TPS2500, TPS2501

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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 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. Also, it

is important to note that this table assumes VIN = 3.3 V and IAUX = 0 A, so Figure 14 through Figure 19 should be

consulted to approximate how IOS shifts with VIN and IAUX. See the Programming the Current-Limit Threshold

Resistor section for additional details.

Table 3. Common RILIM Resistor Selections, VIN = 3.3 V, IAUX = 0 A

Resistor Tolerance

Actual Limits

Desired Nominal

Ideal Resistor

Closest 1%

1% high

IOS(min)

IOS(nom)

IOS(max)

Current Limit (mA)

(k

Ω)

Resistor (k

Ω)

1% low (k

Ω)

(k

Ω)

(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 VIN = 3.3 V to demonstrate

how efficiency is impacted by different inductors.

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