参数资料
型号: LTC4224IMS-1#TRPBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 电源管理
英文描述: 2-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO10
封装: LEAD FREE, PLASTIC, MSOP-10
文件页数: 4/16页
文件大小: 215K
代理商: LTC4224IMS-1#TRPBF
LTC4224-1/LTC4224-2
12
422412fa
APPLICATIONS INFORMATION
Next, assume that there is no load current at start-up,
and calculate the inrush current required to charge the
load capacitor. As there is no gate capacitor, the supplies
start-up in current limit. Compute the time, tSU, it takes
to fully charge the load capacitor:
tSU =
VCC CLOAD
ITRIP
Table 3 lists the worst-case tSU values assuming 30%
tolerance for load capacitances.
Table 3. Worst-Case tSU
VOLTAGE SUPPLY
tSU(MIN)
tSU(MAX)
5V
0.53ms
0.65ms
3.3V
0.23ms
0.29ms
The start-up ECB blanking delay is guaranteed to be at least
2.5ms, which is longer than the tSU tabulated in Table 3.
Hence, both supplies can start up successfully.
Next, verify that the thermal ratings of the selected external
MOSFETs are not exceeded during power-up or an output
short-circuit. Assuming the MOSFET dissipates power only
due to inrush current charging the load capacitor, the energy
dissipated in the MOSFET during power-up is the same
as that stored in the load capacitor after power-up. The
average power dissipated in the MOSFET is given by:
PAVG =
CLOAD VOUT
2
2 tSU
The worst-case PAVGiscalculatedtobe4.6Wforboththe5V
supply and the 3.3V supply. In this example, the FDS6911
MOSFET offers a good solution. Since this MOSFET is a dual
N-channel in a single SO8 package, it must be able to tolerate
the combined power dissipation of both supplies during
the tSU start-up time. The increase in steady-state junction
temperature due to power dissipated in the MOSFET is
ΔT = PAVGZTH where ZTH is the thermal impedance.
Under this condition, the FDS6911 datasheet’s Transient
Thermal Impedance plot indicates that the junction tem-
perature will increase by 6.4°C using ZTHJC = 0.7°C/W
(single pulse). The FDS6911’s on-resistance is 17mΩ at
VGS = 4.5V, 25°C.
The magnitude of the power pulse that results during a
severe overload is calculated to be 9.25W for the 5V sup-
ply and 9.2W for the 3.3V supply under the worst case
conditions. Assuming a worst-case circuit breaker timeout
period of 7.5ms, the junction temperature will increase by
25°C, with one supply short-circuited. If both supplies are
short-circuited, the junction temperature will increase by
50°C in the worst-case. During auto-retry (LTC4224-2), in
the event of persistent faults at both supplies, the ample
four second cooling delay limits the increase in junction
temperature to 50°C. The SOA curves of the FDS6911
indicate that the above conditions are safe.
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