Datasheet

TPS2560
TPS2561
SLVS930A – DECEMBER 2009– REVISED FEBRUARY 2012 www.ti.com
ACCOUNTING FOR RESISTOR TOLERANCE
The previous sections described the selection of R
ILIM
given certain application requirements and the importance
of understanding the current-limit threshold tolerance. The analysis focused only on the TPS2560/61
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 R
ILIM
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 I
OS
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 R
ILIM
Resistor Selections
Resistor Tolerance Actual Limits
Ideal Closest 1%
Desired Nominal
Resistor Resistor
IOS MIN IOS Nom IOS MAX
Current Limit (mA)
1% low (kΩ) 1% high (kΩ)
(kΩ) (kΩ)
(mA) (mA) (mA)
300 186.7 187 185.1 188.9 241.6 299.5 357.3
400 140.0 140 138.6 141.4 328.0 400.0 471.4
600 93.3 93.1 92.2 94.0 504.6 601.5 696.5
800 70.0 69.8 69.1 70.5 684.0 802.3 917.6
1000 56.0 56.2 55.6 56.8 859.9 996.4 1129.1
1200 46.7 46.4 45.9 46.9 1052.8 1206.9 1356.3
1400 40.0 40.2 39.8 40.6 1225.0 1393.0 1555.9
1600 35.0 34.8 34.5 35.1 1426.5 1609.2 1786.2
1800 31.1 30.9 30.6 31.2 1617.3 1812.3 2001.4
2000 28.0 28 27.7 28.3 1794.7 2000.0 2199.3
2200 25.5 25.5 25.2 25.8 1981.0 2196.1 2405.3
2400 23.3 23.2 23.0 23.4 2188.9 2413.8 2633.0
2600 21.5 21.5 21.3 21.7 2372.1 2604.7 2831.9
2800 20.0 20 19.8 20.2 2560.4 2800.0 3034.8
14 Submit Documentation Feedback Copyright © 2009–2012, Texas Instruments Incorporated
Product Folder Link(s): TPS2560 TPS2561