Texas Instruments AN-2150 LM3450A, Руководство пользователя

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+
-
DIM
V
AC
500k
Transition
LM3450A
OPTO2
V
POP2
Rectified AC
ANGLE
DEMOD
SAMPLE/REMAP
D
IN
D
OUT
D
IN
D
OUT
R25 R24
C18 C17
R16
R71
R26
R29
R32
FLT2
FLT1
V
ADJ
V
REF
3V
REF
R23
D22
500 Hz
U8
V
OP2
LM3409HV
EN
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Design Information
Thermal Protection
Using the previously mentioned design methodology, thermal protection is indeed necessary for the open
load condition and for power levels below the specified operating range shown in
Figure 15 . The thermal
protection circuit shown in Figure 14 will reduce the maximum holding current when the temperature rises
too high, thus preventing catastrophic failure of Q1. Keep in mind that the thermal foldback does not
prevent the circuit from operating, it simply reduces the amplitude of the dynamic hold. The only negative
effect of the thermal protection is a possible reduction in contrast ratio, meaning the minimum attainable
output current potentially increases as the dynamic hold level decreases.
The thermal protection is accomplished using a PNP transistor (Q11) and a resistor divider comprised of a
fixed resistor (R45) and an NTC thermistor (R44). As Q1 heats up, R44 decreases causing the collector
voltage of Q11 to decrease, effectively reducing the maximum attainable holding current. Placement of
R44 is critical to ensure the best possible thermal coupling to Q1. The drain of Q1 will have the highest
temperature rise but it is at a much higher voltage than the source where R44 is electrically connected.
Because of this, the best placement for R44 is on the other side of the PCB, directly under the drain of
Q1. The dielectric of the PCB provides adequate electrical insulation while yielding the best thermal
coupling. Obviously, R44 placement in potted solutions is much more forgiving. A 10k
Ω NTC is suggested
for R44 and Q11 can be a basic PNP (i.e. MMBT3906). R45 has to be sized experimentally since the
thermal coupling will vary with each PCB layout. A good starting point for R45 is 15k
Ω .
Definitions
I
IN-MIN-REG
– Regulated Minimum Input Current
I
HOLD-MAX
– Maximum Hold Current
Figure 16. Dimming Decoder Circuit
9.7 Dimming Decoder
Angle Sense
V
AC
is a dual input for both the PFC multiplier and the angle decoder. The resistor divider (R26+R29, R32)
should be sized according to the desired angle detect voltage V
DET
. A general rule of thumb is to set V
DET
=
V
IN-PK
/x where x is a value between 4 and 7. R26+R29 should be chosen to be between 1M Ω and 2M Ω to
limit power dissipation.
(23)
17 SNVA485B – June 2011 – Revised May 2013 AN-2150 LM3450A Evaluation Board
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