Automatically Connect One of Two Power Supplies: A Dual Channel Prioritized Power Path Controller

This article discusses an intelligent power switch that automatically connects one of two input voltage supplies to a common output node.

Linear Technology, now part of Analog Devices, recently released the LTC4418, which they describe as a “dual channel prioritized power path controller.” In other words, this device is designed to automatically (intelligently) connect one of two valid power supplies—in order of their priority—to a commonly shared output node (the load).

 

Figure 1. The LTC4418 IC comes in a 20-pin QFN package. Diagram taken from the datasheet (PDF).

 

As can be seen in the figure below, your design might require a host of external components, and consequently you will come to appreciate any and all design guidance that Linear Tech is willing to provide.

 

Figure 2. This intelligent power switch may require many external components. Diagram taken from the datasheet (PDF).

Two Input Voltage Supplies and Their Priority Levels

The two voltage input supplies (V1 and V2) have the same input voltage range, which is 2.5 to 40 V, and they are independent in the sense that they don’t have to be at the same voltage or even a similar voltage. Also, the voltage supply priority is not a function of the voltage level; rather, the voltage priority is defined by pin assignment, where V1 carries a higher priority level than V2. As an example, the application circuit below shows that the 5V SYS (connected to V1) is the higher priority voltage supply (consider it the primary voltage supply) while the 12V wall adapter, tied to V2, is the lower priority (or secondary) voltage supply.

The LTC4418 automatically switches over to the lower priority voltage supply (V2) whenever the higher priority supply voltage (V1) becomes invalid. Likewise, when V1 once again becomes valid, the device automatically switches back to this supply, regardless of the status of the secondary supply.

 

Figure 3. The voltage priority level is set by pin assignment. In this example, the 5V supply (V1) is higher priority than the 12V supply (V2). From the datasheet (PDF).

 

And if your system utilizes more than two input voltage supplies, then you can simply cascade multiple LTC4418 devices together by use of the CAS (cascade) and EN (enable) pins. In this configuration, the LTC4418 will provide the switchover between more than two inputs (see the following figure). Nice!

 

Figure 4. The datasheet shows how multiple LT4418 ICs can be connected together by use of the CAS and EN pins.

Determining When an Input Voltage Supply Is Valid

The OV (over voltage) and UV (under voltage) window, along with the window validation time, is what determines when a power supply is considered valid. Each of the two input voltage supplies have their own OV/UV window, which are user-adjustable by simply selecting the associated resistors. The validation time (t_VALID) of this OV/UV window can be user-adjustable or can be left at its default value, which is 3.5µs (typical); when the default window validation time value is sufficient, the designer can merely connect the TMR pin to the INTV_CC pin. And when a specific window validation time is required, an external capacitor can be connected between the TMR pin and GND, where the validation time is 16ms/nF. Keep in mind, however, that this one validation time window is used for both voltage-supply channels.

For more detailed information on this topic, see the datasheet's section entitled Selecting Validation Time.

Guidance on Choosing External Components

As mentioned previously, Linear Tech has provided some assistance for choosing the external components. As an example, when it comes to selecting the external P-channel MOSFETs, Linear Tech calls out the four key parameters that should be considered:

1. R_DS(ON): on-resistance
2. BV_DSS(MAX): absolute maximum drain-source breakdown voltage
3. V_GS(TH): threshold voltage
4. SOA: safe operating area

The datasheet also provides a list of suggested P-channel MOSFETs (see the figure below).

 

Figure 5. A list of suggested P-channel MOSFETs, from the datasheet.

Layout Assistance

Layout is often a critical part of the design process, and it is greatly appreciated when a manufacturer provides a substantial quantity of layout guidance. The LTC4418 datasheet has a “Layout Consideration” section that is brief but solid. It also has an example layout diagram that is clear and fairly comprehensive; this diagram even includes LT’s interesting recommendation for connecting the exposed pad to the ground pin (that trapezoidal section connected to pin 8, see below).

 

Figure 6. An LTC4418 layout example, provided by the datasheet.

 

Have you had a chance to use this new power path controller, the LTC4418, from Linear Technology? If so, leave a comment and tell us about your experiences.