参数资料
型号: LTC4214-2CMS#PBF
厂商: Linear Technology
文件页数: 17/32页
文件大小: 277K
描述: IC CTRLR HOTSWAP NEGVOLT 10MSOP
标准包装: 50
类型: 热交换控制器
应用: 通用
内部开关:
电源电压: 6 V ~ 16 V
工作温度: 0°C ~ 70°C
安装类型: 表面贴装
封装/外壳: 10-TFSOP,10-MSOP(0.118",3.00mm 宽)
供应商设备封装: 10-MSOP
包装: 管件
LTC4214-1/LTC4214-2
17
421412f
TIMER commences charging C
T
 (Trace 4) while the analog
current limit loop maintains the fault current at 70mV/R
S
,
which in this case is 3.5A (Trace 2). Note that the back-
plane voltage (Trace 1) sags under load. Timer pull-up is
accelerated by V
OUT
. When C
T
 reaches 3V, GATE turns off,
PWRGD pulls high, the load current drops to zero and the
backplane rings in the positive direction. The transient
associated with the GATE turn off can be controlled with a
snubber to reduce ringing and transient voltage suppres-
sor to clip off large spikes. The choice of RC for the
snubber is usually done experimentally. The value of the
snubber capacitor is usually chosen between 10 to 100
times the MOSFET C
OSS
. The value of the snubber resistor
is typically between 3& to 100&. In many cases, a simple
short-circuit test can be performed to determine the
component values needed.
A low impedance short on one card may influence the
behavior of others sharing the same backplane. The initial
glitch and backplane sag as seen in Figure 5 Trace 1, can
rob charge from output capacitors on adjacent cards.
When the faulty card shuts down, current flows in to
refresh the capacitors. If LTC4214s are used by the other
cards, they respond by limiting the inrush current to a
value of 70mV/R
S
. If C
T
 is sized correctly, the capacitors
will recharge long before C
T
 times out.
POWER GOOD, PWRGD
PWRGD latches low if GATE charges up to within 2.8V of
V
IN
 and DRAIN pulls below V
DRNL
 during start-up. PWRGD
is reset in UVLO, in a UV condition or if C
T
 charges up to
3V. An overvoltage condition has no effect on PWRGD
status. A 50礎 current pulls this pin high during reset.
Various ways of using the PWRGD pin for interfacing with
a Power Module load are shown in the Typical Application
as well as Figures 2, 3, 18 and 19.
MOSFET SELECTION
The external MOSFET switch must have adequate safe
operating area (SOA) to handle short-circuit conditions
until TIMER times out. These considerations take prece-
dence 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
absense of a 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
current 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
V
I
S
CB MIN
L MAX
=
(    )
(    )
(8)
where V
CB(MIN)
 = 44mV represents the guaranteed mini-
mum circuit breaker threshold.
During the initial charging process, the LTC4214 may
operate the MOSFET in current limit, forcing (V
ACL
) be-
tween 60mV to 80mV across R
S
. The minimum inrush
current is given by:
I
mV
R
INRUSHMIN
S
(    )
=
60
(9)
Maximum short-circuit current limit is calculated using
the maximum V
SENSE
. This gives
I
mV
R
SHORTCIRCUIT MAX
S
(
)
=
80
(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
APPLICATIO  S I FOR  ATIO
U
U
U
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