LM3444MM/NOPB National Semiconductor, LM3444MM/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM3444MM/NOPB
• Description: IC LED DRIVER AC-DC 10MSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3444MM/NOPB

• Constant Current: Yes
• Constant Voltage: Yes
• Internal Driver: No
• Type - Primary: General Purpose
• Type - Secondary: High Brightness LED (HBLED)
• Mounting Type: Surface Mount
• Topology: AC DC Offline Switcher, PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Frequency: Adjustable/Selectable
• Voltage - Supply: 8 V ~ 13 V
• Package / Case: 10-TFSOP, 10-MSOP (0.118", 3.00mm Width)
• No. Of Outputs: 1
• Output Current: 100mA
• Operating Temperature Range: -40°C To +125°C
• Driver Case Style: MSOP
• No. Of Pins: 10
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Operating Temperature: -
• Other names: LM3444MM/NOPB; LM3444MM/NOPBTR

Figure 12
the peak voltage (V
The voltage at V
similar to the waveforms of
The purpose of the valley fill circuit is to allow the buck con-
verter to pull power directly off of the AC line when the line
voltage is greater than its peak voltage divided by two (two
stage valley fill circuit). During this time, the capacitors within
the valley fill circuit (C7 and C8) are charged up to the peak
of the AC line voltage. Once the line drops below its peak
divided by two, the two capacitors are placed in parallel and
deliver power to the buck converter. One can now see that if
the peak of the AC line voltage is lowered due to variations in
the line voltage the DC offset (V
est value that voltage V
Example:
Line voltage = 90V
Valley-Fill = two stage
Depending on what type and value of capacitors are used,
some derating should be used for voltage droop when the
capacitors are delivering power to the buck converter. With
this derating, the lowest voltage the buck converter will see is
about 42.5V in this example.
To determine how many LEDs can be driven, take the mini-
mum voltage the buck converter will see (42.5V) and divide it
by the worst case forward voltage drop of a single LED.
Example: 42.5V/3.7V = 11.5 LEDs (11 LEDs with margin)
OUTPUT CAPACITOR
A capacitor placed in parallel with the LED or array of LEDs
can be used to reduce the LED current ripple while keeping
the same average current through both the inductor and the
LED array. With a buck topology the output inductance (L2)
can now be lowered, making the magnetics smaller and less
expensive. With a well designed converter, you can assume
shows the AC waveform. One can easily see that
BUCK
AC
FIGURE 12. AC Line
PEAK
to 135V
with a valley fill stage of two will look
BUCK
) will always be:
Figure
will encounter.
AC
DC
) will lower. V
11.
30127554
DC
is the low-
13
that all of the ripple will be seen by the capacitor, and not the
LEDs. One must ensure that the capacitor you choose can
handle the RMS current of the inductor. Refer to
manufacture’s datasheets to ensure compliance. Usually an
X5R or X7R capacitor between 1 µF and 10 µF of the proper
voltage rating will be sufficient.
SWITCHING MOSFET
The main switching MOSFET should be chosen with efficien-
cy and robustness in mind. The maximum voltage across the
switching MOSFET will equal:
The average current rating should be greater than:
RE-CIRCULATING DIODE
The LM3444 Buck converter requires a re-circulating diode
D10 (see the Typical Application circuit
inductor current during the MOSFET Q2 off-time. The most
efficient choice for D10 is a diode with a low forward drop and
near-zero reverse recovery time that can withstand a reverse
voltage of the maximum voltage seen at V
110V
190V.
The current rating must be at least:
Or:
Design Example
The following design example illustrates the process of cal-
culating external component values.
Known:
1.
2.
3.
4.
Choose:
1.
2.
3.
4.
5.
Calculate:
1.
2.
Input voltage range (90V
Number of LEDs in series = 7
Forward voltage drop of a single LED = 3.6V
LED stack voltage = (7 x 3.6V) = 25.2V
Nominal switching frequency, f
I
Δi (usually 15% - 30% of
120 mA
Valley fill stages (1,2, or 3) = 2
Assumed minimum efficiency = 80%
Calculate minimum voltage V
Calculate maximum voltage V
AC
LED(AVE)
± 20% line, the reverse voltage could be as high as
= 400 mA
I
I
DS-MAX
D
= 1 - (D
= I
LED(-AVE)
MIN
AC
ILED(AVE)
) x I
– 135V
BUCK
BUCK
LED(AVE)
SW-TARGET
(D
) = (0.30 x 400 mA) =
MAX
equals:
Figure
AC
equals:
BUCK
)
)
. For a common
= 250 kHz
2) to carry the
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