LM2696MXAX datasheet(13/19 Pages) NSC

13 / 19 page

Application Information (Continued)

Legend:

1: Min off-time (165 ns typical)

2: On-time (set by the user)

3: Min off-time (165 ns typical)

4: Blanking time (165 ns typical)

5: Extended off-time (12 µs typical)

L: UVP threshold

The last benefit of this scheme is when the short circuit is

removed, and full load is re-applied, the part will automati-

cally recover into the load. The variation in the off-time

removes the constraints of other frequency fold back sys-

tems where the load would typically have to be reduced.

MODES OF OPERATION

Since the LM2696 utilizes a catch diode, whenever the load

current is reduced to a point where the inductor ripple is

greater than two times the load current, the part will enter

discontinuous operation. This is because the diode does not

permit the inductor current to reverse direction. The point at

which this occurs is the critical conduction boundary and can

be calculated by the following equation:

One advantage of the adaptive on-time control scheme is

that during discontinuous conduction mode the frequency

will gradually decrease as the load current decreases. In

DCM the switching frequency may be determined using the

relationship:

It can be seen that there will always be some minimum

switching frequency. The minimum switching frequency is

determined by the parameters above and the minimum load

presented by the feedback resistors. If there is some mini-

mum frequency of operation the feedback resistors may be

sized accordingly.

The adaptive on-time control scheme is effectively a pulse-

skipping mode, but since it is not tied directly to an internal

clock, its pulse will only occur when needed. This is in

contrast to schemes that synchronize to a reference clock

frequency. The constant on-time pulse-skipping/DCM mode

minimizes output voltage ripple and maximizes efficiency.

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FIGURE 4. Fault Condition Timing

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FIGURE 5. Normalized Output Voltage

Versus Load Current

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