Datasheet

TPS2552, TPS2553
TPS2552-1, TPS2553-1
www.ti.com SLVS841E –NOVEMBER 2008– REVISED FEBRUARY 2012
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 focussed only on the TPS2552/53 and
TPS2552-1/53-1 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)
75 SHORT ILIM to IN 50.0 75.0 100.0
120 226.1 226 223.7 228.3 101.3 120.0 142.1
200 134.0 133 131.7 134.3 173.7 201.5 233.9
300 88.5 88.7 87.8 89.6 262.1 299.4 342.3
400 65.9 66.5 65.8 67.2 351.2 396.7 448.7
500 52.5 52.3 51.8 52.8 448.3 501.6 562.4
600 43.5 43.2 42.8 43.6 544.3 604.6 673.1
700 37.2 37.4 37.0 37.8 630.2 696.0 770.8
800 32.4 32.4 32.1 32.7 729.1 800.8 882.1
900 28.7 28.7 28.4 29.0 824.7 901.5 988.7
1000 25.8 26.1 25.8 26.4 908.3 989.1 1081.0
1100 23.4 23.2 23.0 23.4 1023.7 1109.7 1207.5
1200 21.4 21.5 21.3 21.7 1106.0 1195.4 1297.1
1300 19.7 19.6 19.4 19.8 1215.1 1308.5 1414.9
1400 18.3 18.2 18.0 18.4 1310.1 1406.7 1517.0
1500 17.0 16.9 16.7 17.1 1412.5 1512.4 1626.4
1600 16.0 15.8 15.6 16.0 1512.5 1615.2 1732.7
1700 15.0 15.0 14.9 15.2 1594.5 1699.3 1819.4
CONSTANT-CURRENT VS. LATCH-OFF OPERATION AND IMPACT ON OUTPUT VOLTAGE
Both the constant-current devices (TPS2552/53 ) and latch-off devices (TPS2552-1/53-1) operate
identically during normal operation, i.e. the load current is less than the current-limit threshold and the devices
are not limiting current. During normal operation the N-channel MOSFET is fully enhanced, and V
OUT
= V
IN
- (I
OUT
x r
DS(on)
). The voltage drop across the MOSFET is relatively small compared to V
IN
, and V
OUT
≉ V
IN
.
Both the constant-current devices (TPS2552/53 ) and latch-off devices (TPS2552-1/53-1) operate
identically during the initial onset of an overcurrent event. Both devices limit current to the programmed
current-limit threshold set by R
ILIM
by operating the N-channel MOSFET in the linear mode. During current-limit
operation, the N-channel MOSFET is no longer fully-enhanced and the resistance of the device increases. This
allows the device to effectively regulate the current to the current-limit threshold. The effect of increasing the
resistance of the MOSFET is that the voltage drop across the device is no longer negligible (V
IN
≠ V
OUT
), and
V
OUT
decreases. The amount that V
OUT
decreases is proportional to the magnitude of the overload condition. The
expected V
OUT
can be calculated by I
OS
× R
LOAD
, where I
OS
is the current-limit threshold and R
LOAD
is the
magnitude of the overload condition. For example, if I
OS
is programmed to 1 A and a 1 Ω overload condition is
applied, the resulting V
OUT
is 1 V.
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