LM3444MM Datasheet, PDF (12/30 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682C – NOVEMBER 2010 – REVISED MAY 2013
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I_AVE
I_L2 (t)
I_L2-PK
'i_L
I_L2-MIN
t
t_ON
t_OFF
Figure 16. Inductor Current Waveform in CCM
During the off-period denoted by t_OFF, the current through L2 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 approximately 145°C.
Design Guide
DETERMINING DUTY-CYCLE (D)
Duty cycle (D) approximately equals:
V_LED
t_ON
V_BUCK ^{= D =} _{tON + tOFF} ^{= tON x fSW}
(4)
With efficiency considered:
1
K
u
V_LED
V_BUCK
=D
(5)
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 determining 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 current into a capacitor creates a linear charging characteristic.
dv
i = C _dt
(6)
Resistor R4, capacitor C11 and the current through resistor R4 (i_COLL), which is approximately equal to V_LED/R4,
are all fixed. Therefore, dv is fixed and linear, and dt (t_OFF) can now be calculated.
R4
t_OFF = C11 x 1.276V x _VLED
(7)
12
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