LM3406HVMHX/NOPB National Semiconductor, LM3406HVMHX/NOPB Datasheet - Page 23

• Manufacturer Part Number: LM3406HVMHX/NOPB
• Description: IC LED DRVR HP CONS CURR 14TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3406HVMHX/NOPB

• Constant Current: Yes
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Automotive
• Type - Secondary: White LED
• Frequency: 1MHz
• Voltage - Supply: 6 V ~ 75 V
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP Exposed Pad, 14-eTSSOP 14-HTSSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 1.5A
• Internal Switch(s): Yes
• Efficiency: 96%
• For Use With: 876-1003 - LM3406 LED DRIVER EVAL BOARDLM3406MHEVAL - BOARD EVAL FOR LM3406MH
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3406HVMHX

www.national.com
The most filtering (and therefore the highest output capaci-
tance) is needed when ΔI
MAX
required impedance of C
A ceramic capacitor will be used and the required capacitance
is selected based on the impedance at 440 kHz:
This calculation assumes that C
and therefore impedance due to the equivalent series resis-
tance (ESR) and equivalent series inductance (ESL) of of the
device is negligible. The closest 10% tolerance capacitor val-
ue is 1.5 µF. The capacitor used should have an X7R dielec-
tric and should be rated to 50V. The high voltage rating
ensures that C
circuit and a load dump occurs. Several manufacturers pro-
duce ceramic capacitors with these specifications in the 1206
case size. With only 4V of DC bias a 50V rated ceramic ca-
pacitor will have better than 90% of it's rated capacitance,
which is more than enough for this design.
R
Using the expression for R
Sub-1Ω resistors are available in both 1% and 5% tolerance.
A 1%, 0.13Ω device is the closest value, and a 0.33W, 1210
size device will handle the power dissipation of 290 mW. With
the resistance selected, the average value of LED current is
re-calculated to ensure that current is within the ±5% toler-
ance requirement. From the expression for average LED
current:
INPUT CAPACITOR
Controlling input ripple current and voltage is critical in auto-
motive applications where stringent conducted electromag-
netic interference tests are required. Δv
to 300 mV
calculated for the largest t
minimum input voltage. Using the equations from the Input
Capacitors section:
As with the output capacitor, this required value is low enough
to use a ceramic capacitor, and again the effective capaci-
tance will be lower than the rated value with 16V across C
Reviewing plots of %C vs. DC Bias for several capacitors re-
veals that a 3.3 µF, 1210-size capacitor in X7R rated to 50V
loses about 22% of its rated capacitance at 16V, hence two
such caps are needed.
SNS
. Inductor ripple current with one LED is 516 mA
I
F
= 0.2 / 0.13 = 1.54A, 3% above the target current
C
C
O
P-P
Z
IN(MIN)
C
= 1/(2 x
r
D
= [0.3 / (0.516 - 0.3] x 0.35 = 0.35Ω
or less. The minimum required capacitance is
O
= ΔV
= (1.5 x 1.09 x 10
will not be damaged if the LED fails open
R
SNS
π
F
/ ΔI
x 0.49 x 4.4 x 10
R
= 0.2 / 1.5 = 0.133Ω
SNS
O
F
L
is calculated:
SNS
ON
= 0.5V / 2A = 0.25Ω
is highest, which occurs at V
= 0.2 / I
, 1090 ns, which occurs at the
:
O
will be a ceramic capacitor,
-6
F
) / 0.3 = 5.5 µF
5
IN(MAX)
) = 1.03 µF
will be limited
P-P
. The
IN-
IN
.
22
Input rms current is high in buck regulators, and the worst-
case is when the duty cycle is 50%. Duty cycle in a buck
regulator can be estimated as D = V
drops to 9V the duty cycle will be nearly 50%.
Ripple current ratings for 1210 size ceramic capacitors are
typically higher than 2A, so two of them in parallel can tolerate
more than enough for this design.
RECIRCULATING DIODE
To survive an input voltage transient of 40V the Schottky
diode must be rated to a higher voltage. The next highest
standard voltage rating is 60V. Selecting a 60V rated diode
provides a large safety margin for the ringing of the switch
node and also makes cross-referencing of diodes from differ-
ent vendors easier.
The next parameters to be determined are the forward current
rating and case size. The lower the duty cycle the more ther-
mal stress is placed on the recirculating diode. When driving
one LED the duty cycle can be estimated as:
The estimated average diode current is then:
A 2A-rated diode will be used. To determine the proper case
size, the dissipation and temperature rise in D1 can be cal-
culated as shown in the Design Considerations section. V
for a case size such as SMB in a 60V, 2A Schottky diode at
1.5A is approximately 0.4V and the θ
sipation and temperature rise can be calculated as:
C
The bootstrap capacitor C
capacitors with X7R dielectric. A 25V rating is appropriate for
all application circuits. The COMP pin capacitor C
linear regulator filter capacitor C
ceramic capacitors, also with X7R dielectric and a 25V rat-
ings.
EFFICIENCY
To estimate the electrical efficiency of this example the power
dissipation in each current carrying element can be calculated
and summed. One calculation will be detailed for the nominal
input voltage of 13.8V, and these calculations can be repeat-
ed for other numbers of LEDs.
Total output power, P
Conduction loss, P
B
P
, C
C
C
= (I
AND C
F
2
I
IN-RMS
x R
P
F
O
DSON
I
= I
= 1.5 x Sqrt(0.5 x 0.5) = 750 mA
D
P
T
C
D
RISE
= (1 - 0.3) x 1.54 = 1.1A
F
, in the internal MOSFET:
) x D = (1.54
D = 4.1 / 13.8 = 0.3
= 1.1 x 0.4 = 440 mW
x V
O
, is calculated as:
= 0.44 x 75 = 33°C
O
B
= 1.54 x 4.1 = 6.3W
should always be a 22 nF ceramic
2
F
x 0.75) x 0.3 = 530 mW
should always be 100 nF
JA
O
is 75°C/W. Power dis-
/ V
IN
, and when V
C
and the
IN
D