Datasheet

LM22679-ADJ
+
GND
+
VIN
SS
SW
BOOT
FB
GND
IADJ
VIN 4.5V to 42V
C2
22 PF
C1
6.8 PF
R3
C3
10 nF
L1
4.7 PH
C4
180 PF
GND
R
FBT
1.54 k:
R
FBB
976:
D1
60V, 5A
VOUT 3.3V
C6
1 PF
C7
6.8 PF
LM22679/LM22679Q
SNVS581K –FEBRUARY 2013–REVISED NOVEMBER 2012 www.ti.com
APPLICATION INFORMATION
TYPICAL BUCK REGULATOR APPLICATION
Figure 15 shows an example of converting an input voltage range of 5.5V to 42V, to an output of 3.3v at 5A. See
AN-1891 for more information.
Figure 15. Typical Buck Regulator Application
EXTERNAL COMPONENTS
The following guidelines should be used when designing a step-down (buck) converter with the LM22679.
INDUCTOR
The inductor value is determined based on the load current, ripple current, and the minimum and maximum input
voltages. To keep the application in continuous conduction mode (CCM), the maximum ripple current, I
RIPPLE
,
should be less than twice the minimum load current.
The general rule of keeping the inductor current peak-to-peak ripple around 30% of the nominal output current is
a good compromise between excessive output voltage ripple and excessive component size and cost. Using this
value of ripple current, the value of inductor, L, is calculated using the following formula:
(9)
where F
sw
is the switching frequency and V
in
should be taken at its maximum value, for the given application.
The above formula provides a guide to select the value of the inductor L; the nearest standard value will then be
used in the circuit.
Once the inductor is selected, the actual ripple current can be found from the equation shown below:
(10)
Increasing the inductance will generally slow down the transient response but reduce the output voltage ripple.
Reducing the inductance will generally improve the transient response but increase the output voltage ripple.
The inductor must be rated for the peak current, I
PK
, in a given application, to prevent saturation. During normal
loading conditions, the peak current is equal to the load current plus 1/2 of the inductor ripple current.
During an overload condition, as well as during certain load transients, the controller may trip current limit. In this
case the peak inductor current is given by I
CL
, found in the Electrical Characteristics table. Good design practice
requires that the inductor rating be adequate for this overload condition. If the inductor is not rated for the
maximum expected current, it can saturate resulting in damage to the LM22679 and/or the power diode.
This consideration highlights the value of the current limit adjust feature of the LM22679.
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