LM3445-208277EV/NOPB National Semiconductor, LM3445-208277EV/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM3445-208277EV/NOPB
• Description: BOARD EVAL LED DRIVER LM3445
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3445-208277EV/NOPB

• Current - Output / Channel: 500mA
• Outputs And Type: 1, Non-Isolated
• Features: Dimmable with Standard TRIAC Dimmer, Power Factor Correction
• Voltage - Input: 176 ~ 305 VAC
• Utilized Ic / Part: LM3445
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3445-208277EV

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Design Customization Procedure
The LM3445 datasheet outlines a typical implementation and
design procedure for a LM3445 based triac-dimmable LED
driver. One feature includes the use of a PNP transistor to
translate the LED string voltage and maintain a constant off
time. This allows a constant LED current to be maintained
regardless of the number of series LEDs. For input voltages
above 150V
find. An alternate implementation uses a current source to
supply a constant current to charge the off-time capacitor.
This circuit has been implemented on this demonstration
board and is described below:
This method eliminates the need for the LED voltage trans-
lation transistor but requires adjustment to the circuit values
when a different LED string voltage is required and it is de-
sired to keep the LED string average current the same. Sam-
ple calculations illustrating the design procedure are shown
below:
SPECIFICATIONS
Number of LEDs, N = 8
V
V
f
V
Δi = 310 mA
Efficiency Estimate, n = 0.75
I
V
V
C18 = 220 pF
1. Inductor Selection
Based on the design parameters above, calculate the target
inductor value:
Select 1mH
SW
ave
LED
IN
FLTR2
Z
BE
=5.6V
= 0.5V
= 230V
target = 80 kHz
= 350mA
= 3.4V * N
= 750 mV
AC
AC
a suitable PNP transistor becomes difficult to
30103512
8
2. Current Source Component Selection
Calculate t
the additional target parameters we defined:
Using the target t
calculated using:
Finally, R27 can be selected using the current, Zener voltage,
and transistor parameters:
3. Average LED Current
The LM3445 constant off-time control loop regulates the peak
inductor and therefore, peak LED current. The average LED
current is related to the peak inductor current by the following
relationship:
Now that our current source and peak current are defined we
can adjust the peak current detect level to obtain the desired
average output current .
On this demo board the current sense resistors are the par-
allel combination of R20 and R21, equal to R
A 10Ω and 1.8Ω are selected as R20 and R21 for a net re-
sistance of 1.52Ω
OFF
to obtain the target inductor ripple current using
OFF
result the current source output can be
sense
: