LM3405AXMYEVAL National Semiconductor, LM3405AXMYEVAL Datasheet - Page 7

• 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

When the LM3405A starts up, internal circuitry from V
plies a 20mA current to the BOOST pin, flowing out of the
BOOST pin into C3. This current charges C3 to a voltage suf-
ficient to turn the switch on. The BOOST pin will continue to
source current to C3 until the voltage at the feedback pin is
greater than 123mV.
There are various methods to derive V
1.
2.
3.
4.
The first method is shown in the Simplified Block Diagram of
Figure 1. Capacitor C3 is charged via diode D2 by V
a normal switching cycle, when the internal NMOS power
switch is off (T
nus the forward voltage of D2 (V
in the inductor (L1) forward biases the catch diode D1 (V
Therefore the gate drive voltage stored across C3 is:
When the NMOS switch turns on (T
to:
Since the voltage across C3 remains unchanged, V
forced to rise thus reverse biasing D2. The voltage at
V
Depending on the quality of the diodes D1 and D2, the gate
drive voltage in this method can be slightly less or larger than
the input voltage V
variation of the input supply does not cause the gate drive
voltage to fall outside the recommended range:
The second method for deriving the boost voltage is to con-
nect D2 to the output as shown in Figure 3. The gate drive
voltage in this configuration is:
Since the gate drive voltage needs to be in the range of 2.5V
to 5.5V, the output voltage V
range. For the calculation of V
section.
The third method can be used in the applications where both
V
be charged directly from these voltages; instead C3 can be
charged from V
BOOST
IN
and V
From the input voltage (V
From the output voltage (V
From a shunt or series zener diode
From an external distributed voltage rail (V
is then:
V
OUT
BOOST
FIGURE 3. V
are greater than 5.5V. In these cases, C3 cannot
V
OFF
V
BOOST
2.5V < V
IN
BOOST
= 2V
V
) (refer to Figure 2), V
or V
IN
SW
. For best performance, ensure that the
- V
IN
= V
- V
OUT
BOOST
– (R
IN
SW
SW
IN
- V
= V
minus a zener voltage (V
– (R
OUT
DS(ON)
= V
D2
IN
derived from V
OUT
OUT
OUT
)
+ V
should be limited to a certain
D2
IN
DS(ON)
, see OUTPUT VOLTAGE
), during which the current
- V
)
x I
– V
D1
ON
L
D2
) – V
< 5.5V
BOOST
D2
), the switch pin rises
x I
BOOST
+ V
L
+ V
)
D2
D1
:
D1
+ V
equals V
OUT
EXT
D1
)
IN
BOOST
. During
IN
D3
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IN
sup-
) by
D1
mi-
is
).
7
placing a zener diode D3 in series with D2 as shown in Figure
4. When using a series zener diode from the input, the gate
drive voltage is V
An alternate method is to place the zener diode D3 in a shunt
configuration as shown in Figure 5. A small 350mW to
500mW, 5.1V zener in a SOT-23 or SOD package can be
used for this purpose. A small ceramic capacitor such as a
6.3V, 0.1µF capacitor (C5) should be placed in parallel with
the zener diode. When the internal NMOS switch turns on, a
pulse of current is drawn to charge the internal NMOS gate
capacitance. The 0.1µF parallel shunt capacitor ensures that
the V
should be chosen to provide enough RMS current to the zener
diode and to the BOOST pin. A recommended choice for the
zener current (I
BOOST pin supplies the gate current of the NMOS power
switch. It reaches a maximum of around 3.6mA at the highest
gate drive voltage of 5.5V over the LM3405A operating range.
For the worst case I
that case, the maximum boost current will be:
R2 will then be given by:
For example, let V
FIGURE 5. V
FIGURE 4. V
BOOST
R2 = (12V - 5V) / (5.4mA + 1mA) = 1.09kΩ
R2 = (V
voltage is maintained during this time. Resistor R2
BOOST
I
BOOST-MAX
BOOST
ZENER
IN
IN
IN
- V
derived from V
- V
= 12V, V
BOOST
) is 1mA. The current I
derived from V
ZENER
D3
= 1.5 x 3.6mA = 5.4mA
- V
, increase the current by 50%. In
Zener
) / (I
D2
ZENER
+ V
BOOST_MAX
D1
IN
= 5V, I
.
through a Shunt Zener
IN
through a Series
ZENER
+ I
ZENER
BOOST
= 1mA, then:
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)
into the
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