LTC4242 Linear Technology, LTC4242 Datasheet - Page 14

• Manufacturer Part Number: LTC4242
• Description: Dual Slot Hot Swap Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC4242
for the 3.3V auxiliary power paths. Sense resistors R1 to
R4 provide input for current fault detection. Resistors R
to R
control loops. Capacitors C
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 satisfi ed before the
external and internal switches can be turned on.
1. The device’s power supply, V
2. The EN pin must be pulled low to begin the start-up
When these initial conditions are satisfi 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 9µA current source. The voltage at the
GATE pins rises with a slope equal to 9µA/C
supply inrush current is set at C
the capacitance at the supply output.
The gate of the internal switch is slewed resulting in the
3.3V
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.
14
undervoltage lockout threshold. To turn on the exter-
nal/internal switches, the main/auxiliary input supplies
must exceed their UVLO thresholds.
sequence.
G4
AUX
and capacitors C
supply output powering up at an internally set
U
G1
U
G1
to C
to C
G4
G4
L
compensate the current
/C
also control the output
CC
W
G
, must exceed its
• 9µA, where C
G
U
and the
L
G1
is
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/
2. EN going high turns off all switches.
3. A variety of fault conditions will turn off the switches.
4. When thermal shutdown activates, the internal switch
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.
AUXOUTn
PGOODn
12V
3V
internal switches.
These include supply undervoltage and overcurrent
circuit breaker faults.
is shut off.
5V/DIV
5V/DIV
5V/DIV
5V/DIV
5V/DIV
OUTn
OUTn
ENn
Figure 4. Normal Power-Up Sequence
10ms/DIV
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