LM3444MMX/NOPB National Semiconductor, LM3444MMX/NOPB Datasheet - Page 10

• 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: -

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During the off-period denoted by t
continues to flow through the LEDs via D10.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the output switch when the IC junction temperature exceeds
165°C. After thermal shutdown occurs, the output switch
doesn’t turn on until the junction temperature drops to ap-
proximately 145°C.
Design Guide
DETERMINING DUTY-CYCLE (D)
Duty cycle (D) approximately equals:
With efficiency considered:
For simplicity, choose efficiency between 75% and 85%.
CALCULATING OFF-TIME
The “Off-Time” of the LM3444 is set by the user and remains
fairly constant as long as the voltage of the LED stack remains
constant. Calculating the off-time is the first step in determin-
ing the switching frequency of the converter, which is integral
in determining some external component values.
PNP transistor Q3, resistor R4, and the LED string voltage
define a charging current into capacitor C11. A constant cur-
rent into a capacitor creates a linear charging characteristic.
Resistor R4, capacitor C11 and the current through resistor
R4 (i
fixed. Therefore, dv is fixed and linear, and dt (t
be calculated.
Common equations for determining duty cycle and switching
frequency in any buck converter:
Therefore:
COLL
), which is approximately equal to V
OFF
, the current through L2
LED
OFF
/R4, are all
) can now
10
With efficiency of the buck converter in mind:
Substitute equations and rearrange:
Off-time, and switching frequency can now be calculated us-
ing the equations above.
SETTING THE SWITCHING FREQUENCY
Selecting the switching frequency for nominal operating con-
ditions is based on tradeoffs between efficiency (better at low
frequency) and solution size/cost (smaller at high frequency).
The input voltage to the buck converter (V
both line variations and over the course of each half-cycle of
the input line voltage. The voltage across the LED string will,
however, remain constant, and therefore the off-time remains
constant.
The on-time, and therefore the switching frequency, will vary
as the V
practice is to choose a desired nominal switching frequency
knowing that the switching frequency will decrease as the line
voltage drops and increase as the line voltage increases
(Figure
FIGURE 8. Graphical Illustration of Switching Frequency
The off-time of the LM3444 can be programmed for switching
frequencies ranging from 30 kHz to over 1 MHz. A trade-off
between efficiency and solution size must be considered
when designing the LM3444 application.
The maximum switching frequency attainable is limited only
by the minimum on-time requirement (200 ns).
BUCK
8).
voltage changes with line voltage. A good design
vs V
BUCK
BUCK
) changes with
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