參數(shù)資料
型號(hào)： LTC4244CGN-1#TRPBF
廠商： Linear Technology
文件頁數(shù)： 23/28頁
文件大小： 337K
描述： IC CTRLR HOTSWAP PCI 20-SSOP
標(biāo)準(zhǔn)包裝： 2,500
類型： 熱交換控制器
應(yīng)用： CompactPCI?
內(nèi)部開關(guān)： 無
電源電壓： 3.3V，5V，±12V
工作溫度： 0°C ~ 70°C
安裝類型： 表面貼裝
封裝/外殼： 20-SSOP（0.154"，3.90mm 寬）
供應(yīng)商設(shè)備封裝： 20-SSOP
包裝： 帶卷 (TR)
23
LTC4244/LTC4244-1
42441f
the PC board. For 1 ounce copper foil plating, a general rule
is 1 ampere of DC current per via making sure the via is
properly dimensioned so that solder completely fills the
void. For other plating thicknesses, check with your PCB
fabrication facility.
Design Example
As a design example, consider a CPCI Hot Swap applica-
tion with the following power supply requirements:
Table 4. Design Example Power Supply Requirements
VOLTAGE
MAXIMUM DC
LOAD
SUPPLY
SUPPLY CURRENT
CAPACITANCE
12V
450mA
100礔
5V
5A
2200礔
3.3V
7A
2200礔
12V
100mA
100礔
The first step is to select the appropriate values of R
SENSE
for the 5V and 3.3V supplies. Calculating the value of
R
SENSE
 is based on I
LOAD(MAX)
and the lower limit for the
circuit breaker threshold voltage (47mV for both the 5V
and 3.3V circuit breakers). If a 1% tolerance is assumed
for the sense resistors, then 5m& and 7m& resistor
values yield the following minimum and maximum I
TRIP
values:
Table 5. I
TRIP
 vs R
SENSE
R
SENSE
 (1% RTOL)
I
TRIP(MIN)
I
TRIP(MAX)
5m&
9.3A
11.5A
7m&
6.6A
8.2A
So sense resistor values of 7m& and 5m& should suffice
for the 5V and 3.3V supplies, respectively.
The second step is to select MOSFETs for the 5V and 3.3V
supplies. The IRF7457s on resistance is less than 10.5m&
for V
GS
> 4.5V and a junction temperature of 25癈. Since
the maximum load current requirement for the 3.3V sup-
ply is 7A, the steady-state power the device may be
required to dissipate is 514mW. The IRF7457 has a
junction-to-ambient thermal resistance of 50癈/Watt. If a
maximum ambient temperature of 50癈 is assumed, this
yields a junction temperature of 75.7癈. According to the
IRF7457s Normalized On-Resistance vs Junction Tem-
perature curve, the devices on-resistance can be expected
to increase by about 20% over its room temperature value.
Recalculation of the steady-state values of R
ON
 and junc-
tion temperature yields approximately 12.6m& and 81癈,
respectively. The I " R drop across the 3.3V sense resistor
and series MOSFET at maximum load current under these
conditions will be less than 124mV.
The next step is to select appropriate values for C1
 
and
C
TIMER
. Assuming that the total current for the 5V supply
is constrained to less than 6A during power-up (6 ?5V
medium length connector pins at 1A per pin), then the
inrush current shouldnt exceed:
I
INRUSH
 < 6A  I
LOAD(5VOUT)
 = 6A  5A = 1A
(12)
This yields:
C
I
F
I
C
A
F
A
nF
GATE MAX
INRUSHMAX
1
2200
1
100
2200
1
220
>
?/DIV>
?nbsp   >
?/DIV>
?/DIV>
=
(
)
(
)
"
"
(13)
Hence a C1 value of 330nF ?0% should suffice. The value
of C
TIMER
 for this design example will be constrained by
the duration of the 12V supply inrush current, which
according to Equation 2 is:
t
C
V
I
I
t
F
V
mA
mA
ms
ON   VOUT
LOAD
LIMIT MIN    LOAD MAX
ON   VOUT
(
)
(    )
(
)
(
)
"
"

"
"

12
12
2
12
2  100
12
550
450
24
<
?/DIV>
<
?/DIV>
=
(14)
In order to guarantee that the LTC4244s TIMER fault
inhibit period is greater than 24ms, the value of C
TIMER
should be:
C
ms  I
V   V
C
ms
A
V
V
nF
TIMER
TIMER MAX
TIMER MAX
TIMER
>
?/DIV>
>
?/DIV>
=
24
12
24
26
12    1 9
61 8
"

"
  .
.
(
)
(
)
(15)
So a value of 82nF (?0%) should suffice.
APPLICATIO S I FOR ATIO
U
U
U
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