lm3150mhx National Semiconductor Corporation, lm3150mhx Datasheet - Page 14

• Manufacturer Part Number: lm3150mhx
• Description: Simple Switcher Controller, 42v Synchronous Step-down
• Manufacturer: National Semiconductor Corporation

www.national.com
high-side FET. It is charged during the SW off-time. The rec-
ommended value for C
Design Example
1. Define Power Supply Operating Conditions
a. V
b. V
c. Typical Load Current = 12A, Max Load Current = 15A
d. Soft-Start time t
2. Set Output Voltage with Feedback Resistors
R
3. Determine R
t
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 re-
duced and meet the following criteria:
OFF
FB2
OUT
IN-MIN
should meet the following criteria:
= 22.6 kΩ, nearest 1% standard value.
= 3.3V
= 6V, V
t
f
OFF
smax
ON
IN-TYP
D
SS
D
= (1-0.55)/687 kHz = 654 ns
t
and f
OFF
= 0.137/ 200 ns = 687 kHz
min
max
D
D
= 5 ms
R
min
max
BST
= 3.3V/24V = 0.137
FB2
> t
= 12V, V
t
= 3.3V / 6V = 0.55
OFF
S
= V
OFF-MIN
= V
is 0.47 µF. The EN bypass capac-
= 22.455 kΩ
> 725 ns
OUT
OUT
IN-MAX
/V
/V
+ 200 ns
IN-MAX
IN-MIN
= 24V
FIGURE 4. Design Example Schematic
14
itor, C
pin from open drain type of signal.
A switching frequency is arbitrarily chosen at 500 kHz which
should allow for reasonable size components and satisfies
the requirements above.
f
Using f
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 cal-
culation 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 ap-
plied voltage is only 30% of the rated voltage. The chosen
S
R
= 500 kHz
ON
R
EN
= [(3.3 x 12) - 3.3] / (12 x 100 pC x 500 kHz) - 4.3 kΩ
S
ON
R
, recommended value is 1000 pF when driving the EN
R
= 500 kHz R
OND
OND
= [(V
= - [(V
f
= - [(12 - 1) x (12 x 16.5 + 100)] -1000
S
OUT
< (1 - 0.55)/725 ns = 620 kHz
x V
IN
f
ON
- 1) x (V
s
IN
R
R
< (1 - D)/725 ns
) - V
OND
can be calculated as follows:
ON
= 56.2 kΩ
OUT
= -4.3 kΩ
IN
] / (V
x 16.5 + 100)] - 1000
IN
x K x f
30053161
S
) + R
OND