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2015-08-0011

PT0497-4

08/31/15

10

PI5PD2556/2557

Precision Adjustable

Current-Limited Power Switch

Select the closest 1% resistor less than the calculated value: RILIM = 33.2 kΩ. This sets the minimum current-limit

threshold at 3000 mA . Use the IOS equations, Figure 7, and the previously calculated value for RILIM to

calculate the maximum resulting current-limit threshold.

The resulting maximum current-limit threshold is 3592 mA with a 33.2 kΩ 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

Select the closest 1% resistor greater than the calculated value: RILIM = 23.7kΩ. This sets the maximum

current-limit threshold at 5000 mA . Use the IOS equations, Figure 7, and the previously calculated value for

RILIM to calculate the minimum resulting current-limit threshold.

The resulting minimum current-limit threshold is 4316 mA with a 23.7 kΩ 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 PI5PD2556/57 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

Desired Nominal

Current Limit (mA)

Ideal

Resistor

(kΩ)

Closet 1%

Resistor

(kΩ)

Resistor Tolerance

Actual Limits

Ios Min

(mA)

Ios Nor

(mA)

Ios Max

(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

1500

73.6

73.2

72.5

73.9

1276

1508

1725

1750

63.1

63.4

62.8

64

1489

1742

1965

2000

55.2

54.9

54.4

55.4

1737

2012

2252

2250

49.1

48.7

48.2

49.2

1975

2269

2523