Rectified AC

BIAS

AC

LM3450/50A

VCCUV

6V

14V

VCC

AC

RETURN

VCC UVLO

24V

CVCC

QPS

RBS

QSW

LM3450

SNVS681D – NOVEMBER 2010 – REVISED MAY 2013

www.ti.com

THERMAL PROTECTION

With the LM3450A, QPS has to dissipate more power than with the LM3450. During worst case conditions such

as open LED load, the converter will be demanding very little current regardless of the triac position. If the phase

dimmer conduction angle is large and the load is not present, QPS has to dissipate many watts since the dynamic

hold is attempting to regulate the current to ten's of mA. Using the LM3450, this is nominally not a problem since

it is sampling the dynamic hold infrequently. However, the LM3450A is drawing the hold current every cycle

which becomes a problem very quickly. It should be noted that If the input AC line is very noisy, the VAC input to

the decoder could have enough variation in steady state to cause the decoder to think the dimmer is transitioning

all of the time. This would increase the sampling rate dramatically, putting much more thermal strain on the

passFET in LM3450 applications as well.

To mitigate these problems, a thermal protection circuit should be implemented on the LM3450A designs (and

can be on the LM3450 designs as well) as shown in red in Figure 31. The NTC thermistor should be placed on

the opposite side of the PCB directly under the drain of QPS. This will provide the best thermal coupling while

maintaining the necessary high voltage spacing constraints. At startup the NTC is at a high resistance value,

turning the PNP fully on which provides the dynamic hold path. As the NTC heats up the resistance decreases

and the base voltage increases. Eventually, the PNP will transition into linear mode and the effective resistance

from collector to emitter will increase. This will decrease the maximum holding current, thereby decreasing the

thermal stress on QPS. Given enough headroom, the circuit should reach thermal equilibrium in a safe controlled

manner.

Since this method of thermal protection linearly reduces the maximum hold current with increasing temperature,

the foldback will not be perceptible to the consumer. Instead, the result of the foldback will simply be a reduction

of contrast ratio, meaning the minimum achievable LED current will increase as the temperature increases

beyond the foldback level.

Figure 35. Primary Bias Circuitry

PRIMARY BIAS SUPPLY

The LM3450/50A requires a supply voltage at VCC, not to exceed 25V. The device has VCC under-voltage lockout

(UVLO) with rising and falling thresholds of 12.9V and 7.9V respectively. A 24V Zener diode should be placed

from the VCC pin to GND to protect the device from substantial spikes that could cause damage.

24

Submit Documentation Feedback

Copyright © 2010–2013, Texas Instruments Incorporated

Product Folder Links: LM3450