LM3445-220VEVAL National Semiconductor, LM3445-220VEVAL Datasheet - Page 13

• Manufacturer Part Number: LM3445-220VEVAL
• Description: LM3445 Triac Dimmable Offline LED Driver; Qty per Container: 1/
• Manufacturer: National Semiconductor

OVERVIEW OF CONSTANT OFF-TIME CONTROL
A buck converter’s conversion ratio is defined as:
Constant off-time control architecture operates by simply
defining the off-time and allowing the on-time, and therefore
the switching frequency, to vary as either V
The output voltage is equal to the LED string voltage (V
and should not change significantly for a given application.
The input voltage or V
input line varies. The length of the on-time is determined by
the sensed inductor current through a resistor to a voltage
reference at a comparator. During the on-time, denoted by
t
increase. During the on-time, current flows from V
through the LEDs, through L2, Q2, and finally through R3 to
ground. At some point in time, the inductor current reaches a
maximum (I
the ISNS pin. This sensed voltage across R3 is compared
ON
, MOSFET switch Q2 is on causing the inductor current to
L2-PK
) determined by the voltage sensed at R3 and
BUCK
in this analysis will vary as the
FIGURE 11. LM3445 Buck Regulation Circuit
IN
or V
O
changes.
BUCK
LED
),
,
13
against the voltage of dim decoder output, FLTR2, at which
point Q2 is turned off by the controller.
During the off-period denoted by t
continues to flow through the LEDs via D10.
FIGURE 12. Inductor Current Waveform in CCM
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OFF
, the current through L2
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