LTC4223-1/LTC4223-2
11
422312f
APPLICATIONS INFORMATION
Several conditions must be met before the external and
internal switches are allowed to turn on. First V
CC
and
the input supplies (12V
IN
, AUXIN) must exceed their
undervoltage lockout thresholds. Next TIMER must be
<0.2V and
?/DIV>
E
?/DIV>
N must be pulled low.
Once these conditions are met, a debounce timing cycle is
initiated when AUXON or 12ON pin is toggled from low to
high. These two control pins turn on/off the 3.3V auxiliary
and 12V supplies. At the end of the debounce cycle, the
ON pins and fault status are checked. If both ON pins are
high and fault is cleared, the 3.3V auxiliary supply starts
up rst followed by the 12V supply. Note that the turn-on
delay for the AUXON and 12ON pins is 15約 and 30約.
Figure 2 shows the two supplies turning on in sequence
after
?/DIV>
E
?/DIV>
N goes low.
By default, the internal pass transistor turns on rst if
both ON pins are high and start-up conditions met. The
output is current limited at 240mA by its internal ACL
ampli er as the load current charging the output capaci-
tor increases. This causes the TIMER to ramp up with a
10糀 pull-up. Normally the AUXOUT voltage exceeds its
power-good threshold before TIMER time-out and then
?/DIV>
A
?/DIV>
U
?/DIV>
X
?/DIV>
P
?/DIV>
G
?/DIV>
O
?/DIV>
O
?/DIV>
D pulls low.
Once AUXOUT signals power is good and the TIMER pin
returns to <0.2V, the external MOSFET is then allowed to
turn on by charging up the GATE with a 10糀 current source
(Figure 2). The voltage at the GATE pin rises with a slope
equal to 10糀/C
G
and the supply inrush current owing
into the load capacitor C
L1
(see Figure 14) is limited to:
I
C
C
礎
INRUSH
L
G
=
1
10
"
The 12V output follows the GATE voltage when the MOSFET
turns on. If the voltage across the current sense resistor
R
S
becomes too high, the inrush current is limited by the
internal current limit circuitry. Once the output, 12V
OUT
exceeds its power-good threshold,
?/DIV>
1
?/DIV>
2
?/DIV>
P
?/DIV>
G
?/DIV>
O
?/DIV>
O
?/DIV>
D also pulls
low.
If only the 12ON pin is high at the end of debounce cycle,
the external MOSFET turns on rst. After that, if AUXON
pulls high, the internal switch turns on only after the 12V
output signals power is good and TIMER <0.2V.
Figure 2. Normal Power-Up Sequence
Table 1. 12V and Auxiliary Supply Turn-Off Conditions
CONDITION
RESULT
CLEARED BY
AUX
12V
AUXON Goes
Low
Turns Off
No Effect AUXON High
12ON Goes Low
No Effect
Turns Off 12ON High
?/DIV>
E
?/DIV>
N Goes High
Turns Off
Turns Off
?/DIV>
E
?/DIV>
N Low
UVLO on V
CC
Turns Off
Turns Off V
CC
> UVLO
UVLO on AUXIN
Turns Off
No Effect AUXIN > UVLO
UVLO on 12V
IN
No Effect
Turns Off 12V
IN
> UVLO
AUX Overcurrent
Fault
Turns Off
Turns Off AUXON and 12ON Low,
?/DIV>
E
?/DIV>
N High-to-Low, UVLO
on V
CC
12V Overcurrent
Fault
No Effect
Turns Off 12ON Low,
?/DIV>
E
?/DIV>
N High-to-
Low, UVLO on V
CC
Thermal
Shutdown
Turns Off
Turns Off AUXON and 12ON Low,
?/DIV>
E
?/DIV>
N High-to-Low, UVLO
on V
CC
, Temperature <
120癈
20ms/DIV
12PGOOD
5V/DIV
EN
5V/DIV
AUXOUT
5V/DIV
TIMER
2V/DIV
12V
OUT
5V/DIV
AUXPGOOD
5V/DIV
422312 F02
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