LM3150MHE/NOPB National Semiconductor, LM3150MHE/NOPB Datasheet - Page 16

• Manufacturer Part Number: LM3150MHE/NOPB
• Description: IC REG SWITCH CTRL 6-42V 14TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM3150MHE/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: Adj to 0.6V
• Current - Output: 12A
• Frequency - Switching: 1MHz
• Voltage - Input: 6 ~ 42 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP Exposed Pad, 14-eTSSOP 14-HTSSOP
• Package: 14TSSOP EP
• Output Voltage: 0.6(Min) V
• Output Current: 12(Max) A
• For Use With: 551600142-002 - WEBENCH BUILD IT BOARD LM3150
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM3150MHETR

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tion temperature rise above ambient temperature and θ
30°C/W, can be estimated by:
The system calculated Pdh of 0.674W is much less than the
FET Pdmax of 4.1W and therefore the RJK0305DPB max al-
lowable power dissipation criteria is met.
Low-Side MOSFET
Primary loss is conduction loss given by:
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:
OUT
Pdl = Iout
DS(ON)
BYP
IN-MAX
C
. For this example the rms input current is approximately
g
IN
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-
= 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:
2
LIM
x R
Pdmax = 125°C / 30°C/W = 4.1W
= (10.4 x 0.014) / (75 x 10
LIM
DS(ON)
DS(ON)
is:
5 ms > 0.412 ms
x (1-D) = 12
. The plateau voltage of the FET V
LIM
, is calculated by estimating the
CL
= 10.4A. Then the following
2
x 0.01 x (1-0.275) = 1W
-6
) = 1.9 kΩ
JA
GS
=
16
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.
SS
= 0.068 µF, which is the next closest standard value.
C
SS
= (7.7 µA x 5 ms) / 0.6V = 0.064 µF