MAX16841_13 Datasheet, PDF (12/18 Pages) Maxim Integrated Products – Controller IC for Dimmable Offline LED Lamps

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:
Lm = ^{170V × D2}
I_IN × f_SW × 2
I_IN
=
P_IN × 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 =
2 × I_IN × V_INmax
Lm × f_SW
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:
f_Zmin
=
2
×
π
V_LED
× L × I_Lmax
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:
R17 = ^{ILmax × R20}
G_m × V_P−P
C4 =
5
2 × π × f_Zmin × R17
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:
C3 =
1
2 × π × 5 × f_Zmin × R17
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
V_LED
× ^NP
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:
R17 = ^{ILmmax × R20}
G_m × V_P−P
C4 =
5
2 × π × f_Zmin × R17
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:
C3 =
1
2 × π × 5 × f_Zmin × R17
Maxim Integrated
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