Datasheet

LTC4223-1/LTC4223-2
14
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; fi xed
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 amplifi er 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 fi lter delay is given by
C
T
• 123[ms/μF].
After a successful power-up cycle, the ACL amplifi er 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 signifi cant 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 fi lter
delay, and pull the gate down. Then the ACL amplifi er 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 fi lter delay.
Undervoltage Fault
An undervoltage fault occurs if either AUXIN or 12V
IN
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, V
CC
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 V
CC
is restored.
Figure 7. Short-Circuit Fault on 3.3V
AUX
OutputFigure 6. Short-Circuit Fault on 12V Output
5μs/DIV
12V
GATE
5V/DIV
12V
OUT
5V/DIV
422312 F06
ΔV
SENSE
200mV/DIV
5μs/DIV
AUXOUT
5V/DIV
FAULT
5V/DIV
422312 F07
I
LOAD
1A/DIV