Applications having two energy sources such as PCIe cards, Tablets and Portable battery powered equipment require preference of one source to another. For example, mains power (wall-adapter) has the priority over the internal battery back-up power. These applications demand for switchover from mains power to backup power only when main input voltage falls below a user defined threshold. The TPS25948xx devices provide a simple solution for priority power multiplexing needs.

Figure 8-14 shows a typical priority power multiplexing implementation using TPS259480x devices. When primary (priority) power source (IN1) is present and within the valid range (not in UV/OV condition), the primary path device path powers the OUT bus irrespective of whether auxiliary supply voltage (V_IN2) is greater than, equal to or less than primary supply voltage (V_IN1). The device in auxiliary path is held in off condition by forcing its OVLO pin to high using the SPLYGD signal from the primary path device.

Once the primary supply voltage falls outside the user-defined valid operating range (UV/OV condition), the primary path device de-asserts the SPLYGD which signals the auxiliary path device to turn on and the system starts operating from the auxiliary supply. During this transition, the auxiliary path device bypasses its dVdt limited startup and performs a fast recovery to start delivering power within t_SWOV.

When the primary supply is restored, the primary path device turns on fully at a defined slew rate and then asserts its SPLYGD pin high to turn the auxiliary path device off, allowing a seamless transition from auxiliary to the primary supply with minimal output voltage droop and with no shoot-through current.

A key consideration in power MUXing applications is the minimum voltage the output bus droops to during the switchover from one supply to another. This in turn depends on multiple factors including the output load current (I_LOAD), output bus hold-up capacitance (C_OUT) and switchover time (t_SW).

While switching from primary supply (V_IN1) to auxiliary supply (V_IN2), the minimum bus voltage can be calculated using Equation 15. Here, the switchover time (t_SW) is equal to the fast OVLO recovery time (t_SWOV) taken by the TPS259480x variants to turn on fully and start delivering current to the load.

While switching from auxiliary supply (V_IN2) to primary supply (V_IN1), the minimum bus voltage can be calculated using Equation 16. Here the maximum switchover time is equal to the RCB recovery time (t_SWRCB), depending on whether V_IN1 is equal to or lower than V_IN2 to start with.

The SPLYGD pins of the devices can be used as a digital indication to identify which of the 2 supplies is active and delivering power to the load.

Figure 8-14 Priority Power MUXing With 2 × TPS259480x - Option 1

This configuration provides the most compact priority power MUXing solution with multiple benefits, including active current limit protection on both channels as well as overvoltage protection on primary channel. It also provides the fastest switchover time from primary to auxiliary, but at the cost of a slightly increased quiescent current on the auxiliary path while primary path is active. Also, it uses the fewest external components, but at the cost of bypassing overvoltage protection on auxiliary channel.

The following waveforms illustrate the TPS259480x performance in a priority power MUXing configuration.

Figure 8-15 TPS259480x Power MUX - Switchover Between Primary and Auxiliary Supplies

Figure 8-16 TPS259480x Power MUX - Switchover Between Primary and Auxiliary Supplies

ere is a possible variation to the above configuration in case overvoltage protection is needed on both channels. This needs an additional signal N-FET to drive the OVLO pin of the auxiliary path device as shown in Figure 8-17. The switchover times are similar to the previous configuration.

Figure 8-17 Priority Power MUXing With 2 × TPS259480x - Option 2

Another variation of the previous configuration ensures minimum quiescent current on the auxiliary channel while primary channel is active, but at the cost of additional N-FET to drive the EN/UVLO pin of auxiliary path device as shown in Figure 8-18. At the same time, it has a higher switchover delay from primary to auxiliary supply as compared to the previous configuration.

Figure 8-18 Priority Power MUXing With 2 × TPS259480x - Option 3

While switching from a higher supply rail to lower supply rail, the minimum bus voltage can be calculated using Equation 17. Here, the switchover time is equal to the time taken by the device to come out of reverse current blocking state (t_SWRCB).

While switching from a lower supply rail to higher supply rail, the minimum bus voltage can be calculated using Equation 18. Here, the switchover time (t_SW) is the time taken by the device to turn on fully and start delivering current to the load, which is equal to the device turn-on time (t_ON), which in turn includes the turn-on delay (t_D,ON) and rise time (t_R) determined by the dVdt capacitor (C_dVdt) and bus voltage.