LM3450_1 Datasheet, PDF (10/26 Pages) National Semiconductor (TI) – Evaluation Board Maximum LED Stack Voltage

FIGURE 3. Bias Circuitry
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BIAS SUPPLIES & CAPACITANCES
Bias Supplies
The primary bias supply shown in Figure 3 enables instant
turn-on through Q1 while providing an auxiliary winding for
high efficiency steady state operation. The two bias paths are
each connected to V_CC through a diode (D8, D9) to ensure
the higher of the two is providing V_CC current. The LM3450
BIAS pin helps to ensure that the auxiliary winding is always
providing V_CC during normal operation.
Since there is optical isolation, a secondary bias supply is also
necessary. This is accomplished with another auxiliary wind-
ing, diode (D4), and capacitance (C4) which creates another
flyback output that scales with the regulated output (similar to
the auxiliary primary bias winding). During transient condi-
tions, the regulated output voltage may dip or spike due to the
low bandwidth of the regulation loop. To ensure the sec-
ondary bias moves highly proportional to the regulated fly-
back output, the output winding is tapped to provide the
secondary bias.
It is also advantageous to linear regulate down to approxi-
mately 9V, from the 12V bias supplies, for every opto-isolator
supply rail (V_POP1, V_OP1, V_OP2) . This will stabilize the opto-
isolator rail over the entire operating range, preventing noise
coupling into COMP and the dimming input of the LM3409.
The primary and secondary bias outputs for both versions of
the evaluation board are set to 12V at the nominal input volt-
age. A transformer turns ratio and corresponding turns cal-
culation is used to size the primary auxiliary winding and
determine the tap point for the secondary winding:
Input Capacitance
The input capacitor of the flyback (C1) has to be able to pro-
vide energy during the worst-case switching period at the
peak of the AC input. C1 should be a high frequency, high
stability capacitor (usually a metallized film capacitor, either
polypropylene or polyester) with an AC rating equal to the
maximum input voltage. C1 should also have a DC voltage
rating exceeding the maximum peak input voltage + half of
the peak to peak input voltage ripple specification. The mini-
mum required input capacitance is calculated given the same
ripple specification:
Output Capacitance
Since the LM3450 is a power factor controller, C1 is mini-
mized and the output capacitor (C11) serves as the main
energy storage device. C11 should be a high quality elec-
trolytic capacitor that can tolerate the large current pulses
associated with CRM operation. The voltage rating should be
at least 25% greater than the regulated output voltage and,
given the desired voltage ripple, the minimum output capaci-
tance is calculated:
The minimum primary bias supply capacitance is calculated,
given a minimum V_CC ripple specification, to keep V_CC above
UVLO at the worst-case current:
Definitions
Δv_IN-PK – Peak Input Voltage Switching Ripple
Δv_OUT – Nominal Output Voltage Ripple
Δv_CC – Nominal Primary Bias Ripple
V_CC – Primary Bias Capacitance
n_AUX – Primary to Auxiliary Turns Ratio
N_A – Number of Auxiliary Turns
f_2L – Twice Line Frequency
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