LM3450 Datasheet, PDF (25/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
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SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
Figure 35 shows how the LM3450/50A provides a quick way to generate the necessary primary bias supply at
start-up. Since the AC line peak voltage is always higher than the rating of the controller, all designs require an
N-channel MosFET (passFET). The passFET (Q_PS) is connected with its drain attached to the rectified AC. The
gate of Q_PS is connected to the BIAS pin which has a stack of 2 Zener diodes internal to the device. These
diodes are then biased from the rectified AC line through series resistance (R_BS). The source of Q_PS is held at a
V_GS below the Zener voltage and current flows through Q_PS to charge up whatever capacitance is present. If the
capacitance is large enough, the source voltage will remain relatively constant over the line cycle and this
becomes the input bias supply at V_CC.
This bias circuit enables instant turn-on. However, once the circuit is operational it is desirable to bootstrap V_CC
to an auxiliary winding of the inductor or transformer (also used for ZCD). The two bias paths are each
connected to V_CC through a diode to ensure the higher of the two is providing V_CC current. This bootstrapping
greatly improves efficiency when quick start-up is necessary.
To ensure that the auxiliary winding is powering V_CC at all times except start-up, the LM3450/50A has a dual
BIAS mode. The BIAS voltage at startup is 20V through two Zener diodes. When the V_CC UVLO rising threshold
is exceeded and the device turns on, the BIAS pin voltage is reduced to 14V (bottom 6V Zener is shorted). Once
the V_CC UVLO falling threshold is reached again, the BIAS pin will return to 20V to attempt to restart the device.
It should be noted that the large hysteresis of V_CC UVLO and the dual BIAS mode allow for a large variation of
the auxiliary bias circuitry easing the design of the magnetics.
SOFTSTART
As in any off-line system, softstart is an important part of the design. Since the LM3450/50A are used with phase
dimming applications, the typical startup problems are magnified since a phase dimmer is frequently turned on
and off rapidly. This requires a softstart mechanism that quickly resets when the LM3450/50A turns off. Since the
LM3450/50A has two distinct functional parts (PFC and phase decoder), ideally both should be softstarted
simultaneously. This will ensure the most controlled start-up possible.
The circuit in Figure 36 provides this exact functionality. Both V_ADJ and COMP are diode or'ed into an RC
charging circuit fed from V_CC. The reset mechanism is accomplished using an 18V Zener from BIAS, a current
limiting resistor and an NPN transistor. The reset is activated when VCC uvlo falling is triggered and BIAS
transitions to 20V. This discharges the RC to zero and as soon as V_CC uvlo rising is passed BIAS transitions to
14V again, releasing the clamp on the RC softstart circuit. The RC will charge up to the 3.9V Zener clamp (which
is above the dynamic range of COMP and V_ADJ and become effectively out of the circuit until the next turn-off.
LM3450/50A
BIAS COMP ^VADJ ^VCC
18V
500k
3.9V
47uF
20k
Figure 36. Dual Softstart Circuit
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