LTC4223-1/LTC4223-2

15

422312f

APPLICATIONS INFORMATION

Power-Good Monitor

Internal circuitry monitors the output voltages, AUXOUT

respective open drain outputs, ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D and ⎯1⎯2⎯P⎯G⎯O⎯O⎯D.

Several conditions must be met before the power-good

outputs assert low.

1. The monitored output should be above its power-

good threshold and hysteresis.

2. The input supply is above undervoltage lockout.

3. ⎯E⎯N is low.

4. The associated ON pin is high.

5. Thermal shutdown is not activated.

If any of the supply outputs falls below its power-good

threshold for more than 20μs, the respective power-good

output will be pulled high by the external pull-up resistor

or internal 10μA pull-up.

Resetting Faults (LTC4223-1)

Any supply faults tripping the circuit breaker are latched

and ⎯F⎯A⎯U⎯L⎯T asserts low. For the latched-off version

(LTC4223-1), to reset a fault latch due to overcurrent or

thermal shutdown on auxiliary supply, pull both AUXON and

12ON pins low together for at least 100μs, after which the

⎯F⎯A⎯U⎯L⎯T will go high. Toggling both the ON pins high together

again initiates the debounce timing cycle, thereafter the

auxiliary supply starts up first followed by 12V supply. To

skip the debounce timing cycle, first pull only AUXON low

then high for at least 50μs before toggling 12ON low then

high. The fault latch clears on the falling edge of 12ON

and the auxiliary supply powers up. Thereafter, the 12V

supply powers up if 12ON pulls high.

To reset a fault on the 12V supply and re-start the output,

toggle only the 12ON pin low and then high again. Tog-

gling the ⎯E⎯N pin high then low again or bringing the bias

will initiate the debounce timing cycle and reset all fault

undervoltage threshold will not reset the fault latches. For

the auto-retry version (LTC4223-2), the latched fault will

be cleared automatically after a cool-off timing cycle.

Auto-Retry after a Fault (LTC4223-2)

At time point 1 in Figure 8, if a fault latched-off the 3.3V

auxiliary supply after power-up, a cool-off cycle begins.

The TIMER capacitor charges up to 1.235V with a 10μA

current and then discharges with a 2μA current to 0.2V at

time point 3. This is followed by a debounce timing cycle

whereby the fault latch is cleared, and ⎯F⎯A⎯U⎯L⎯T pulls high

when TIMER reaches its threshold at time point 4. At the

end of debounce cycle, the internal switch is allowed to

turn on. If the output short persists, the auxiliary supply

powers up into a short with active current limiting. At time

point 7, the fault filter delay begins with TIMER ramping

up with a 10μA current. If the TIMER times out at time

point 8, ⎯F⎯A⎯U⎯L⎯T will be pulled low and a new cool-off cycle

begins with TIMER ramping down with a 2μA current.

The whole process repeats itself until the output-short

is removed.

In Figure 9, a fault latches off the 12V supply at time point

1; a cool-off cycle begins by discharging the TIMER ca-

pacitor with 2μA current from 1.235V to 0.2V threshold.

At time point 2 a new debounce timing cycle is initiated

where the fault latch is cleared, and ⎯F⎯A⎯U⎯L⎯T pulls high when

TIMER reaches its threshold at time point 3. At the end of

the debounce cycle, the 12V GATE is allowed to start up. If

the output short persists, the 12V supply powers up into a

short with active current limiting. At time point 6, the fault

filter delay begins with TIMER ramping up with a 200μA

current. The TIMER times out at time point 7, ⎯F⎯A⎯U⎯L⎯T pulls

low and a new cool-off cycle begins with TIMER ramping

down with a 2μA current. The whole process repeats itself

until the output-short is removed.

The auto-retry duty cycle is given by:

DutyCycle

t

tt

t

FILTER

COOL

DEBOUNCE

FILT

=

++

•%

100

E

ER

duty cycle for auxiliary and 12V supply is 6.5% and 0.5%

respectively.