LM3444MMX/NOPB National Semiconductor, LM3444MMX/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM3444MMX/NOPB
• Description: IC LED DRIVER AC-DC 10MSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3444MMX/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)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Operating Temperature: -

Worst case scenario for minimum on time is when V
its maximum voltage (AC high line) and the LED string voltage
(V
The maximum voltage seen by the Buck Converter is:
INDUCTOR SELECTION
The controlled off-time architecture of the LM3444 regulates
the average current through the inductor (L2), and therefore
the LED string current. The input voltage to the buck converter
(V
each half-cycle of the input line voltage. The voltage across
the LED string is relatively constant, and therefore the current
through R4 is constant. This current sets the off-time of the
converter and therefore the output volt-second product
(V
product makes it possible to keep the ripple through the in-
ductor constant as the voltage at V
The equation for an ideal inductor is:
Given a fixed inductor value, L, this equation states that the
change in the inductor current over time is proportional to the
voltage applied across the inductor.
During the on-time, the voltage applied across the inductor is,
Since the voltage across the MOSFET switch (Q2) is rela-
tively small, as is the voltage across sense resistor R3, we
can simplify this to approximately,
FIGURE 9. LM3444 External Components of the Buck
LED
BUCK
LED
) is at its minimum value.
x off-time) remains constant. A constant volt-second
V
) changes with line variations and over the course of
L(ON-TIME)
= V
BUCK
Converter
- (V
LED
+ V
BUCK
DS(Q2)
30127540
varies.
+ I
L2
x R3)
BUCK
is at
11
During the off-time, the voltage seen by the inductor is ap-
proximately:
The value of V
the LED stack voltage will remain constant. If we rewrite the
equation for an inductor inserting what we know about the
circuit during the off-time, we get:
Re-arranging this gives:
From this we can see that the ripple current (Δi) is proportional
to off-time (t
by V
These equations can be rearranged to calculate the desired
value for inductor L2.
Where:
Finally:
Refer to “Design Example” section of the datasheet to better
understand the design process.
SETTING THE LED CURRENT
The LM3444 constant off-time control loop regulates the peak
inductor current (I
average LED current (I
rent is regulated by regulating the peak inductor current.
LED
divided by a constant (L2).
OFF
L(OFF-TIME)
) multiplied by a voltage which is dominated
V
L2
L(ON-TIME)
). The average inductor current equals the
V
L(OFF-TIME)
AVE
will be relatively constant, because
). Therefore the average LED cur-
= V
BUCK
= V
LED
- V
LED
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