LM3445_10 Datasheet, PDF (18/26 Pages) National Semiconductor (TI) – Triac Dimmable Offline LED Driver

The off-time of the LM3445 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 LM3445 application.
The maximum switching frequency attainable is limited only
by the minimum on-time requirement (200 ns).
Worst case scenario for minimum on time is when V_BUCK is at
its maximum voltage (AC high line) and the LED string voltage
(V_LED) is at its minimum value.
The maximum voltage seen by the Buck Converter is:
INDUCTOR SELECTION
The controlled off-time architecture of the LM3445 regulates
the average current through the inductor (L2), and therefore
the LED string current. The input voltage to the buck converter
(V_BUCK) changes with line variations and over the course of
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_LED x off-time) remains constant. A constant volt-second
product makes it possible to keep the ripple through the in-
ductor constant as the voltage at V_BUCK varies.
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,
V_L(ON-TIME) = V_BUCK - (V_LED + V_DS(Q2) + I_L2 x R3)
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,
V_L(ON-TIME) = V_BUCK - V_LED
During the off-time, the voltage seen by the inductor is ap-
proximately:
V_L(OFF-TIME) = V_LED
The value of V_L(OFF-TIME) will be relatively constant, because
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_OFF) multiplied by a voltage which is dominated
by V_LED divided by a constant (L2).
These equations can be rearranged to calculate the desired
value for inductor L2.
Where:
30060340
FIGURE 17. LM3445 External Components of the Buck
Converter
The equation for an ideal inductor is:
Finally:
Refer to “Design Example” section of the datasheet to better
understand the design process.
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