LM3450 Datasheet, PDF (15/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

www.ti.com
V_SW
V_ZCD
VRING
VR
VIN
LM3450
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
t
1.3V
t
Figure 22. ZCD Waveforms for Flyback Design
ZERO CURRENT DETECTION
ZCD is implemented with a 100kΩ resistor from the ZCD pin to a coupled winding on the transformer or inductor
as shown in Figure 21. This winding is also used to bootstrap V_CC after start-up. When Q_SW turns off, the voltage
at the ZCD pin (V_ZCD) increases as energy is transferred through the auxiliary winding. The circuit arms when
V_ZCD exceeds 1.5V. Then, when the energy is fully transferred, V_ZCD decreases towards zero. When V_ZCD falls
below 1.3V, the transformer is assumed to be demagnetized, the circuit disarms, and Q_SW is turned back on as
shown in Figure 22. The ZCD pin voltage will remain low until Q_SW is turned off via peak detection and the cycle
repeats.
SWITCHING FREQUENCY
With a constant on-time and variable off-time, there is a variable switching frequency:
f_SW = D(t) x _{¨¨©§LVx}^IN_I^-PP-KPK_{¸¸¹·}
(6)
Figure 20 shows that the minimum switching frequency occurs at the peak of the rectified AC waveform, while
the maximum switching frequency occurs at the valley.
ERROR AMPLIFIER
The LM3450/50A internal error amplifier is used for non-isolated designs (boost) where the output voltage can be
directly sensed, via a resistor divider, at the FB pin. The FB pin is the inverting input of the trans-conductance
amplifier which is regulated to 2.5V. The COMP pin is the output of the amplifier and external compensation is
placed from COMP to GND in the form of a single capacitor (C_CMP) as shown in Figure 21, a series resistor and
capacitor, or both. The output of the amplifier sources or sinks current as necessary to force the inputs of the
amplifier to be equal. The compensation method depends upon the transient performance desired and requires a
loop gain analysis. This analysis can be somewhat complex and cumbersome. A detailed analysis can be found
Application Notes AN-2098 (literature number SNVA463) and/or AN-2150 (literature number SNVA485).
Copyright © 2010–2013, Texas Instruments Incorporated
Product Folder Links: LM3450
Submit Documentation Feedback
15