Datasheet

FB
Vout
R
FBB
R
FBT
LM22672
SNVS588L –SEPTEMBER 2008–REVISED APRIL 2013 www.ti.com
(16)
A good value for R
FBB
is 1k Ω. This will help to provide some of the minimum load current requirement and
reduce susceptibility to noise pick-up. The top of R
FBT
should be connected directly to the output capacitor or to
the load for remote sensing. If the divider is connected to the load, a local high-frequency bypass should be
provided at that location.
For output voltages of 5V, the -5.0 option should be used. In this case no divider is needed and the FB pin is
connected to the output. The approximate values of the internal voltage divider are as follows: 7.38k from the FB
pin to the input of the error amplifier and 2.55k from there to ground.
Both the -ADJ and -5.0 options can be used for output voltages greater than 5V, by using the correct output
divider. As mentioned in the Internal Loop Compensation section, the -5.0 option is optimized for output voltages
of 5V. However, for output voltages greater than 5V, this option may provide better loop bandwidth than the -ADJ
option, in some applications. If the -5.0 option is to be used at output voltages greater than 5V, the following
equation should be used to determine the resistor values in the output divider:
(17)
Again a value of R
FBB
of about 1k Ω is a good first choice.
Figure 22. Output Voltage Divider
A maximum value of 10 kΩ is recommended for the sum of R
FBB
and R
FBT
to maintain good output voltage
accuracy for the -ADJ option. A maximum of 2 kΩ is recommended for the -5.0 option. For the -5.0 option, the
total internal divider resistance is typically 9.93 kΩ.
In all cases the output voltage divider should be placed as close as possible to the FB pin of the LM22672; since
this is a high impedance input and is susceptible to noise pick-up.
Power Diode
A Schottky type power diode is required for all LM22672 applications. Ultra-fast diodes are not recommended
and may result in damage to the IC due to reverse recovery current transients. The near ideal reverse recovery
characteristics and low forward voltage drop of Schottky diodes are particularly important for high input voltage
and low output voltage applications common to the LM22672. The reverse breakdown rating of the diode should
be selected for the maximum V
IN
, plus some safety margin. A good rule of thumb is to select a diode with a
reverse voltage rating of 1.3 times the maximum input voltage.
Select a diode with an average current rating at least equal to the maximum load current that will be seen in the
application.
Circuit Board Layout
Board layout is critical for the proper operation of switching power supplies. First, the ground plane area must be
sufficient for thermal dissipation purposes. Second, appropriate guidelines must be followed to reduce the effects
of switching noise. Switch mode converters are very fast switching devices. In such cases, the rapid increase of
input current combined with the parasitic trace inductance generates unwanted L di/dt noise spikes. The
magnitude of this noise tends to increase as the output current increases. This noise may turn into
electromagnetic interference (EMI) and can also cause problems in device performance. Therefore, care must be
taken in layout to minimize the effect of this switching noise.
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