Datasheet
OUT OUT IN(MIN) RIPPLE(BUCK-BOOST)
2(PEAK)
IN(MIN) TOL
I (V V ) I
I
V 2(1 L )
Ku
RIPPLE(BUCK)
OUT
1(PEAK)
TOL
I
I
I
2(1 L )
K
IN(MIN) OUT
OUT IN(MIN) RIPPLE
V (V )
L1 Buck-Boost Mode
(V V ) f I
u u
OUT IN(MAX) OUT
IN(MAX) RIPPLE
V (V V )
L1 Buck Mode
V f I
u u
LM5118, LM5118-Q1
www.ti.com
SNVS566H –APRIL 2008–REVISED JANUARY 2014
(10)
(11)
where
• V
OUT
is the output voltage
• V
IN(MAX)
is the maximum input voltage
• f is the switching frequency
• I
RIPPLE
is the selected inductor peak to peak ripple current (1.2 A selected for this example)
• V
IN(MIN)
is the minimum input voltage
The resulting inductor values are:
• L1 = 28 µH, Buck Mode
• L1 = 9.8 µH Buck-Boost mode
A 10 µH inductor was selected which is a compromise between these values, while favoring the buck-boost
mode. As will be illustrated in the compensation section below, the inductor value should be as low as possible to
move the buck-boost right-half-plane zero to a higher frequency. The ripple current is then rechecked with the
selected inductor value using Equation 10 and Equation 11,
• I
RIPPLE(BUCK)
= 3.36 A
• I
RIPPLE(BUCK-BOOST)
= 1.17 A
Because the inductor selected is lower than calculated for the Buck mode, the minimum load current for CCM in
buck mode is 1.68 A at maximum VIN.
With a 10 µH inductor, the worst case peak inductor currents can be estimated for each case, assuming a 20%
inductor value tolerance and assuming 80% efficiency of the converter.
(12)
(13)
where
• η is efficiency
• L
TOL
is the inductor tolerance
For this example, Equation 12 and Equation 13 yield:
• I
1(PEAK)
= 5.62 A
• I
2(PEAK)
= 13.4 A
An acceptable current limit setting would be 6.7 A for buck mode since the LM5118 automatically doubles the
current limit threshold in buck-boost mode. The selected inductor must have a saturation current rating at least
as high as the buck-boost mode cycle-by-cycle current limit threshold, in this case at least 13.5 A. A 10 µH, 15 A
inductor was chosen for this application.
Copyright © 2008–2014, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Links: LM5118 LM5118-Q1