lm3150mhx National Semiconductor Corporation, lm3150mhx Datasheet - Page 16

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

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Pdl is also less than the Pdmax specified on the RJK0305DPB
MOSFET datasheet.
However, it is not always necessary to use the same MOS-
FET for both the high-side and low-side. For most applications
it is necessary to choose the high-side MOSFET with the low-
est gate charge and the low-side MOSFET is chosen for the
lowest allowed R
vs Q
The current limit resistor, R
R
ature of 100°C. By choosing to go into current limit when the
average output load current is 20% higher than the output
load current of 12A while the inductor ripple current ratio is
1/3 of the load current will make I
calculation of R
Let R
8. Calculate Input Capacitance
The input capacitor should be chosen so that the voltage rat-
ing is greater than the maximum input voltage which for this
example is 24V. Similar to the output capacitor, the voltage
rating needed will depend on the type of capacitor chosen.
The input capacitor should also be able to handle the input
rms current, which is a maximum of approximately 0.5 x
I
0.5 x 12A = 6A.
The minimum capacitance with a maximum 5% input ripple
ΔV
To handle the large input rms current 2 ceramic capacitors
are chosen at 10 µF each with a voltage rating of 50V and
case size of 1210. Each ceramic capacitor is capable of han-
dling 3A of rms current. A aluminum electrolytic of 5 times the
combined input capacitance, 5 x 20 µF = 100 µF, is chosen
to provide input voltage filter damping because of the low ESR
ceramic input capacitors.
C
imum V
9. Calculate Soft-Start Capacitor
The soft start-time should be greater than the input voltage
rise time and also satisfy the following equality to maintain a
smooth transition of the output voltage to the programmed
regulation voltage during startup. The desired soft-start time,
tss, of 5 ms also needs to satisfy the equality in equation 12,
by using the chosen component values through the previous
steps as shown below:
Since the desired soft-start time satisfies the equality in equa-
tion 12, the soft start capacitor is calculated as:
Let C
This should be a ceramic cap with a voltage rating greater
than 10V.
OUT
Pdl = Iout
DS(ON)
BYP
IN-MAX
C
. For this example the rms input current is approximately
g
IN
SS
LIM
= 0.1µF ceramic with a voltage rating greater than max-
curve must be less than VCC - 750 mV.
= [12 x 0.275 x (1-0.275)] / [500 kHz x 0.6] = 8 µF
IN
of the low-side FET at the maximum junction temper-
= 0.068 µF, which is the next closest standard value.
= 1.91 kΩ which is the next standard value.
R
5 ms > (3.3V x 300 µF) / (1.2 x 12A - 12A)
= (0.05 x 12) = 0.6V:
C
2
LIM
SS
x R
= (10.4 x 0.014) / (75 x 10
= (7.7 µA x 5 ms) / 0.6V = 0.064 µF
DS(ON)
LIM
DS(ON)
is:
5 ms > 0.412 ms
x (1-D) = 122 x 0.01 x (1-0.275) = 1W
. The plateau voltage of the FET V
LIM
, is calculated by estimating the
CL
= 10.4A. Then the following
-6
) = 1.9 kΩ
GS
16
10. C
C
C
C
VCC
EN
BST
= 1000 pF ceramic with a voltage rating greater than 10V
VCC
= 0.47 µF ceramic with a voltage rating greater than 10V
= 1 µF ceramic with a voltage rating greater than 10V
, C
EN
, and C
BST