LM3444
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SNVS682C –NOVEMBER 2010–REVISED MAY 2013
Common equations for determining duty cycle and switching frequency in any buck converter:
1
fSW
=
tOFF + tON
tON
VLED
VBUCK
D =
=
tON + tOFF
tOFF
tON + tOFF
D‘ =
(8)
(9)
Therefore:
D
tON
1 - D
tOFF
and
fSW =
,
fSW
=
With efficiency of the buck converter in mind:
VLED
= h ì D
VBUCK
(10)
(11)
Substitute equations and rearrange:
VLED
≈
«
’
◊
1
h
1 -
ì
∆
÷
VBUCK
fSW
=
tOFF
Off-time, and switching frequency can now be calculated using the equations above.
SETTING THE SWITCHING FREQUENCY
Selecting the switching frequency for nominal operating conditions is based on tradeoffs between efficiency
(better at low frequency) and solution size/cost (smaller at high frequency).
The input voltage to the buck converter (VBUCK) changes with both line variations and over the course of each
half-cycle of the input line voltage. The voltage across the LED string will, however, remain constant, and
therefore the off-time remains constant.
The on-time, and therefore the switching frequency, will vary as the VBUCK voltage changes with line voltage. A
good design practice is to choose a desired nominal switching frequency knowing that the switching frequency
will decrease as the line voltage drops and increase as the line voltage increases (Figure 17).
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