nominal range.

 

Example: In the schematic, the ON Semiconductor (formerly Fairchild) BSS84 MOSFET acts like a switch that can turn OFF the “always ON” VCC power supply via the software-controlled signal “Control”. The MOSFET pin connections are:

 

  • Gate pin is connected to “Control”.
  • Source pin is connected to the 3.0-V power supply rail.
  • Drain pin is connected to the flash VCC pin.

 

While “Control” disconnects power from the flash device, ensure that CS# stays HIGH. Here is how “Control” manages the power supply to the flash chip:

 

  • As “Control” decreases the gate voltage towards VSS (ground), the MOSFET turns ON allowing current to flow from Source to Drain and the flash chip is powered.
  • As “Control” increases the gate voltage towards VCC, the MOSFET turns OFF, which disconnects the VCC power supply from the flash device.

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