10.3.1.2 Priority Power MUX Operation

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 back-up power or auxiliary power. These applications demand for switch over from mains power to back-up power only when main input voltage falls below a user defined threshold. The devices provide a simple solution for priority power multiplexing needs.

Figure 79 shows a typical priority power multiplexing implementation using devices. When primary power IN1 is present, the device in IN1 path powers the OUT bus irrespective of whether auxiliary power IN2 is greater than or less than IN1. Once the voltage on the IN1 rail falls below the user-defined threshold, the device IN1 issues a signal to switch over to auxiliary power IN2. The transition happens seamlessly in less than 125 µs, with minimal voltage droop on the bus. The voltage droop during transition is a function of load current and bus capacitance (see Equation 36).

Equation 36.

where

• V_(droop) in Volts, I_(Load) is load current in Ampere, C_(BUS) is bus capacitance in µF

When the main voltage supply (IN1) is not present or during brown-out conditions, the device in auxiliary supply rail (IN2) provides power to the output. When IN1 recovers, the device connected to IN1 is turned on at defined slew rate and the device in IN2 path is turned off, allowing a seamless transition from auxiliary to the main voltage supply with minimal droop and with no shoot-through current.

Priority power multiplexing can be done either between two similar rails (such as 12 V Primary to 12 V Aux, 3.3 V Primary to 3.3 V Aux) or between dissimilar rails (such as 12 V Primary to 5 V Aux or 3.3 V Aux; or vice versa).

A. C_IN: Optional and only for noise suppression.

B. Master controls the slave using priority signal for switch over to Auxiliary power.

Figure 79. Priority Power Multiplexing Implementation

Figure 80 and Figure 81 show typical switch-over waveforms of Priority Muxing implementation using the TPS25942 or TPS25944 for 11.5 V Primary and 14.5 V Auxiliary Bus.

Figure 82 and Figure 83 show typical switch-over waveforms of Priority Muxing implementation using the TPS25942 or TPS25944 for 12 V Primary and 3.3 V Auxiliary Bus.

V(IN1) = 11.5 V	R(ILIM1) = 24.6 kΩ,	C(OUT) = 150 µF
V(IN2) = 14.5 V	R(ILIM2) = 33.2 kΩ	C(dVdT) = 1.2 nF
RL = 5.6 Ω	R(IMON) = 16.2 kΩ	V(UVLO-High) = 10.8 V

Figure 80. IN1 Power Recovery: Change Over from Auxiliary IN2 to Primary Power IN1

V(IN1) = 12 V	R(ILIM1) = 24.6 kΩ	C(OUT) = 150 µF
V(IN2) = 3.3 V	R(ILIM2) = 33.2 kΩ	C(dVdT) = 1.2 nF
RLoad = 5.6 Ω	R(IMON) = 16.2 kΩ	V(UVLO-High) = 10.8 V

Figure 82. IN1 Power Recovery: Change Over from Auxiliary IN2 to Main Power IN1

V(IN1) = 11.5 V	R(ILIM1) = 24.6 kΩ	C(OUT) = 150 µF
V(IN2) = 14.5 V	R(ILIM2) = 33.2 kΩ	C(dVdT) = 1.2 nF
RL = 5.6 Ω	R(IMON) = 16.2 kΩ	V(UVLO-Low) = 10.2 V

Figure 81. IN1 Brownout Condition: Change Over from Main IN1 to Auxiliary Power IN2

V(IN1) = 12 V	R(ILIM1) = 24.6 kΩ	C(OUT) = 150 µF
V(IN2) = 3.3 V	R(ILIM2) = 33.2 kΩ	C(dVdT) = 1.2 nF
RL = 5.6 Ω	R(IMON) = 16.2 kΩ	V(UVLO-Low) = 10.2 V

Figure 83. IN1 Brownout Condition: Change Over from Main IN1 to Auxiliary Power IN2