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参数资料
| 型号： | LTC4253AIGN#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 20/34页 |
| 文件大小： | 383K |
| 描述： | IC HOT SWAP CONTRLR -48V 16-SSOP |
| 标准包装： | 100 |
| 类型： | 热交换控制器 |
| 应用： | 通用 |
| 内部开关： | 无 |
| 电源电压： | 8.2 V ~ 14.5 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-SSOP（0.154"，3.90mm 宽） |
| 供应商设备封装： | 16-SSOP |
| 包装： | 管件 |
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LTC4253/LTC4253A
20
425353aff
For more information www.linear.com/4253
MOSFET SELECTION
The external MOSFET switch must have adequate safe op-
erating area (SOA) to handle short-circuit conditions until
TIMER times out. These considerations take precedence
over DC current ratings. A MOSFET with adequate SOA for
a given application can always handle the required current
but the opposite may not be true. Consult the manufacturers
MOSFET data sheet for safe operating area and effective
transient thermal impedance curves.
MOSFET selection is a 3-step process by assuming the
absence of soft-start capacitor. First, R
S
is calculated and
then the time required to charge the load capacitance is
determined. This timing, along with the maximum short-
circuit current and maximum input voltage, defines an
operating point that is checked against the MOSFETs
SOA curve.
To begin a design, first specify the required load current
and Ioad capacitance, I
L
and C
L
. The circuit breaker cur-
rent trip point (V
CB
/R
S
) should be set to accommodate
the maximum load current. Note that maximum input
current to a DC/DC converter is expected at V
SUPPLY(MIN)
.
R
S
is given by:
R
S
=
V
CB(MIN)
I
L(MAX)
(8)
where V
CB(MIN)
= 40mV (45mV for the LTC4253A) repre-
sents the guaranteed minimum circuit breaker threshold.
During the initial charging process, the LTC4253/LTC4253A
may operate the MOSFET in current limit, forcing (V
ACL
)
between 80mV to 120mV (V
ACL
is 54mV to 66mV for the
LTC4253A) across R
S
. The minimum inrush current is
given by:
I
INRUSH(MIN)
=
V
ACL(MIN)
R
S
(9)
Maximum short-circuit current limit is calculated using
the maximum V
SENSE
. This gives
I
SHORTCIRCUIT(MAX)
=
V
ACL(MAX)
R
S
(10)
The TIMER capacitor C
T
must be selected based on the
slowest expected charging rate; otherwise TIMER might
time out before the load capacitor is fully charged. A value
for C
T
is calculated based on the maximum time it takes
the load capacitor to charge. That time is given by:
t
CL(CHARGE)
=
C V
I
=
C
L
V
SUPPLY(MAX)
I
INRUSH(MIN)
(11)
The maximum current flowing in the DRAIN pin is given by:
I
DRN(MAX)
=
V
SUPPLY(MAX)
V
DRNCL
R
D
(12)
Approximating a linear charging rate, I
DRN
drops from
I
DRN(MAX)
to zero, the I
DRN
component in Equation (3)
can be approximated with 0.5 " I
DRN(MAX)
. Rearranging
the equation, TIMER capacitor C
T
is given by:
C
T
=
t
CL(CHARGE)
(200礎 + 4 I
DRN(MAX)
)
4V
(13)
Returning to Equation (3), the TIMER period is calcu-
lated and used in conjunction with V
SUPPLY(MAX)
and
I
SHORTCIRCUIT(MAX)
to check the SOA curves of a prospec-
tive MOSFET .
As a numerical design example for the LTC4253, consider
a 30W load, which requires 1A input current at 36V . If
V
SUPPLY(MAX)
= 72V and C
L
= 100礔 , R
D
= 1M? Equation
(8) gives R
S
??0m? Equation (13) gives C
T
= 414nF .
To account for errors in R
S
, C
T
, TIMER current (200礎),
TIMER threshold (4V), R
D
, DRAIN current multiplier and
DRAIN voltage clamp (V
DRNCL
), the calculated value should
be multiplied by 1.5, giving the nearest standard value of
C
T
??80nF .
If a short-circuit occurs, a current of up to 120mV/40m=?A
will flow in the MOSFET for 6.3ms as dictated by C
T
= 680nF
in Equation (3). The MOSFET must be selected based on
this criterion. The IRF530S can handle 100V and 3A for
10ms and is safe to use in this application.
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