Time (s)

Continuous Conduction Mode (CCM)

Inductor Current

2

2.5

3

3.5

4

4.5

5

5.5

LOAD CURRENT (A)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

LM2696

SNVS375B – OCTOBER 2005 – REVISED APRIL 2013

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The last benefit of this scheme is when the short circuit is removed, and full load is re-applied, the part will

automatically recover into the load. The variation in the off-time removes the constraints of other frequency fold

back systems where the load would typically have to be reduced.

Figure 22. Normalized Output Voltage

Versus Load Current

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:

(19)

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:

(20)

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

minimum 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.

Several diagrams are shown in Figure 23 illustrating continuous conduction mode (CCM), discontinuous

conduction mode (DCM), and the boundary condition.

16

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