LTC4240IGN#PBF Linear Technology, LTC4240IGN#PBF Datasheet - Page 19

• Manufacturer Part Number: LTC4240IGN#PBF
• Description: IC CTRLR HOTSWAP CPCI I2C 28SSOP
• Manufacturer: Linear Technology
• Type: Hot-Swap Controllerr

Specifications of LTC4240IGN#PBF

• Applications: CompactPCI™
• Internal Switch(s): No
• Voltage - Supply: 3.3V, 5V, ±12V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP (0.150", 3.95mm Width)
• Family Name: LTC4240
• Package Type: SSOP N
• Operating Supply Voltage (min): 2.55/4.45/10.8/-10.5V
• Operating Supply Voltage (max): 5/12/14/-14V
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Product Depth (mm): 3.99mm
• Product Height (mm): 1.5mm
• Mounting: Surface Mount
• Pin Count: 28
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
Once the TIMER voltage has reached 5.5V, all of the
supplies will be latched off if any supply enters current
limit for at least 35 s. The 35 s delay prevents quick
current spikes—for example, from a fan turning on—
from causing false trips of the circuit breaker.
During normal operation, the 5V and 3.3V supplies are
protected from overcurrent and short-circuit conditions
by dual-level circuit breakers. In the event that either
supply current exceeds the nominal limit, an internal timer
is started. If the supply is still overcurrent after 35 s, the
circuit breaker trips and all the supplies are turned off
(Figure 7). If a short-circuit occurs on 5V
the supply current exceeds three times the set limit, an
analog loop will limit the current to 3 times the value set
by R
35 s, the LTC4240 latches off (Figure 8). It will stay in the
latched off state until it is reset using the OFF/ON pin or by
using the I
cycling any of the power supplies.
5V
5V
IN
IN
100mV/DIV
100mV/DIV
SENSE
–5V
–5V
5V/DIV
5V/DIV
5V/DIV
5V/DIV
FAULT
FAULT
SENSE
SENSE
GATE
GATE
2
and 55mV. If the short persists for more than
C interface. The LTC4240 can also be reset by
Figure 8. Short-Circuit Fault on 5V
Figure 7. Overcurrent Fault on 5V
U
U
20 s/DIV
20 s/DIV
W
OUT
or 3V
4240 F07
4240 F08
U
OUT
and
The current limit and the foldback current level for the 5V
and 3.3V outputs are both a function of the external sense
resistor (R1 for 3V
A sense resistor is connected between 5V
5V
sense resistor is connected between 3V
3V
current level are given by Equations 4 and 5:
where XV
Equation 4 is the current limit for XV
5 shows the I
assume voltage on TIMER pin is greater than 5.5V.
XV
pins connecting 5V
The current limit for the internal 12V switch is set at
–1200mA folding back to –350mA and the –12V switch at
500mA folding back to 250mA.
Selecting R
An equivalent circuit for the 5V and 3.3V circuit breakers
is shown in Figure 9. The sense resistor and the circuit
breaker threshold voltage determine the fault current that
turns off the external FETs. Sense resistors with a 1%
tolerance are recommended. Due to part to part and
temperature variations for both the sense resistor value
and the circuit breaker threshold voltage, the actual cur-
rent limit threshold will exhibit some variation. To calcu-
late the smallest value of current that will trip the fault
comparator, use the largest value of the sense resistor and
the smallest value of the threshold voltage. A 0.005 1%
sense resistor (on the 3.3V supply, for example) with
typical temperature coefficients would increase to ap-
proximately 0.0051 (nominal value multiplied by the 1%
tolerance and the TC at 70 C). Since the minimum value of
the threshold voltage is 50mV, this implies a current limit
of 9.8A. To arrive at the largest value of the current limit
that will turn off the external FETs, the nominal value of the
SENSE
SENSE
I
I
OUT
LIMIT XVOUT
FOLDBACK XVOUT
= 3V
(Pin 23). The current limit and the current foldback
(
(Pin 20) for the 5V supply. For the 3.3V supply, a
OUT
OUT
SENSE
(
= 5V
LIMIT
)
or 5V
OUT
OUT
R
IN
)
for shorted outputs. Both equations
SENSE XVOUT
OUT
, R
and R2 for 5V
or 3V
R
55
SENSE
. Note that since there are only 8
SENSE XVOUT
mV
(
OUT
11
mV
(
.
0.007 for 5V
)
OUT
OUT
)
LTC4240
, see front page).
IN
IN
XV
(Pin 22) and
(Pin 21) and
IN
. Equation
IN
.
19
4240f
( )
( )
4
5