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參數資料
| 型號： | TPS2557DRB |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 電源管理 |
| 英文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO8 |
| 封裝： | PLASTIC, SON-8 |
| 文件頁數： | 4/22頁 |
| 文件大小： | 942K |
| 代理商： | TPS2557DRB |
ILIM
OSmax
0.947
ILIM
OSmax
0.947
OSmax
R
(k ) = 33.2k
99038V
I
(mA) =
R
k
99038V
I
(mA) =
33.2
k
I
(mA) = 3592mA
W
0.947
ILIM
OSmax
99038V
OSmax
R
k
1
0.947
ILIM
OSmax
ILIM
I
(mA) = 5000mA
I
(mA) =
99038V
R
(k ) =
I
mA
R
(k ) = 23.4k
W
÷
÷
÷
÷
è
23.7
ILIM
OSmin
1.0708
ILIM
OSmin
1.0708
OSmin
R
(k ) = 23.7k
127981V
I
(mA) =
R
k
127981V
I
(mA) =
k
I
(mA) = 4316mA
W
SLVS931 – NOVEMBER 2009
www.ti.com
(3)
The resulting maximum current-limit threshold is 3592 mA with a 33.2 k
resistor.
APPLICATION 2: DESIGNING BELOW A MAXIMUM CURRENT LIMIT
Some applications require that current limiting must occur below a certain threshold. For this example, assume
that the desired upper current-limit threshold must be below 5000 mA to protect an up-stream power supply. Use
(4)
Select the closest 1% resistor greater than the calculated value: RILIM = 23.7k. This sets the maximum
current-limit threshold at 5000 mA . Use the IOS equations, Figure 19, and the previously calculated value for
RILIM to calculate the minimum resulting current-limit threshold.
(5)
The resulting minimum current-limit threshold is 4316 mA with a 23.7 k
resistor.
ACCOUNTING FOR RESISTOR TOLERANCE
The previous sections described the selection of RILIM given certain application requirements and the importance
of understanding the current-limit threshold tolerance. The analysis focused only on the TPS2556/57
performance and assumed an exact resistor value. However, resistors sold in quantity are not exact and are
bounded by an upper and lower tolerance centered around a nominal resistance. The additional RILIM resistance
tolerance directly affects the current-limit threshold accuracy at a system level. The following table shows a
process that accounts for worst-case resistor tolerance assuming 1% resistor values. Step one follows the
selection process outlined in the application examples above. Step two determines the upper and lower
resistance bounds of the selected resistor. Step three uses the upper and lower resistor bounds in the IOS
equations to calculate the threshold limits. It is important to use tighter tolerance resistors, e.g. 0.5% or 0.1%,
when precision current limiting is desired.
Table 1. Common RILIM Resistor Selections
Resistor Tolerance
Actual Limits
Ideal
Closest 1%
Desired Nominal
Resistor
IOS MIN
IOS Nom
IOS MAX
Current Limit (mA)
1% low (k
)
1% high (k
)
(k
)
(k
)
(mA)
750
146.9
147
145.5
148.5
605
749
886
1000
110.2
110
108.9
111.1
825
1002
1166
1250
88.2
88.7
87.8
89.6
1039
1244
1430
12
Copyright 2009, Texas Instruments Incorporated
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