Datasheet

TPS2500, TPS2501
www.ti.com SLVS886C –OCTOBER 2008–REVISED AUGUST 2010
worst-case resistor tolerance assuming 1% resistor values. Step 1 follows the selection process outlined in the
application examples above. Step 2 determines the upper and lower resistance bounds of the selected resistor.
Step 3 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. Also, it
is important to note that this table assumes V
IN
= 3.3 V and I
AUX
= 0 A, so Figure 14 through Figure 19 should be
consulted to approximate how I
OS
shifts with V
IN
and I
AUX
. See the Programming the Current-Limit Threshold
Resistor section for additional details.
Table 3. Common R
ILIM
Resistor Selections, V
IN
= 3.3 V, I
AUX
= 0 A
Resistor Tolerance Actual Limits
Desired Nominal Ideal Resistor Closest 1%
1% high I
OS(min)
I
OS(nom)
I
OS(max)
Current Limit (mA) (kΩ) Resistor (kΩ)
1% low (kΩ)
(kΩ) (mA) (mA) (mA)
300 94.98 95.30 94.35 96.25 198.2 299.0 401.7
400 71.19 71.50 70.79 72.22 273.0 398.3 524.8
500 56.93 57.60 57.02 58.18 347.4 494.2 641.7
600 47.42 47.50 47.03 47.98 430.6 599.0 767.7
700 40.64 40.20 39.80 40.60 518.5 707.6 896.5
800 35.55 35.70 35.34 36.06 591.8 796.6 1001.2
900 31.59 31.60 31.28 31.92 678.0 899.7 1121.5
1000 28.42 28.70 28.41 28.99 754.7 990.4 1226.5
1100 25.84 26.10 25.84 26.36 839.0 1088.9 1339.7
1200 23.68 23.70 23.46 23.94 934.1 1199.0 1465.5
1300 21.85 22.10 21.88 22.32 1009.8 1285.5 1563.9
1400 20.29 20.50 20.30 20.71 1098.0 1385.7 1677.1
Thermal Sense
The TPS2500 self-protects using two independent thermal sensing circuits that monitor the operating
temperatures of the boost converter and power switch independently and disable operation if the temperature
exceeds recommended operating conditions. The boost converter and power switch each have an ambient
thermal sensor that disables operation if the measured junction temperature in that part of the circuit exceeds
150°C. The boost converter continues to operate even if the power switch is disabled due to an overtemperature
condition.
Component Recommendations
The main functions of the TPS2500 are integrated and meet recommended operating conditions with a wide
range of external components. The following sections give guidelines and trade-offs for external component
selection. The recommended values given are conservative and intended over the full range of recommended
operating conditions.
Boost Inductor
Connect the boost inductor from IN to SW. The inductance controls the ripple current through the inductor. A
2.2-mH inductor is recommended, and the minimum and maximum inductor values are constrained by the
integrated features of the TPS2500. The minimum inductance is limited by the peak inductor-current value. The
ripple current in the inductor is inversely proportional to the inductance value, so the output voltage may fall out
of regulation if the peak inductor current exceeds the cycle-by-cycle current-limit comparator (3 A minimum).
Using a nominal 2.2-mH inductor allows full recommended current operation even if the inductance is 20% low
(1.76 mH) due to component variation. The maximum inductance value is limited by the internal compensation of
the boost-converter control loop. A maximum 4.7-mH (typical) inductor value is recommended to maintain
adequate phase margin over the full range of recommended operating conditions.
The following chart shows the efficiency vs AUX current of two different inductors at V
IN
= 3.3 V to demonstrate
how efficiency is impacted by different inductors.
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