the LM3544 can be configured to switch the power connec-

tion of a device between two separate sources is shown in

Figure 8, below.

In the example, the logic signals EN and SRC control the

power supply connections. If the EN signal is low, all four

LM3544 switches will be open and neither supply will be

connected to the bus. If EN and SRC are both high, switch 1

and switch 3 close, connecting the main power supply to the

two bus power outputs. When EN is high and SRC is low, the

auxiliary supply is connected to the power outputs through

switches 2 and 4.

In order to allow the power-switching circuit to smoothly

transition from one supply to another, delay networks must

be placed before the enable pins. These delay networks

account for the turn-on delay of the LM3544 switches by

slowing the fall time of the enable signals. Rise times of

these signals are not affected by the delay networks. Glitch

free transition is assured as long as both supplies remain

within operating specifications for a minimum of 5 ms after

the logic signal SRC is toggled.

10120830

FIGURE 8. Using the LM3544 in a Wake-on-USB Application

www.national.com

12