Datasheet
Irmsco = 12 x
12
0.3
Irmsco = I
OUT
x
12
r
EN
SS
HG
SW
L
LM3151/2/3
FB
VCC
PGND
SGND
BST
LG
C
BYP
C
SS
V
IN
VIN
C
OUT
V
OUT
C
BST
C
VCC
V
IN
M2
M1
C
EN
C
IN
LM3151, LM3152, LM3153
SNVS562G –SEPTEMBER 2008–REVISED MARCH 2011
www.ti.com
Design Example
Figure 15. Design Example Schematic
1.Define Power Supply Operating Conditions
a. V
OUT
= 3.3V
b. V
IN-MIN
= 6V, V
IN-TYP
= 12V, V
IN-MAX
= 24V
c. Typical Load Current = 12A, Max Load Current = 15A
d. Soft-Start time t
SS
= 5 ms
2. Determine which IC Controller to Use
The LM3151 and LM3152 allow for the full input voltage range. However, from buck converter basic theory, the
higher switching frequency will allow for a smaller inductor. Therefore, the LM3152-3.3 500 kHz part is chosen so
that a smaller inductor can be used.
3. Determine Inductor Required
a. ET = (24-3.3) x (3.3/24) x (1000/500) = 5.7 V µs
b. From the inductor nomograph a 12A load and 5.7 V µs calculation corresponds to a L44 type of inductor.
c. Using the inductor designator L44 in Table 1 the Coilcraft HA3778-AL 1.65 µH inductor is chosen.
4. Determine Output Capacitance
The voltage rating on the output capacitor should be greater than or equal to the output voltage. As a rule of
thumb most capacitor manufacturers suggests not to exceed 90% of the capacitor rated voltage. In the case of
multilayer ceramics the capacitance will tend to decrease dramatically as the applied voltage is increased
towards the capacitor rated voltage. The capacitance can decrease by as much as 50% when the applied
voltage is only 30% of the rated voltage. The chosen capacitor should also be able to handle the rms current
which is equal to:
(2)
For this design the chosen ripple current ratio, r = 0.3, represents the ratio of inductor peak-to-peak current to
load current Iout. A good starting point for ripple ratio is 0.3 but it is acceptable to choose r between 0.25 to 0.5.
The nomographs in this datasheet all use 0.3 as the ripple current ratio.
(3)
I
rmsco
= 1A
t
ON
= (3.3V/12V) / 500 kHz = 550 ns
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