11

LTC1472

1472fa

Output Capacitors

400

µs rise time. Therefore, capacitors as large as 150µF

can be driven without producing voltage spikes on the

0.1

µF to1µFcapacitorfornoisereductionandsmoothing.

transitions between output voltages without creating spikes

on the system power supply lines.

Supply Sequencing

Because the 5V supply is the source of power for both the

power supplies such that the 5V supply is powered before

or simultaneous to the application of 3.3V or 12V power.

It is interesting to note however, that all of the switches in

the LTC1472 are NMOS transistors which require charge

pumps to generate gate voltages higher than the supply

rails for full enhancement. Because the gate voltages start

a 0V when the supplies are first activated, the switches

always start in the off state and do not produce glitches at

the output when powered.

Some PCMCIA switch matrix products employ PMOS

switches for 12V VPP control and great care must be taken

to ensure that the 5V control logic is powered before the

12V supply is turned on. If this sequence is not followed,

the PMOS VPP switch gate may start at ground potential

and the VPP output may be inadvertently forced to 12V.

Although, not advisable, it is possible to power the 12V

power. Only about 50

these conditions.

If the 5V supply must be turned off, it is important to

program all switches to the Hi-Z or 0V state at least 100

µs

before the 5V power is removed to ensure that all NMOS

switch gates are fully discharged to 0V.

current drops to < 1

µA with all the switches turned off and

therefore no 5V power is consumed in the standby mode.