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参数资料
| 型号： | LTC4225CGN-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 16/24页 |
| 文件大小： | 346K |
| 描述： | IC CTLR HOT SWAP DUAL 24-SSOP |
| 标准包装： | 55 |
| 类型： | 热交换控制器 |
| 应用： | 通用 |
| 内部开关： | 无 |
| 电流限制： | 可调 |
| 电源电压： | 2.9 V ~ 18 V |
| 工作温度： | 0°C ~ 70°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-SSOP（0.154"，3.90mm 宽） |
| 供应商设备封装： | 24-SSOP |
| 包装： | 管件 |
LTC4225-1/LTC4225-2
16
422512f
Next, verify that the thermal ratings of the selected
MOSFET , Si7336ADP , are not exceeded during power-up
or an output short.
Assuming the MOSFET dissipates power due to inrush
current charging the load capacitor, C
L
, at power-up, the
energy dissipated in the MOSFET is the same as the energy
stored in the load capacitor, and is given by:
E
CL
=
1
2
C
L
V
IN
2
For C
L
= 1600礔 , the time it takes to charge up C
L
is
calculated as:
t
CHARGE
=
C
L
V
IN
I
INRUSH
=
1600礔 12V
1A
=19ms
The inrush current is set to 1A by adding capacitance,
C
HG
, at the gate of the Hot Swap MOSFET .
C
HG
=
C
L
I
HGATE(UP)
I
INRUSH
=
1600礔 10礎
1A
=16nF
Choose a practical value of 15nF for C
HG
.
The average power dissipated in the MOSFET is calculated
as:
P
AVG
=
E
CL
t
CHARGE
=
1
2
1600礔 12V
( )
2
19ms
=6W
The MOSFET selected must be able to tolerate 6W for
19ms during power-up. The SOA curves of the Si7336ADP
provide for 1.5A at 30V (45W) for 100ms. This is suffi-
cient to satisfy the requirement. The increase in junction
temperature due to the power dissipated in the MOSFET
is T = P
AVG
" Zth
JC
where Zth
JC
is the junction-to-case
thermal impedance. Under this condition, the Si7336ADP
data sheet indicates that the junction temperature will
increase by 4.8癈 using Zth
JC
= 0.8癈/W (single pulse).
The duration and magnitude of the power pulse during an
output short is a function of the TMR capacitance, C
T
, and
the LTC4225s active current limit. The short-circuit dura-
tion is given as C
T
" 12[ms/礔] = 0.56ms for C
T
= 0.047礔 .
The maximum short-circuit current is calculated using the
maximum active current limit threshold V
SENSE(ACL)(MAX)
and minimum R
S
value.
I
SHORT(MAX)
=
V
SENSE(ACL)(MAX)
R
S(MIN)
=
75mV
3.96m&
=18.9A
So, the maximum power dissipated in the MOSFET is
18.9A " 12V = 227W for 0.56ms. The Si7336ADP data
sheet indicates that the worst-case increase in junction
temperature during this short-circuit condition is 22.7癈
using Zth
JC
= 0.1癈/W (single pulse). Choosing C
T
=
0.047礔 will not cause the maximum junction temperature
of the MOSFET to be exceeded. The SOA curves of the
Si7336ADP provide for 15A at 30V (450W) for 1ms. This
also satisfies the requirement.
Next, select the resistive divider at the ON1 and ON2 pins
to provide an undervoltage threshold of 9.6V for the 12V
supply. First, choose the bottom resistors, R1 and R3, to be
20k. Then, calculate the top resistor value for R2 and R4:
R
TOP
=
V
IN(UVTH)
V
ON(TH)
1
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
R
BOTTOM
R
TOP
=
9.6V
1.235V
1
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
20k =135k
Choose the nearest 1% resistor value of 137k for R2
and R4. In addition, there is a 0.1礔 bypass (C1) at the
INTV
CC
pin and a 10nF filter capacitor (C
F
) at the ON pin
to prevent the supply glitches from turning off the Hot
Swap MOSFET .
PCB Layout Considerations
For proper operation of the LTC4225s circuit breaker, Kelvin
connection to the sense resistor is strongly recommended.
The PCB layout should be balanced and symmetrical to
minimize wiring errors. In addition, the PCB layout for the
sense resistor and the power MOSFET should include good
thermal management techniques for optimal device power
dissipation. A recommended PCB layout is illustrated in
Figure 7.
applicaTions inForMaTion
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