TPS2551DRVT datasheet(18/28 Pages) TI

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-k

ILIM

IOC(max)

IOC(typ)

IOC(min)

IOS(min)

APPLICATION 1: DESIGNING ABOVE A MINIMUM CURRENT-LIMIT

APPLICATION 2: DESIGNING BELOW A MAXIMUM CURRENT-LIMIT

TPS2550

TPS2551

SLVS736 – FEBRUARY 2008

Figure 21. Current-Limit Threshold Vs.RILIM

Some applications require that current-limiting cannot occur below a certain threshold. For this example, assume

that 1 A must be delivered to the load so that the minimum desired current-limit threshold is 1000 mA. Use the

IOS equations and Figure 21 to select RILIM.

• I

OS(min) (mA) = 1000 mA

• I

OS(min) (mA) = (39700 V) / (RILIM (kΩ))

1.342

• R

ILIM (kΩ) = [(39700 V) / (IOS(min) (mA))]

1/1.342

• R

ILIM = 15.54 kΩ

Select the closest 1% resistor less than the calculated value: RILIM = 15.4 kΩ. This sets the minimum current-limit

threshold at 1 A . Use the IOC equations, Figure 21, and the previously calculated value for RILIM to calculate the

maximum resulting current-limit threshold.

• R

ILIM = 15.4 kΩ

• I

OC(max) (mA) = (24500 V) / (RILIM (kΩ))

0.975

• I

OC(max) (mA) = (24500 V) / (15 (kΩ))

0.975

• I

OC(max) = 1703 mA

The resulting maximum current-limit theshold is 1.7 A with a 15.4 k

Ω resistor.

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 1.25 A to protect an up-stream power supply. Use

the IOC equations and Figure 21 to select RILIM.

• I

OC(max) (mA) = 1250 mA

• I

OC(max) (mA) = (24500 V) / (RILIM (kΩ))

0.975

• R

ILIM (kΩ) = [(24500 V) / (IOC(max) (mA))]

1/0.975

• R

ILIM = 21.15 kΩ

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