LM3402HVMR/NOPB National Semiconductor, LM3402HVMR/NOPB Datasheet - Page 21

• Manufacturer Part Number: LM3402HVMR/NOPB
• Description: IC LED DRVR HP CONST CURR 8-PSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3402HVMR/NOPB

• Constant Current: Yes
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Automotive
• Type - Secondary: High Brightness LED (HBLED), White LED
• Frequency: 1MHz
• Voltage - Supply: 6 V ~ 75 V
• Mounting Type: Surface Mount
• Package / Case: 8-PSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 500mA
• Internal Switch(s): Yes
• Efficiency: 96%
• Primary Input Voltage: 75V
• No. Of Outputs: 1
• Output Voltage: 73V
• Output Current: 500mA
• Voltage Regulator Case Style: PSOP
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC (15-Dec-2010)
• Rohs Compliant: Yes
• For Use With: 551600000-001A/NOPB - BOARD WEBENCH SO8/SOP LM3404/2551600003-001A - BOARD WEBENCH MSOP LM3402LM3402HVEVAL - BOARD EVALUATION FOR LM3402HV
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3402HVMR

To select R
the Controlled On-time Overview section can be re-written as:
Minimum on-time occurs at the maximum V
110% = 26.4V. R
The closest 1% tolerance resistor is 59.0 kΩ. The switching
frequency of the circuit can then be found using the equation
relating R
USING AN OUTPUT CAPACITOR
The inductor will be the largest component used in this design.
Because the application does not require any PWM dimming,
an output capacitor can be used to greatly reduce the induc-
tance needed without worry of slowing the potential PWM
dimming frequency. The total solution size will be reduced by
using an output capacitor and small inductor as opposed to
one large inductor.
OUTPUT INDUCTOR
Knowing that an output capacitor will be used, the inductor
can be selected for a larger current ripple. The desired max-
imum value for Δi
Minimum inductance is selected at the maximum input volt-
age. Re-arranging the equation for current ripple selection
yields the following:
The closest standard inductor value is 33 µH. Off-the-shelf
inductors rated at 33 µH are available from many magnetics
manufacturers.
Inductor datasheets should contain three specifications which
are used to select the inductor. The first of these is the aver-
age current rating, which for a buck regulator is equal to the
average load current, or I
by a specified temperature rise in the inductor, normally 40°
C. For this example, the average current rating should be
greater than 350 mA to ensure that heat from the inductor
does not reduce the lifetime of the LED or cause the LM3402
to enter thermal shutdown.
The second specification is the tolerance of the inductance
itself, typically ±10% to ±30% of the rated inductance. In this
example an inductor with a tolerance of ±20% will be used.
With this tolerance the typical, minimum, and maximum in-
ductor current ripples can be calculated:
L
MIN
R
ON
= [(26.4 – 3.7) x 300 x 10
Δi
f
SW
L(TYP)
ON
= (300 x 10
ON
= 3.7 / (59000 x 1.34 x 10
to f
the expression relating t
= [(26.4 – 3.7) x 300 x 10
SW
ON
L
:
is ±30%, or 0.6 x 350 mA = 210 mA
is therefore calculated as:
-9
x 26.4) / 1.34 x 10
= 206 mA
F
. The average current rating is given
-9
] / (0.6 x 0.35) = 32.4 µH
P-P
ON
-10
-9
to input voltage from
) = 468 kHz
-10
] / 33 x 10
IN
= 59105 Ω
, which is 24V x
-6
P-P
.
21
The third specification for an inductor is the peak current rat-
ing, normally given as the point at which the inductance drops
off by a given percentage due to saturation of the core. The
worst-case peak current occurs at maximum input voltage
and at minimum inductance, and can be determined with the
equation from the Design Considerations section:
For this example the peak current rating of the inductor should
be greater than 479 mA. In the case of a short circuit across
the LED array, the LM3402 will continue to deliver rated cur-
rent through the short but will reduce the output voltage to
equal the CS pin voltage of 200 mV. Worst-case peak current
in this condition is equal to:
In the case of a short at the switch node, the output, or from
the CS pin to ground the short circuit current limit will engage
at a typical peak current of 735 mA. In order to prevent in-
ductor saturation during these short circuits the inductor’s
peak current rating must be above 735 mA. The device se-
lected is an off-the-shelf inductor rated 33 µH ±20% with a
DCR of 96 mΩ and a peak current rating of 0.82A. The phys-
ical dimensions of this inductor are 7.0 x 7.0 x 4.5 mm.
R
The current sensing resistor value can be determined by re-
arranging the expression for average LED current from the
LED Current Accuracy section:
Sub-1Ω resistors are available in both 1% and 5% tolerance.
A 1%, 0.75Ω resistor will give the best accuracy of the aver-
age LED current. To determine the resistor size the power
dissipation can be calculated as:
Standard 0805 size resistors are rated to 125 mW and will be
suitable for this application.
SNS
Δi
L(LED-SHORT)
Δi
Δi
L(MAX)
L(MIN)
I
L(PEAK)
I
= [(26.4 – 3.7) x 300 x 10
= [(26.4 – 3.7) x 300 x 10
L(PEAK)
P
R
SNS
= [(26.4 – 0.2) x 300 x 10
SNS
= 0.35 + 0.258 / 2 = 479 mA
P
= 0.35
= 0.35 + 0.149 = 499 mA
= 0.74Ω, t
SNS
= 172 mA
= 258 mA
= 298 mA
= (I
2
x 0.75 = 92 mW
F
)
2
SNS
x R
P-P
P-P
P-P
= 220 ns
SNS
-9
-9
] / 39.6 x 10
] / 26.4 x 10
-9
] / 26.4 x 10
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