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MAX16841
Controller IC for Dimmable Offline LED Lamps
Choose the transformer’s magnetizing inductance (Lm)
in such a way so that the transformer operates in DCM
above 120V AC input. DCM operation at higher voltages
reduces switching losses in the Q4 MOSFET. Use the fol-
lowing equation to calculate Lm:
where G m is the transconductance of the internal error
amplifier and V P-P is 2.4V.
Place the compensation pole formed by R17 and
C3 at 5 x f Zmin :
Lm =
170V × D 2
I IN × f SW × 2
C3 =
1
2 × π × 5 × f Zmin × R17
I IN = IN
2 × I IN × V INmax
Lm × f SW
× P
V LED N
R17 = Lmmax
C4 =
P ×π
340V
where D is the switching duty cycle at 170V DC and f SW
is the switching frequency.
In DCM conditions, the peak current in Lm can be
calculated with the help of the following equation:
I P =
where V INmax is the maximum peak input voltage.
Feedback Compensation
Loop Compensation for
Nonisolated Buck (R17, C3, C4)
The switching converter small-signal transfer function
contains a pole at origin and a zero. The zero location
is inversely related to inductor current and inductance
value. The minimum frequency of the zero location is:
Loop Compensation
for Flyback Driver (R17, C3, C4)
The switching converter small-signal transfer function is
identical to the buck transfer function. The zero location
is inversely related to primary-magnetizing inductance
and its current. The minimum frequency of the zero
location is:
f Zmin
=
2 × π × Lm × I Lmax N S
Design the loop compensation in such a way so that the
loop crossover is near f Zmin . Place the compensation
zero formed by R17 and C4 at f Zmin /5. R20 is given by:
I × R20
G m × V P ? P
5
2 × π × f Zmin × R17
where Lm is the magnetizing inductance of the flyback
f Zmin =
V LED
2 × π × L × I Lmax
transformer, G m is the transconductance of the internal
error amplifier, and V P-P is 2.4V.
Place the compensation pole formed by R17 and
Design the loop compensation in such a way so that the
C3 at 5 x f Zmin :
R17 = Lmax
loop crossover is near f Zmin . Place the compensation
zero formed by R17 and C4 at f Zmin /5. R20 is given by:
I × R20
G m × V P ? P
C3 =
1
2 × π × 5 × f Zmin × R17
Maxim Integrated
C4 =
5
2 × π × f Zmin × R17
12
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