Datasheet

LM22672-ADJ
+
GND
+
VIN
EN
SW
BOOT
FB
GND
RT/SYNC
VIN 4.5V to 35V
C2
22 PF
C1
2.2 PF
EN
SYNC
R3
C3
10 nF
L1
22 PH
C4
120 PF
GND
R
FBT
1.54 k:
R
FBB
976:
D1
60V, 1A
VOUT 3.3V
C6
1 PF
SS
LM22672
SNVS588L –SEPTEMBER 2008–REVISED APRIL 2013 www.ti.com
Figure 21. Typical Buck Regulator Application
EXTERNAL COMPONENTS
The following guidelines should be used when designing a step-down (buck) converter with the LM22672.
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:
where
• F
sw
is the switching frequency and V
in
should be taken at its maximum value, for the given application. (11)
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:
(12)
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 LM22672 and/or the power diode.
Input Capacitor
The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the regulator current
during switch on-time. Low ESR ceramic capacitors are preferred. Larger values of input capacitance are
desirable to reduce voltage ripple and noise on the input supply. This noise may find its way into other circuitry,
sharing the same input supply, unless adequate bypassing is provided. A very approximate formula for
determining the input voltage ripple is shown below:
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