Datasheet

L2
R3
C11
Q3
D10
I
COLL
R4
V
BUCK
C12
9
10
11
12
13 4
5
6
7
8
BLDR
ASNS
ISNS
DIM
COFF
GND
VCC
FLTR1
GND
FLTR2
LM3448
U1
14 3NCNC
15 2SWSW
16 1SWSW
LM3448
www.ti.com SNOSB51C –SEPTEMBER 2011–REVISED MAY 2013
Figure 20. Simplified Buck Regulation Circuit
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
, the SW
FET is on causing the inductor current to increase (see Figure 21). During the on-time, current flows from V
BUCK
through the LEDs, L2, the LM3448's internal SW FET and finally through R3 to ground. At some point in time the
inductor current reaches a maximum (I
L2-PK
) determined by the voltage at the ISNS pin. This sensed voltage
across R3 is compared against the dim decoder voltage on FLTR2 at which point the SW FET is turned off by
the regulator. During the off-period denoted by t
OFF
, the current through L2 continues to flow through the LEDs
via D10. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor R4, capacitor C11 and
transistor Q3 provide a linear current ramp that in conjunction with the COFF comparator threshold sets the
constant off-time for a given output voltage.
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