Datasheet

LM22677
SNVS582M –SEPTEMBER 2008–REVISED APRIL 2013 www.ti.com
Duty-Cycle Limits
Ideally the regulator would control the duty cycle over the full range of zero to one. However due to inherent
delays in the circuitry, there are limits on both the maximum and minimum duty cycles that can be reliably
controlled. This in turn places limits on the maximum and minimum input and output voltages that can be
converted by the LM22677. A minimum on-time is imposed by the regulator in order to correctly measure the
switch current during a current limit event. A minimum off-time is imposed in order the re-charge the bootstrap
capacitor. The following equation can be used to determine the approximate maximum input voltage for a given
output voltage:
(3)
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 LM22677 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 LM22677 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)
8 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM22677