The Fairchild FPF1320 dual-input single-output power switch is a unique IC that switches a load between two power sources. It behaves as a power multiplexer (or mux), using a digital select signal to determine which of the two 1.5 V to 5.5 V input power rails to connect to the output, and can handle a maximum of 1.5 A while blocking unwanted reverse current from flowing from output to input or between inputs. Each input channel is controlled by an internal MOSFET switch, avoiding the voltage drop that occurs when diodes are used to OR multiple power supplies together.
However, the FPF1320 is only available in a tiny 1 mm × 1.5 mm BGA (ball grid array) package, making it difficult for a student or hobbyist to use. To address that issue, this board mounts the chip onto a compact carrier with 0.1? pin spacing that makes it easy to use with standard solderless breadboards, and it also serves as a breakout for a USB Micro-B connector that can be used to supply one of the input power rails.
The board is configured so that the ?B? power rail is preferred and will be selected by default when that power source is present. USB power is connected to this preferred rail, so the default behavior is for the board to select USB power when USB is connected and the alternate ?A? power rail when it is not. This enables the common application of having a device that can be powered by USB or an external power supply, automatically choosing the appropriate source based on what is connected. The default behavior can be changed through a surface-mount jumper and overridden through the power select (SEL) input.
Since this board is a carrier for the FPF1320, we recommend careful reading of the FPF1320 datasheet (1MB pdf).
Using the power multiplexer
VINA: Power supply input A. Can be connected to a 1.5 V to 5.5 V power source.
VINB: Power supply input B. Can be connected to a 1.5 V to 5.5 V power source. Connected to VBUS by default through surface-mount jumper. This power rail is preferred by default.
VBUS: USB 5 V supply. Connected to VINB by default through surface-mount jumper.
VOUT Multiplexed power output.
GND: Common ground for power supplies, load, and USB. (All of the board?s GND pins are internally connected with each other and with GND from the USB connector.)
SEL: Select input that determines which power source is selected: LOW (< 0.65 V) selects VINA, HIGH (> 1.15 V) selects VINB. On-board pull-up resistor to VINB gives it the default behavior of selecting VINB when VINB is connected and selecting VINA when it is not.
EN: Enable input; when driven low, disconnects VOUT from both VINA and VINB. Pulled high through a pull-up resistor and diodes to both VINA and VINB so the IC is enabled by default whenever either power supply is present.
- D?: Signals broken out from USB connector.
Because of the way the board connects SEL by default, the output voltage will briefly drop to the logic level threshold of the SEL pin (about 1 V) when the multiplexer is transitioning from the VINB source to the VINA source. The ?Power source selection? section below discusses this behavior in more detail, as well as some possibilities for modifying it.
The EN pin can be driven low to turn off the output and put the FPF1320 in a low-power state. The board pulls EN high through a 100 k? pull-up resistor and a pair of diodes to both VINA and VINB, so it is enabled by default when a suitable voltage is present on either input.
Power connections without USB
Power connections with USB
Warning: It is important to be aware that while this board has three power inputs, two of those three are typically tied together (there are only two independent power inputs to the FPF1320). As such, you must take care to avoid creating a short circuit between USB power and an external power supply. For example, when the board is in its default configuration (i.e. with VBUS tied to VINB), any external power supply must be connected to VINA; connecting it to VINB will create a short between this power supply and USB power when USB is connected.
The remaining lines from the USB connector (D?, D+, ID, and GND) are broken out to a set of pins adjacent to VOUT, allowing this module to also function as a basic USB Micro-B connector breakout board.
Power source selection
To avoid such a pronounced drop when switching from VINB to VINA, you can bias the voltage on SEL. The board connects SEL to VINB through a 100 k? resistor, so adding a resistor between SEL and GND creates a voltage divider that lowers the SEL voltage and in turn causes the board to switch sooner. For example, adding a 30 k? resistor between SEL and GND lowers SEL to about 1.15 V when VINB is 5 V, and in our tests, it caused the FPF1320 to switch over to VINA as soon as VINB fell to about 4 V.
Note that the SEL input does not have a precision threshold, so even this method might be unsuitable for especially voltage-sensitive applications. In such cases, the selection behavior can be overridden by directly connecting a digital signal (such as an output from a comparator or a microcontroller monitoring VINB) to the SEL pin.
with USB Micro-B connector carrier with USB Micro-B
with included optional header pins. connector in a breadboard.
- Size: 0.8? × 0.55? × 0.15?1
- Weight: 0.8 g2
- Minimum operating voltage: 1.5 V
- Maximum operating voltage: 5.5 V
- Maximum output current: 1.5 A
- 1. Without included optional headers. This measurement includes the USB Micro-B connector, which extends 0.05? past the edge of the PCB.
- 2. Without included optional headers.
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