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| 型号： | LTC4240IGN#TR |
| 厂商： | Linear Technology |
| 文件页数： | 19/28页 |
| 文件大小： | 493K |
| 描述： | IC CTRLR HOT SWAP CPCI 28-SSOP |
| 标准包装： | 2,500 |
| 类型： | 热交换控制器 |
| 应用： | CompactPCI? |
| 内部开关： | 无 |
| 电源电压： | 3.3V，5V，±12V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-SSOP（0.154"，3.90mm 宽） |
| 供应商设备封装： | 28-SSOP |
| 包装： | 带卷 (TR) |
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LTC4240
19
4240f
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祍. The 35祍 delay prevents quick
current spikesfor 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祍, the
circuit breaker trips and all the supplies are turned off
(Figure 7). If a short-circuit occurs on 5V
OUT
or 3V
OUT
and
the supply current exceeds three times the set limit, an
analog loop will limit the current to 3 times the value set
by R
SENSE
and 55mV. If the short persists for more than
35祍, 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
2
C interface. The LTC4240 can also be reset by
cycling any of the power supplies.
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
OUT
and R2 for 5V
OUT
, see front page).
A sense resistor is connected between 5V
IN
(Pin 21) and
5V
SENSE
(Pin 20) for the 5V supply. For the 3.3V supply, a
sense resistor is connected between 3V
IN
(Pin 22) and
3V
SENSE
(Pin 23). The current limit and the current foldback
current level are given by Equations 4 and 5:
mV
R
mV
R
LIMIT XVOUT
SENSE XVOUT
FOLDBACK XVOUT
SENSE XVOUT
(
)
(
)
(
)
( )
( )
=
=
55
4
11
5
where XV
OUT
= 5V
OUT
or 3V
OUT
.
Equation 4 is the current limit for XV
OUT
H XV
IN
. Equation
5 shows the I
LIMIT
for shorted outputs. Both equations
assume voltage on TIMER pin is greater than 5.5V.
XV
OUT
= 3V
OUT
or 5V
OUT
. Note that since there are only 8
pins connecting 5V
IN
, R
SENSE
e 0.007& for 5V
IN
.
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
SENSE
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癈). 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
APPLICATIO S I FOR ATIO
U
U
U
Figure 7. Overcurrent Fault on 5V
Figure 8. Short-Circuit Fault on 5V
GATE
5V/DIV
5V
IN
5V
SENSE
100mV/DIV
FAULT
5V/DIV
20祍/DIV
4240 F07
GATE
5V/DIV
5VIN5VSENSE
100mV/DIV
FAULT
5V/DIV
20祍/DIV
4240 F08
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