Datasheet
6
OUT
IN(MIN) OUT
LIMIT(BUCK-BOOST)
50 10 V
2.5
C15 f (V V )
I
10 R13
u u
u u
u
6
OUT
IN(MAX)
LIMIT(BUCK)
50 10 V
1.25
C15 f V
I
10 R13
u u
u u
u
C15 = C
RAMP
=
L x 10
-6
2 x R
SENSE
(BUCK-BOOST)
IN(MIN) OUT RIPPLE(BUCK-BOOST)
OUT
BUCK-BOOST
IN(MIN)
2.5 (1 M)
R13
V V I
I
10 K
V 2
§ ·
¨ ¸
¨ ¸
K
© ¹
(BUCK)
RIPPLE(BUCK)
OUT
BUCK
1.25(1 M)
R13
I
I
10 K
2
§ ·
¨ ¸
K
© ¹
BUCK BOOST
IN(MIN)
10
K 1
V
t
BUCK
IN(MAX) OUT
10
K 1
V V
t
LM5118, LM5118-Q1
SNVS566H –APRIL 2008–REVISED JANUARY 2014
www.ti.com
R13 = R
SENSE
To select the current sense resistor, begin by calculating the minimum K values for each mode using
Equation 14 and Equation 15. K represents the slope compensation of the controller and is different for each
mode, K
BUCK
and K
BUCK-BOOST
.
(14)
(15)
• K
BUCK
= 1.16
• K
BUCK-BOOST
= 3
Use Equation 16 and Equation 17 to calculate RSENSE for each mode of operation. A design margin, M, should
be selected between 10%-30% to allow for component tolerances. For this design M was selected to be 10%.
(16)
(17)
• R13
(BUCK)
= 19.75mΩ
• R13
(BUCK-BOOST)
= 15.5mΩ
An R
SENSE
value of no more than 15.5mΩ must be used to ensure the required maximum output current in the
buck-boost mode. A standard value of 15mΩ was selected for this design.
C15 = C
RAMP
With the inductor value selected, the value of C3 necessary for the emulation ramp circuit is:
(18)
With the inductance value (L1) selected as 10 µH, the calculated value for C
RAMP
is 333 pF. A standard value of
330 pF was selected.
Inductor Current Limit Calculation
The current limit for each mode can be calculated using Equation 19 and Equation 20. If the peak current limit is
less than the calculated inductor peak current the R13 and C15 need to be recalculated. This can be done by
increasing the previous K values or M and reiterating the calculations.
(19)
• I
LIMIT(BUCK)
= 7.795 A
(20)
• I
LIMIT(BUCK-BOOST)
= 14.29 A
22 Submit Documentation Feedback Copyright © 2008–2014, Texas Instruments Incorporated
Product Folder Links: LM5118 LM5118-Q1