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LM3445
www.ti.com
LM3445MM
D10
C12
V BUCK
L2
SNVS570L – JANUARY 2009 – REVISED MAY 2013
R4
Q3
I COLL
GATE
8
Q2
4
COFF
ISNS
7
R3
GND
6
C11
Figure 21. LM3445 Buck Regulation Circuit
OVERVIEW OF CONSTANT OFF-TIME CONTROL
A buck converter’s conversion ratio is defined as:
V IN = D = t ON + t OFF = t ON x f SW
V O t ON
(3)
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 IN or V O changes. The output voltage is equal to the LED
string voltage (V LED ), and should not change significantly for a given application. The input voltage or V BUCK in
this analysis will vary as the 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 ON , MOSFET
switch Q2 is on causing the inductor current to increase. During the on-time, current flows from V BUCK , through
the LEDs, through L2, Q2, and finally through R3 to ground. At some point in time, the inductor current reaches a
maximum (I L2-PK ) determined by the voltage sensed at R3 and the ISNS pin. This sensed voltage across R3 is
compared against the voltage of dim decoder output, FLTR2, at which point Q2 is turned off by the controller.
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