LTC1421 datasheet(14/24 Pages) LINER

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LTC4223-1/LTC4223-2

422312f

APPLICATIONS INFORMATION

There are two different modes of fault time-out for the 3.3V

auxiliary supply: adjustable delay through TIMER capacitor

during power-up when ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D not asserted; ﬁxed

25μs delay after power-up when ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D asserted

low. Under the situation whereby AUXON toggles low then

high for short duration after power-up while ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D

still pulling low due to output load capacitor, 25μs fault

time-out applies.

When the auxiliary supply is powered up into an output

short, the ACL ampliﬁer will regulate the gate of the

internal pass transistor to produce 240mA output cur-

rent. At this time a 10μA pull-up current starts charging

up the TIMER pin capacitor until it exceeds its threshold

(1.235V). The internal pass transistor then turns off and

⎯F⎯A⎯U⎯L⎯T pulls low. Thereafter, the TIMER is discharged by

a 2μA pull-down current. The fault ﬁlter delay is given by

CT • 123[ms/μF].

After a successful power-up cycle, the ACL ampliﬁer pro-

tects the auxiliary supply from overcurrent by pulling down

the gate of the internal pass transistor rapidly as shown

in Figure 7. Thereafter, the gate recovers and servos the

output current to about 240mA for 25μs before pulling

down to ground gently, turning the transistor off. At this

time, ⎯F⎯A⎯U⎯L⎯T pulls low and the 12V external MOSFET is

also turned off by the 1mA GATE pull-down current.

Whenever the 3.3V auxiliary supply trips off due to an

overcurrent fault, the 12V supply also shuts off. The

auxiliary supply is, however, unaffected by faults on the

12V supply. In either case ⎯F⎯A⎯U⎯L⎯T latches low when the

affected channels turn off, and ⎯F⎯A⎯U⎯L⎯T is cleared by tog-

gling the ON pins. Faults are cleared automatically in the

LTC4223-2 auto-retry version.

If there is signiﬁcant supply lead inductance, a severe

output short may collapse the input to ground before the

LTC4223 can bring the current under control. In this case

the undervoltage lockout will activate after a 12μs ﬁlter

delay, and pull the gate down. Then the ACL ampliﬁer will

take control and regulate the output in active current limit.

Under this situation, the fault time-out is set by TIMER

delay instead of 25μs ﬁlter delay.

Undervoltage Fault

An undervoltage fault occurs if either AUXIN or 12VIN falls

below its undervoltage threshold for longer than 12μs. This

turns off the affected supply’s switch instantly, but does

not clear the fault latches. Further, an undervoltage fault

on one supply does not affect the operation of the other

supply. If the bias supply input, VCC falls below its UVLO

threshold for more than 80μs, all supply switches are turned

off and the fault latches are cleared. Operation resumes

from a fresh start-up cycle when VCC is restored.

Figure 7. Short-Circuit Fault on 3.3VAUX Output

Figure 6. Short-Circuit Fault on 12V Output

μs/DIV

12VGATE

5V/DIV

12VOUT

5V/DIV

422312 F06

ΔVSENSE

200mV/DIV

μs/DIV

AUXOUT

5V/DIV

FAULT

5V/DIV

422312 F07

ILOAD

1A/DIV

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