18V

3.9V

500k

47uF

20k

COMP

LM3450/50A

BIAS

VADJ VCC

LM3450

www.ti.com

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 (QPS) is connected with its drain attached to the rectified AC. The

gate of QPS 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 (RBS). The source of QPS is held at a

VGS below the Zener voltage and current flows through QPS 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 VCC.

This bias circuit enables instant turn-on. However, once the circuit is operational it is desirable to bootstrap VCC

to an auxiliary winding of the inductor or transformer (also used for ZCD). The two bias paths are each

connected to VCC through a diode to ensure the higher of the two is providing VCC current. This bootstrapping

greatly improves efficiency when quick start-up is necessary.

To ensure that the auxiliary winding is powering VCC 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 VCC 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 VCC 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 VCC 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 VADJ and COMP are diode or'ed into an RC

charging circuit fed from VCC. 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 VCC 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 VADJ and become effectively out of the circuit until the next turn-off.

Figure 36. Dual Softstart Circuit

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