AN2067 Datasheet, PDF (9/25 Pages) STMicroelectronics – LED manufacturers propose white LEDs inside a monolithic chip

AN2067
White LED power supply
2.2.3
2.2.4
The allowed maximum frequency is limited by the dynamic behavior of the PWM current
generator (refer to Section 2.2.4). The best way to use this power supply, is to set the lowest
frequency convenient for human eye versus flicker. The higher the period, the higher the
dimming range.
Audible noise
When the power supply with a PWM signal is used, some audible noise may be heard,
especially if the frequency of the external signal is inside the audible range.
This noise is emitted by the core of the transformer, and is a normal way to work. This noise
is proportional to the output power transferred through it.
This noise can be reduced by the optimization of the transformer.
Dynamic behavior of the current regulation loop
As explained above, the power supply has to generate a PWM current, at a sufficient
frequency in order to avoid flicker, with the possibility to adjust the duty cycle. The time
response of the regulation loop should be short so to be linear (from 0% up to 100%
characteristics of the dimming range).
Thanks to LED threshold, its current can be pulsed and the voltage can be maintained to
avoid charging and discharging the output capacitance. Since the output filtering
capacitance is quite wide, this helps the dynamic behavior of the current generator.
The dynamic behavior of this current generator is limited by several root causes in the whole
schematic.
The first origin of limitation is the RC couple R101 + R107 / C113. When the reference level
changes on the error amplifier (TSM101 pin 5), the op-amp moves to the new operating
point, by charging the node TSM101 pin 3. This is limited by the charging of the capacitor
C113 through the resistor R101 + R107.
Secondly, the filtering capacitance C121 must be large enough to properly sustain the
supply voltage of the op-amp and the resistor R102. During a transient, the op-amp has to
compensate its output level for the loop stabilization. The dynamic behavior is limited by the
discharging/charging of the capacitor C113 through the resistor R102, once the supply
voltage of the amplifier is stable during that transient.
Third, the serial resistance of the load, the shunt resistance R100, and the serial resistor of
the capacitor C112 limit the time response of the current generator. When the output current
switches from off (low current level) to on (high current level), the output voltage has to
increase as per below formula:
Δv_out = ΔI_out ⋅ (R_sload + R_shunt)
The output current capability is limited until the charging of the capacitance C112 is not
completed. When this capacitor is charged, then the current capability is available for the
load itself.
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