LTC1421 datasheet(12/24 Pages) LINER

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LTC4223-1/LTC4223-2

422312f

APPLICATIONS INFORMATION

Turn-Off Sequence

The switches can be turned off by various conditions and

this is summarized in Table 1.

When the 12ON pin goes low, the external switch is turned

off with the GATE pin pulled to ground by 1mA current

sink. The ⎯1⎯2⎯P⎯G⎯O⎯O⎯D pin pulls high indicating that power is

no longer good, while an internal N-channel transistor dis-

charges the output to ground. Similarly, when the AUXON

pin goes low, the internal switch is turned off, ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D

pulls high while its output is discharged to ground through

an internal N-channel transistor. Figure 3 shows the two

supplies being turned off by ⎯E⎯N going high.

Card Presence Detect

In an AMC system, ⎯P⎯S⎯1 and ⎯P⎯S⎯0 signals are used to

detect the presence of a card upon insertion or removal.

Normally ⎯P⎯S⎯1 is connected to the ⎯E⎯N pin with a pull-up

resistor. If AUXON or 12ON is high when the ⎯E⎯N pin goes

low, indicating a board insertion, a timing cycle for contact

debouncing is initiated. Upon insertion, any bounces on the

⎯E⎯N pin will re-start the timing cycle. When TIMER ﬁnally

reaches its threshold during ramp up, the fault latches

will be cleared. If the ⎯E⎯N pin remains low at the end of the

timing cycle, the switches are allowed to turn on.

If the ⎯E⎯N pin is toggled from low to high, indicating board

removal, all the switches will be turned off after a 20μs delay.

Any latched faults will not be cleared. However, removing

the card could cause the ⎯E⎯N pin voltage to bounce, clearing

the fault latches undesirably. This is prevented by blanking

the bounces internally with a TIMER ramp up period given

by CT • 123[ms/μF] as shown in Figure 4.

Timer Functions

An external capacitor CT connected from the TIMER pin

to ground is used to perform several functions.

1. Ignore contact debouncing during card insertion when

the device is enabled. The debounce cycle is given

by ramping up CT with 10μA current to TIMER high

threshold (1.235V) and then ramping down with 2μA

current to below TIMER low threshold (0.2V). This gives

an average debounce cycle time of CT • 741[ms/μF].

After that, if any ON pin is pulled high and ⎯E⎯N pin is

low, the switches can be turned on.

2. Blanking contact bounce on the ⎯E⎯N pin that might

trigger unwanted fault clearing during card removal.

The blanking time is given by CT • 123[ms/μF].

3. Fault ﬁltering during auxiliary supply power-up in analog

current limit. TIMER pulls up with 10μA and pulls down

with 2μA. The ﬁlter time is given by CT • 123[ms/μF].

4. 12V supply fault ﬁltering during and after power-up

in analog current limit. TIMER pulls up with 200μA

and pulls down with 2μA. The ﬁlter time is given by

CT • 6[ms/μF].

5. For cooling off during an auto-retry cycle after

an overcurrent fault on auxiliary or 12V supply

(LTC4223-2). The cool-off time is given by CT • 1482

[ms/μF] after an auxiliary supply fault and CT • 1358[ms/

μF] after a 12V supply fault.

Figure 3. Normal Power-Down Sequence

Figure 4. Debouncing by TIMER during Card Removal

100ms/DIV

12PGOOD

5V/DIV

AUXOUT

5V/DIV

12VOUT

5V/DIV

AUXPGOOD

5V/DIV

422312 F03

5ms/DIV

FAULT

2V/DIV

TIMER

1V/DIV

422312 F04

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