V

FLT 2

V

RAMP

V

SW

V

GATE

time

1.2V

1V

T

T

EN

HOLD

LM3447

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

www.ti.com

Valley switching is implemented by connecting the transformer auxiliary winding to the AUX input of LM3447

monitoring the current sourced out of the AUX pin when the voltage at the auxiliary winding of the transformer is

negative with respect to the GND node. The voltage at this node is clamped at approximately 100mV by the

internal circuitry to protect the device during negative voltage excursions of the auxiliary winding. The waveforms

in Figure 24(b) illustrate the sequence of events that have to occur for the LM3447 to initiate a new switching

cycle.

An internal 14.5

set by the internal ramp rise time, is used to set the maximum frequency. After this timer expires the switching

catch timer starts immediately after the Ramp signal drops and sets the lowest operating frequency.

voltage and varies over the half line cycle. As a result, the AUX circuit shows increased sensitivity where the

valley detect signal, VAL, overlaps the Ramp period. Here, the switching point is observed to randomly jump

between two adjacent valleys, causing a discrete change in the switching period. Such perturbations in switching

frequency cause the switching ripple component of the input current to increase and interfere with phase

dimming performance. Therefore, valley switching is disabled and hard switching operation with fixed Ramp

period is initiated on detection of external phase dimmers, as shown in Figure 25. The valley switching operation

is controlled by the FLT2 input. The operation is disabled when the VFLT2 falls below 1V and is enabled again

when it rises above 1.2V. A 200mV hysteresis is provided for noise immunity.

A second function of AUX pin is to program the output overvoltage protection or open-LED detection feature. The

output voltage is monitored by sampling the voltage at the auxiliary winding. The voltage is sampled after a fixed

delay of 1.84

μs, from the falling edge of the GATE drive signal. The leading edge blanking circuit helps reject the

voltage transients caused by the leakage energy of the transformer thus preventing false tripping of OVP. The

an overvoltage fault, the switch is turned off for 812ms before attempting to restart the circuit. During this fault

cleared, the 812ms fault timer is disengaged and the control loop is activated to resume normal operation.

Figure 25. Waveforms Illustrating Valley Switching Enable and Disable Sequence

maximum source current from the AUX pin to 200

μA and is based on the maximum input voltage and the

transformer primary to auxiliary turns-ratio;

16

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