LM3406HVEVAL National Semiconductor, LM3406HVEVAL Datasheet - Page 18

• Manufacturer Part Number: LM3406HVEVAL
• Description: LM3406/06HV 1.5A Constant Current Buck Regulator for Driving High Power LEDs; Qty per Container: 1/
• Manufacturer: National Semiconductor

www.national.com
R
A moderate switching frequency of 500 kHz will balance the
requirements of inductor size and overall power efficiency.
The LM3406 will allow some shift in switching frequency when
V
culation for R
series, where V
is done with the high accuracy expression listed in the Ap-
pendix.
The closest 1% tolerance resistor is 143 kΩ. The switching
frequency and on-time of the circuit should be checked for
one, three and five LEDs using the equations relating R
t
t
pressions listed in the Appendix.
OUTPUT INDUCTOR
Since an output capacitor will be used to filter some of the AC
ripple current, the inductor ripple current can be set higher
than the LED ripple current. A value of 40%
many buck converters:
With the target ripple current determined the inductance can
be chosen:
The closest standard inductor value is 22 µH. The average
current rating should be greater than 1.5A to prevent over-
ON
ON
ON
O
changes due to the number of LEDs in series, so the cal-
values have been calculated using the high accuracy ex-
to f
and t
L
MIN
SW
. As with the R
ON
= [(24 – 11.8) x 1.01 x 10
ON
O
= 11.8V. Note that the actual R
is done at the mid-point of three LEDs in
Δi
f
f
t
t
L
f
SW(3 LEDs)
SW(5 LEDs)
ON(3 LEDs)
ON(5 LEDs)
SW(1 LED)
t
ON(1 LED)
= 0.4 x 1.5 = 0.6A
R
ON
ON
= 144 kΩ
calculation, the actual f
= 362 kHz
= 504 kHz
= 555 kHz
= 1014 ns
= 1512 ns
= 528 ns
-6
] / (0.6) = 20.5 µH
P-P
P-P
ON
is typical in
calculation
SW
ON
and
and
18
heating in the inductor. Inductor current ripple should be
calculated for one, three and five LEDs:
The peak LED/inductor current is then estimated. This calcu-
lation uses the worst-case ripple current which occurs with
three LEDs.
In order to prevent inductor saturation the inductor’s peak
current rating must be above 1.8A. A 22 µH off-the shelf in-
ductor rated to 2.1A (peak) and 1.9A (average) with a DCR of
59 mΩ will be used.
USING AN OUTPUT CAPACITOR
This application does not require high frequency PWM dim-
ming, allowing the use of an output capacitor to reduce the
size and cost of the output inductor while still meeting the 10%
P-P
capacitor the equation from Buck Regulators with Output Ca-
pacitors is re-arranged to yield the following:
The dynamic resistance, r
taking the tangent line to the V
datasheet.
target for LED ripple current. To select the proper output
Δi
Δi
Δi
L(3 LEDs)
L(5 LEDs)
L(1 LED)
Figure 14
I
L(PEAK)
= [(24 - 11.8) x 1.01 x 10
= [(24 - 19.7) x 1.51 x 10
= [(24 - 4.1) x 5.28 x 10
I
L(PEAK)
= 1.5 + 0.5 x 0.56 = 1.78A
shows an example r
= 478 mA
= 560 mA
= 295 mA
= I
D
L
,of one LED can be calculated by
+ 0.5 x Δi
F
P-P
P-P
P-P
vs. I
L(MAX)
-7
F
-6
-6
] / 22 x 10
D
curve in the LED
] / 22 x 10
] / 22 x 10
calculation.
-6
-6
-6