Datasheet

LM22671
www.ti.com SNVS589M –SEPTEMBER 2008–REVISED APRIL 2013
Where F
sw
is the switching frequency and T
ON
is the minimum on-time; both found in the Electrical
Characteristics table. If the frequency adjust feature is used, that value should be used for F
sw
. Nominal values
should be used. The worst case is lowest output voltage, and highest switching frequency. If this input voltage is
exceeded, the regulator will skip cycles, effectively lowering the switching frequency. The consequences of this
are higher output voltage ripple and a degradation of the output voltage accuracy.
The second limitation is the maximum duty cycle before the output voltage will "dropout" of regulation. The
following equation can be used to approximate the minimum input voltage before dropout occurs:
(4)
The values of T
OFF
and R
DS(ON)
are found in the Electrical Characteristics table. The worst case here is highest
switching frequency and highest load. In this equation, R
L
is the D.C. inductor resistance. Of course, the lowest
input voltage to the regulator must not be less than 4.5V (typ.).
Current Limit
The LM22671 has current limiting to prevent the switch current from exceeding safe values during an accidental
overload on the output. This peak current limit is found in the Electrical Characteristics table under the heading of
I
CL
. The maximum load current that can be provided, before current limit is reached, is determined from the
following equation:
(5)
Where L is the value of the power inductor.
When the LM22671 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:
(6)
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:
(7)
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 15 through Figure 17, 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 LM22671 is in frequency synchronization mode.
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