Datasheet

Irmsco = 12 x
12
0.3
Irmsco = I
OUT
x
12
r
LM3150
www.ti.com SNVS561D –SEPTEMBER 2008–REVISED MARCH 2011
t
OFF
= (1-0.55)/687 kHz = 654 ns
t
OFF
should meet the following criteria:
t
OFF
> t
OFF-MIN
+ 200 ns
t
OFF
> 725 ns
At the maximum switching frequency of 687 kHz, which is limited by the minimum on-time, the off-time of 654 ns
is less than 725 ns. Therefore the switching frequency should be reduced and meet the following criteria:
f
s
< (1 - D)/725 ns
f
S
< (1 - 0.55)/725 ns = 620 kHz
A switching frequency is arbitrarily chosen at 500 kHz which should allow for reasonable size components and
satisfies the requirements above.
f
S
= 500 kHz
Using f
S
= 500 kHz R
ON
can be calculated as follows:
R
ON
= [(V
OUT
x V
IN
) - V
OUT
] / (V
IN
x K x f
S
) + R
OND
R
OND
= - [(V
IN
- 1) x (V
IN
x 16.5 + 100)] - 1000
R
OND
= - [(12 - 1) x (12 x 16.5 + 100)] -1000
R
OND
= -4.3 kΩ
R
ON
= [(3.3 x 12) - 3.3] / (12 x 100 pC x 500 kHz) - 4.3 kΩ
R
ON
= 56.2 kΩ
Next, check the desired minimum input voltage for R
ON
using Figure 13. This design will meet the desired
minimum input voltage of 6V.
4. 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.
5. 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:
For this design the chosen ripple current ratio, r = 0.3, represents the ratio of inductor peak-to-peak current to
load current I
OUT
. 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.
I
rmsco
= 1A
t
ON
= (3.3V/12V)/500 kHz = 550 ns
Minimum output capacitance is:
C
Omin
= 70 / (f
s
2
x L)
C
Omin
= 70 / (500 kHz
2
x 1.65 µH) = 169 µF
The maximum ESR allowed to prevent over-voltage protection during normal operation is:
ESR
max
= (80 mV x L x A
f
) / ET
A
f
= V
OUT
/ 0.6 without a feed-forward capacitor
A
f
= 1 with a feed-forward capacitor
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