Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- DESCRIPTION
- ABSOLUTE MAXIMUM RATINGS
- OPERATING RANGE
- ELECTRICAL CHARACTERISTICS
- TYPICAL PERFORMANCE CHARACTERISTICS
- BLOCK DIAGRAM
- APPLICATION INFORMATION
- CONSTANT ON-TIME CONTROL OVERVIEW
- INTERNAL OPERATION UNDER-VOLTAGE COMPARATOR
- ON-TIME GENERATOR SHUTDOWN
- CURRENT LIMIT
- N-CHANNEL HIGH SIDE SWITCH AND DRIVER
- THERMAL SHUTDOWN
- COMPONENT SELECTION
- FREQUENCY SELECTION
- INDUCTOR SELECTION
- OUTPUT CAPACITOR
- RIPPLE FEED FORWARD
- FEEDBACK RESISTORS
- INPUT CAPACITOR
- AVIN CAPACITOR
- SOFT-START CAPACITOR
- EXTVCC CAPACITOR
- SHUTDOWN
- CBOOT CAPACITOR
- PGOOD RESISTOR
- CATCH DIODE
- BYPASS CAPACITOR
- EXTERNAL OPERATION STARTUP
- UNDER- & OVER-VOLTAGE CONDITIONS
- CURRENT LIMIT
- MODES OF OPERATION
- LINE REGULATION
- TRANSIENT RESPONSE
- EFFICIENCY
- PRE-BIAS LOAD STARTUP
- THERMAL CONSIDERATIONS
- LAYOUT CONSIDERATIONS
- Revision History

I
AVERAGE
Time (s)
Continuous Conduction Mode (CCM)
Inductor Current
f
SW
=
2 L V
OUT
I
OUT
T
ON
2
V
IN
(V
IN
- V
OUT
)
I
BOUNDARY
=
(V
IN
- V
OUT
) D
2 L f
SW
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
NORMALIZED OUTPUT VOLTAGE
LM2696
SNVS375B –OCTOBER 2005–REVISED APRIL 2013
www.ti.com
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.
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