LM3485MM datasheet(11/15 Pages) NSC

11 / 15 page

Functional Description (Continued)

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. Adding a capacitance (C

ADJ) in par-

allel with R

ADJ results in soft-start. CADJ and RADJ create an

RC time constant forcing current limit to activate at a lower

current. The output voltage will ramp more slowly when

using the soft-start functionality.

The C

ADJ also filters unwanted noise so that the ISENSE

comparator will not be accidentally triggered. A value of

100pF to 1nF is recommended in most applications. These

low values for C

ADJ will have little to no effect on soft-start.

There are example start-up plots for C

ADJ equal to 1nF and

10nF in the Typical Performance Characteristics.

External Sense Resistor

The V

DS of a PFET will tend to vary significantly over tem-

perature. This will result an equivalent variation in current

limit. To improve current limit accuracy an external sense

resistor can be connected from V

IN to the source of the

PFET, as shown in

Figure 5.

20034627

FIGURE 5. Current Sensing by External Resistor

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