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LTC4242
14
4242f
for the 3.3V auxiliary power paths. Sense resistors R1 to
R4 provide input for current fault detection. Resistors R
G1
 
to R
G4
 and capacitors C
G1
 to C
G4
 compensate the current
control loops. Capacitors C
G1
 to C
G4
 also control the output
power-up rate and the inrush current while resistors R5
to R8 prevent high frequency oscillations in N-channel
MOSFETs, Q1 to Q4 respectively.
The following conditions must be satis ed before the
external and internal switches can be turned on.
1. The devices power supply, V
CC
, must exceed its
undervoltage lockout threshold. To turn on the exter-
nal/internal switches, the main/auxiliary input supplies
must exceed their UVLO thresholds.
2. The EN pin must be pulled low to begin the start-up
sequence.
When these initial conditions are satis ed, the ON pins are
checked. The LTC4242 features per slot ON pins, the AUXON
and ON, to allow independent control of the main input
supplies (12V and 3.3V) and the 3.3V auxiliary supplies. If
the ON pin is high, the switches turn on. If ON is low, the
switches turn on when the ON pin is brought high. Figure 4
shows all supplies turning on after EN goes low.
Each of the external switches is turned on by charging
the GATE with a 9礎 current source. The voltage at the
GATE pins rises with a slope equal to 9礎/C
G
 and the
supply inrush current is set at C
L
/C
G
 " 9礎, where C
L
 is
the capacitance at the supply output.
The gate of the internal switch is slewed resulting in the
3.3V
AUX
 supply output powering up at an internally set
rate of about 1.25V/ms.
The circuit breaker (ECB) of the input supplies is armed
after the input supplies clear UVLO. Once the supplies
have been turned on and the outputs are within tolerance,
PGOOD for the main input supplies and AUXPGOOD for
the auxiliary input supplies (available for the QFN only)
are pulled low.
Turn-Off Sequence
The switches can be turned off by a variety of conditions.
1. The ON/AUXON pin going low would turn off the main/
internal switches.
2. EN going high turns off all switches.
3. A variety of fault conditions will turn off the switches.
These include supply undervoltage and overcurrent
circuit breaker faults.
4. When thermal shutdown activates, the internal switch
is shut off.
When ON goes low, the main switches are turned off with
a 1mA current pulling down the gate to ground. When
the main supplies are shut off, the PGOOD signal pulls
high and the outputs are discharged to ground through
internal switches. Similarly, when an auxiliary supply is
turned off, the AUXPGOOD signal pulls high and its output
is discharged to ground through internal switches. Figure
5 shows all supplies being turned off by EN going high.
APPLICATIO  S I FOR  ATIO
U
U
U
Figure 4. Normal Power-Up Sequence
AUXOUTn
5V/DIV
12V
OUTn
5V/DIV
3V
OUTn
5V/DIV
PGOODn
5V/DIV
10ms/DIV
4242 F04
ENn
5V/DIV
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