Datasheet

Rectified AC
BIAS
AC
LM3450/50A
V
CCUV
6V
14V
V
CC
AC
RETURN
V
CC
UVLO
24V
C
VCC
Q
PS
R
BS
Q
SW
LM3450
SNVS681D –NOVEMBER 2010–REVISED MAY 2013 www.ti.com
THERMAL PROTECTION
With the LM3450A, Q
PS
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, Q
PS
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 Q
PS
. 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 Q
PS
. 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 V
CC
, not to exceed 25V. The device has V
CC
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 V
CC
pin to GND to protect the device from substantial spikes that could cause damage.
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