Datasheet

0.0 0.2 0.4 0.6 0.8 1.0 1.2
5
10
15
20
25
30
35
40
45
INPUT VOLTAGE (v)
SHORT CIRCUIT VOLTAGE (v)
SAFE OPERATING AREA
LM22679/LM22679Q
www.ti.com SNVS581K –FEBRUARY 2013–REVISED NOVEMBER 2012
When the LM22679 enters current limit, the output voltage will drop and the peak inductor current will be fixed at
I
CL
at the end of each cycle. The switching frequency will remain constant while the duty cycle drops. The load
current will not remain constant, but will depend on the severity of the overload and the output voltage.
For very severe overloads ("short-circuit"), the regulator changes to a low frequency current foldback mode of
operation. The frequency foldback is about 1/5 of the nominal switching frequency. This will occur when the
current limit trips before the minimum on-time has elapsed. This mode of operation is used to prevent inductor
current "run-away", and is associated with very low output voltages when in overload. The following equation can
be used to determine what level of output voltage will cause the part to change to low frequency current foldback:
(4)
Where F
sw
is the normal switching frequency and V
in
is the maximum for the application. If the overload drives
the output voltage to less than or equal to V
x
, the part will enter current foldback mode. If a given application can
drive the output voltage to ≤V
x
, during an overload, then a second criterion must be checked. The next equation
gives the maximum input voltage, when in this mode, before damage occurs:
(5)
Where V
sc
is the value of output voltage during the overload and F
sw
is the normal switching frequency. If the
input voltage should exceed this value, while in foldback mode, the regulator and/or the diode may be
damaged. It is important to note that the voltages in these equations are measured at the inductor. Normal trace
and wiring resistance will cause the voltage at the inductor to be higher than that at a remote load. Therefore,
even if the load is shorted with zero volts across its terminals, the inductor will still see a finite voltage. It is this
value that should be used for V
x
and V
sc
in the calculations. In order to return from foldback mode, the load must
be reduced to a value much lower than that required to initiate foldback. This load "hysteresis" is a normal aspect
of any type of current limit foldback associated with voltage regulators.
The safe operating area, when in short circuit mode, is shown in Figure 12. Operating points below and to the
right of the curve represent safe operation.
Figure 12. SOA
Current Limit Adjustment
A key feature of the LM22679 is the ability to adjust the peak switch current limit. This can be useful when the full
current capability of the regulator is not required for a given application. A smaller current limit may allow the use
of power components with lower current ratings, thus saving space and reducing cost. A single resistor between
the IADJ pin and ground controls the current limit in accordance with Figure 13. The current limit mode is set
during start-up of the regulator. When V
IN
is applied, a weak pull-up is connected to the IADJ pin and, after
approximately 100 µs, the voltage on the pin is checked against a threshold of about 0.8V. With the IADJ pin
open, the voltage floats above this threshold, and the current limit is set to the default value of 7.1A (typ). With a
resistor present, an internal reference holds the pin voltage at 0.8V; the resulting current sets the current limit.
The accuracy of the adjusted current limit will be slightly worse than that of the default value; +35%/ -25% is to
be expected. Resistor values should not exceed the limits shown in Figure 13.
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