LM3405AXMYEVAL National Semiconductor, LM3405AXMYEVAL Datasheet - Page 10

• Manufacturer Part Number: LM3405AXMYEVAL
• Description: BOARD EVAL FOR LM3405 8MSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3405AXMYEVAL

• Current - Output / Channel: 750mA
• Outputs And Type: 1, Non-Isolated
• Features: Dimmable
• Voltage - Input: 5 ~ 18V
• Utilized Ic / Part: LM3405
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Cycle-by-Cycle Current Limit,
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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The catch diode (D1) forward voltage drop and the voltage
drop across the internal NMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
V
The diode forward drop (V
depending on the quality of the diode. The lower V
higher the operating efficiency of the converter.
The inductor value determines the output ripple current (Δi
as defined in Figure 2). Lower inductor values decrease the
size of the inductor, but increases the output ripple current.
An increase in the inductor value will decrease the output rip-
ple current. The ratio of ripple current to LED current is
optimized when it is set between 0.3 and 0.4 at 1A LED cur-
rent. This ratio r is defined as:
One must also ensure that the minimum current limit (1.2A)
is not exceeded, so the peak current in the inductor must be
calculated. The peak current (I
as:
When the designed maximum output current is reduced, the
ratio r can be increased. At a current of 0.2A, r can be made
as high as 0.7. The ripple ratio can be increased at lighter
loads because the net ripple is actually quite low, and if r re-
mains constant the inductor value can be made quite large.
An equation empirically developed for the maximum ripple
ratio at any current below 2A is:
Note that this is just a guideline.
The LM3405A operates at a high frequency allowing the use
of ceramic output capacitors without compromising transient
response. Ceramic capacitors allow higher inductor ripple
without significantly increasing LED current ripple. See the
output capacitor and feed-forward capacitor sections for more
details on LED current ripple.
Now that the ripple current or ripple ratio is determined, the
inductance is calculated by:
where f
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Induc-
tor saturation will result in a sudden reduction in inductance
and prevent the regulator from operating correctly. Because
of the operating frequency of the LM3405A, ferrite based in-
ductors are preferred to minimize core losses. This presents
little restriction since the variety of ferrite based inductors is
huge. Lastly, inductors with lower series resistance (DCR) will
provide better operating efficiency. For recommended induc-
SW
can be approximated by:
SW
is the switching frequency and I
r = 0.387 x I
V
I
SW
LPK
= I
= I
D1
F
) can range from 0.3V to 0.7V
F
LPK
x R
+ Δi
OUT
) in the inductor is calculated
DS(ON)
L
-0.3667
/2
F
is the LED current.
D1
is, the
L
,
10
tor selection, refer to Circuit Examples and Recommended
Inductance Range in Table 1.
*Maximum over full range of V
INPUT CAPACITOR (C1)
An input capacitor is necessary to ensure that V
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage
rating, RMS current rating, and ESL (Equivalent Series In-
ductance). The input voltage rating is specifically stated by
the capacitor manufacturer. Make sure to check any recom-
mended deratings and also verify if there is any significant
change in capacitance at the operating input voltage and the
operating temperature. The input capacitor maximum RMS
input current rating (I
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle D, is closest to 0.5. The
ESL of an input capacitor is usually determined by the effec-
tive cross sectional area of the current path. A large leaded
capacitor will have high ESL and an 0805 ceramic chip ca-
pacitor will have very low ESL. At the operating frequency of
the LM3405A, certain capacitors may have an ESL so large
that the resulting inductive impedance (2
than that required to provide stable operation. It is strongly
recommended to use ceramic capacitors due to their low ESR
and low ESL. A 10µF multilayer ceramic capacitor (MLCC) is
a good choice for most applications. In cases where large ca-
pacitance is required, use surface mount capacitors such as
Tantalum capacitors and place at least a 1µF ceramic capac-
itor close to the V
X7R or X5R dielectrics. Consult capacitor manufacturer
datasheet to see how rated capacitance varies over operating
conditions.
OUTPUT CAPACITOR (C2)
The output capacitor is selected based upon the desired re-
duction in LED current ripple. A 1µF ceramic capacitor results
in very low LED current ripple for most applications. Due to
the high switching frequency, the 1µF capacitor alone (without
feed-forward capacitor C4) can filter more than 90% of the
inductor current ripple for most applications where the sum of
LED dynamic resistance and R1 is larger than 1Ω. Since the
internal compensation is tailored for small output capacitance
with very low ESR, it is strongly recommended to use a ce-
ramic capacitor with capacitance less than 3.3µF.
1.0A
0.6A
0.2A
I
F
TABLE 1. Recommended Inductance Range
Inductance Range and Inductor Current Ripple
Inductance
Inductance
Inductance
Δi
Δi
Δi
L
L
L
IN
/ I
/ I
/ I
F
F
F
pin. For MLCCs it is recommended to use
*
*
*
RMS-IN
IN
and V
) must be greater than:
6.8µH-15µH
10µH-22µH
15µH-27µH
6.8µH
116%
10µH
15µH
51%
58%
OUT
.
10µH
15µH
22µH
36%
39%
79%
π
fL) will be higher
IN
does not
15µH
22µH
27µH
24%
26%
65%