LM3489

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SNVS443A – MAY 2006 – REVISED JUNE 2008

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

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

continuously.

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

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

The current limit comparator has approximately 100ns 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 100ns. 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 fold back

phenomenon inherent in this current limit architecture. See Figure 5.

Figure 5. Current Limit Fold Back Phenomenon

At high input voltages (>28V) 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).

Additionally, a resistor can be placed between the ISENSE pin and the switch node. Any value in the range of

220

Ω to 600Ω is recommended.

START UP

The current limit circuit is active during start-up. During start-up the PFET will stay on until either the current limit

or the feedback comparator is tripped

If the current limit comparator is tripped first then the fold back characteristic should be taken into account. Start-

up into full load may require a higher current limit set point or the load must be applied after start-up.

One problem with selecting a higher current limit is inrush current during start-up. Increasing the capacitance

current limit to activate at a lower current. The output voltage will ramp more slowly when using this technique.

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