Rainbow Electronics MAX16841 - Manual (Page 12)

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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:
2
IN SW
IN
IN
170V D
Lm
I f 2
P
I
340V
Ч
=
Ч Ч
Ч π
=
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:
IN INmax
P
SW
2 I V
I
Lm f
Ч Ч
=
Ч
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:
LED
Zmin
Lmax
V
f
2 L I
=
× π Ч Ч
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:
Lmax
m P P
Zmin
I R20
R17
G V
5
C4
2 f R17
−
Ч
=
Ч
=
Ч π Ч Ч
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
:
Zmin
1
C3
2 5 f R17
=
× π Ч Ч Ч
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:
LED P
Zmin
Lmax S
V N
f
2 Lm I N
= Ч
Ч π Ч Ч
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:
Lmmax
m P P
Zmin
I R20
R17
G V
5
C4
2 f R17
−
Ч
=
Ч
=
Ч π Ч Ч
where Lm is the magnetizing inductance of the flyback
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
C3 at 5 x f
Zmin
:
Zmin
1
C3
2 5 f R17
=
× π Ч Ч Ч