Datasheet
LM22677-ADJ
+
GND
+
VIN
EN
SW
BOOT
FB
GND
RT/SYNC
VIN 4.5V to 42V
C2
22 PF
C1
6.8 PF
EN
SYNC
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
C7
6.8 PF
100 1k 10k 100k 1M 10M
0
5
10
15
20
25
30
35
40
COMPENSATOR GAIN (dB)
FREQUENCY (Hz)
-ADJ
-5.0
LM22677
SNVS582M –SEPTEMBER 2008–REVISED APRIL 2013
www.ti.com
The Q factor depends on the parasitic resistance of the power stage components and is not typically in the
control of the designer. Of course, loop compensation is only one consideration when selecting power stage
components; see the Application Information section for more details.
Figure 19. Compensator Gain
In general, hand calculations or simulations can only aid in selecting good power stage components. Good
design practice dictates that load and line transient testing should be done to verify the stability of the application.
Also, Bode plot measurements should be made to determine stability margins. Application note AN-1889
SNVA364 shows how to perform a loop transfer function measurement with only an oscilloscope and function
generator.
Application Information
TYPICAL BUCK REGULTOR APPLICATION
Figure 20 shows an example of converting an input voltage range of 5.5V to 42V, to an output of 3.3v at 5 Amp.
See AN-1892 for more information.
Figure 20. Typical Buck Regulator Application
EXTERNAL COMPONENTS
The following guidelines should be used when designing a step-down (buck) converter with the LM22677.
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:
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