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
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
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
LM22677
www.ti.com
SNVS582M –SEPTEMBER 2008–REVISED APRIL 2013
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.
If the frequency synchronization feature is used, the current limit frequency fold-back is not operational, and the
system may not survive a hard short-circuit at the output.
The safe operating areas, when in short circuit mode, are shown in Figure 14 through Figure 16, for different
switching frequencies. Operating points below and to the right of the curve represent safe operation. Note that
these curves are not valid when the LM22677 is in frequency synchronization mode.
Figure 14. SOA at 300 kHz Figure 15. SOA at 500 kHz
Figure 16. SOA at 800 kHz
Soft-Start
The soft-start feature allows the regulator to gradually reach steady-state operation, thus reducing start-up
stresses. The internal soft-start feature brings the output voltage up in about 500 µs. This time is fixed and can
not be changed. Soft-start is reset any time the part is shut down or a thermal overload event occurs.
Switching Frequency Adjustment and Synchronization
The LM22677 will operate in three different modes, depending on the condition of the RT/SYNC pin. With the
RT/SYNC pin floating, the regulator will switch at the internally set frequency of 500 kHz (typ.). With a resistor in
the range of 25 kΩ to 200 kΩ, connected from RT/SYNC to ground, the internal switching frequency can be
adjusted from 1MHz to 200 kHz. Figure 17 shows the typical curve for switching frequency vs. the external
resistance connected to the RT/SYNC pin. The accuracy of the switching frequency, in this mode, is slightly
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