LM3485

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SNVS178H – JANUARY 2002 – REVISED DECEMBER 2015

Feature Description (continued)

comparator triggers the 9-µs one shot pulse generator forcing the driver to turn the PFET off. The driver turns the

PFET back on after 9 µs. If the current has not reduced below the set threshold, the cycle will repeat

continuously.

comparator will not be accidentally triggered. A value of 100 pF to 1 nF is recommended in most applications.

Higher values can be used to create a soft-start function (see Start Up).

The current limit comparator has approximately 100 ns of blanking time. This ensures that the PFET is fully on

when the current is sensed. However, under extreme conditions such as cold temperature, some PFETs may not

fully turn on within the blanking time. In this case, the current limit threshold must be increased. If the current limit

function is used, the on time must be greater than 100 ns. Under low duty cycle operation, the maximum

operating frequency will be limited by this minimum on time.

During current limit operation, the output voltage will drop significantly as will operating frequency. As the load

current is reduced, the output will return to the programmed voltage. However, there is a current limit foldback

phenomenon inherent in this current limit architecture. See Figure 25.

Figure 25. Current Limit Fold Back Phenomenon

At high input voltages (>28 V) increased undershoot at the switch node can cause an increase in the current limit

threshold. To avoid this problem, a low Vf Schottky catch diode must be used (see Catch Diode Selection (D1)).

Additionally, a resistor can be placed between the ISENSE pin and the switch node. Any value up to

approximately 600

Ω is recommended.

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