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参数资料
| 型号： | LTC4214-1IMS#TR |
| 厂商： | Linear Technology |
| 文件页数： | 18/32页 |
| 文件大小： | 277K |
| 描述： | IC CTRLR HOTSWAP NEG LV 10MSOP |
| 标准包装： | 2,500 |
| 类型： | 热交换控制器 |
| 应用： | 通用 |
| 内部开关： | 无 |
| 电源电压： | 6 V ~ 16 V |
| 工作温度： | -40°C ~ 85°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： | 带卷 (TR) |
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LTC4214-1/LTC4214-2
18
421412f
for C
T
is calculated based on the maximum time it takes the
load capacitor to charge. That time is given by:
t
C V
I
C V
I
CL CHARGE
L SUPPLY MAX
INRUSH MIN
(
)
( )
( )
"
"
=
=
(11)
The maximum current flowing in the DRAIN pin is given
by:
I
V
V
R
DRN MAX
SUPPLY MAX
DRNCL
D
( )
( )
=
(12)
Approximating a linear charging rate as 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 equa-
tion, TIMER capacitor C
T
is given by:
C
t
A
I
V
T
CL CHARGE
DRN MAX
=
?nbsp +
(
)
(
)
(
)
"
"
40
4
3
(13)
Returning to Equation (3), the TIMER period is calculated
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, consider a 10W load,
which requires 1.1A input current at 10.8V. If
V
SUPPLY(MAX)
= 13.2V and C
L
= 100礔, R
D
= 475k, Equa-
tion (8) gives R
S
= 40m&; Equation (13) gives C
T
= 34nF.
To account for errors in R
S
, C
T
, TIMER current (40礎),
TIMER threshold (3V), 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
= 56nF.
If a short-circuit occurs, a current of up to 80mV/
40m& = 2A will flow in the MOSFET for 0.9ms as dictated
by C
T
= 56nF in Equation (3). The MOSFET must be
selected based on this criterion. The IRF7413 can handle
20V and 2A for 9ms and is safe to use in this application.
Computing the maximum soft-start capacitor value during
soft-start to a load short is complicated by the nonlinear
MOSFETs SOA characteristics and the R
SS
C
SS
response.
An overly conservative but simple approach begins with
the maximum circuit breaker current, given by:
I
mV
R
CB MAX
S
(
)
=
56
(14)
From the SOA curves of a prospective MOSFET, determine
the time allowed, t
SOA(MAX)
. C
SS
is given by:
C
t
R
SS
SOA MAX
SS
=
(
)
. "
1 61
(15)
In the above example, 56mV/40m& gives 1.4A. t
SOA(MAX)
for the IRF7413 is 8ms for 1.4A at 30V. From Equation
(15), C
SS
= 68nF. Actual board evaluation showed that
C
SS
= 33nF was appropriate. The ratio (R
SS
" C
SS
) to
t
CL(CHARGE)
is a good gauge as a large ratio may result in
the time-out period expiring. This gauge is determined
empirically with board level evaluation.
SUMMARY OF DESIGN FLOW
To summarize the design flow, consider the application
shown in Figure 2. It was designed for 12W for a 10V to
14V supply.
Calculate the maximum load current: 12W/10V = 1.2A;
allowing for 75% converter efficiency, I
IN(MAX)
= 1.6A.
Calculate R
S
: from Equation (8) R
S
= 25m&.
Calculate I
SHORTCIRCUIT(MAX)
: from Equation (10)
I
SHORTCIRCUIT(MAX)
= 3.2A.
Select a MOSFET that can handle 3.2A at 14V: IRF7413.
Calculate C
T
: from Equation (13) C
T
= 24nF. Select
C
T
= 47nF, which gives the circuit breaker time-out period
t
MAX
= 0.7ms.
Consult MOSFET SOA curves: the IRF7413 can handle
3.2A at 20V for 3.5ms, so it is safe to use in this
application.
Calculate C
SS
: using Equations (14) and (15) select
C
SS
= 22nF.
FREQUENCY COMPENSATION
The LTC4214 typical frequency compensation network for
the analog current limit loop is a series R
C
(10&) and C
C
connected to V
EE
. Figure 6 depicts the relationship be-
APPLICATIO S I FOR ATIO
U
U
U
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