LM3405AXMYEVAL National Semiconductor, LM3405AXMYEVAL Datasheet - Page 11

• 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

Given the availability and quality of MLCCs and the expected
output voltage of designs using the LM3405A, there is really
no need to review other capacitor technologies. A benefit of
ceramic capacitors is their ability to bypass high frequency
noise. A certain amount of switching edge noise will couple
through the parasitic capacitances in the inductor to the out-
put. A ceramic capacitor will bypass this noise. In cases where
large capacitance is required, use Electrolytic or Tantalum
capacitors with large ESR, and verify the loop performance
on the bench. Like the input capacitor, multilayer ceramic ca-
pacitors are recommended X7R or X5R. Again, verify actual
capacitance at the desired operating voltage and tempera-
ture.
Check the RMS current rating of the capacitor. The maximum
RMS current rating of the capacitor is:
One may select a 1206 size ceramic capacitor for C2 since
its current rating is typically higher than 1A, more than enough
for the requirement.
FEED-FORWARD CAPACITOR (C4)
The feed-forward capacitor (designated as C4) connected in
parallel with the LED string is required to provide multiple
benefits to the LED driver design. It greatly improves the large
signal transient response and suppresses LED current over-
shoot that may otherwise occur during PWM dimming; it also
helps to shape the rise and fall times of the LED current pulse
during PWM dimming thus reducing EMI emission; it reduces
LED current ripple by bypassing some of inductor ripple from
flowing through the LED. For most applications, a 1µF ce-
ramic capacitor is sufficient. In fact, the combination of a 1µF
feed-forward ceramic capacitor and a 1µF output ceramic ca-
pacitor leads to less than 1% current ripple flowing through
the LED. Lower and higher C4 values can be used, but bench
validation is required to ensure the performance meets the
application requirement.
Figure 12 shows a typical LED current waveform during PWM
dimming without feed-forward capacitor. At the beginning of
each PWM cycle, overshoot can be seen in the LED current.
Adding a 1µF feed-forward capacitor can totally remove the
overshoot as shown in Figure 13.
FIGURE 12. PWM Dimming without Feed-Forward
Capacitor
30015269
11
CATCH DIODE (D1)
The catch diode (D1) conducts during the switch off-time. A
Schottky diode is required for its fast switching time and low
forward voltage drop. The catch diode should be chosen such
that its current rating is greater than:
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To im-
prove efficiency, choose a Schottky diode with a low forward
voltage drop.
BOOST DIODE (D2)
A standard diode such as the 1N4148 type is recommended.
For V
small-signal Schottky diode is recommended for better effi-
ciency. A good choice is the BAT54 small signal diode.
BOOST CAPACITOR (C3)
A 0.01µF ceramic capacitor with a voltage rating of at least
6.3V is sufficient. The X7R and X5R MLCCs provide the best
performance.
POWER LOSS ESTIMATION
The main power loss in LM3405A includes three basic types
of loss in the internal power switch: conduction loss, switching
loss, and gate charge loss. In addition, there is loss associ-
ated with the power required for the internal circuitry of IC.
The conduction loss is calculated as:
If the inductor ripple current is fairly small (for example, less
than 40%) , the conduction loss can be simplified to:
The switching loss occurs during the switch on and off tran-
sition periods, where voltage and current overlap resulting in
power loss. The simplest means to determine this loss is to
empirically measure the rise and fall times (10% to 90%) of
the voltage at the switch pin.
Switching power loss is calculated as follows:
FIGURE 13. PWM Dimming with a 1µF Feed-Forward
BOOST
P
SW
circuits derived from voltages less than 3.3V, a
= 0.5 x V
P
COND
IN
I
D1
= I
x I
Capacitor
= I
F
F
2
x f
F
x R
x (1-D)
SW
DS(ON)
x ( T
RISE
x D
+ T
FALL
30015270
www.national.com
)