Texas Instruments LM3445 Evaluation Board LM3445-120VFLBK/NOPB LM3445-120VFLBK/NOPB Datenbogen
Produktcode
LM3445-120VFLBK/NOPB
L2
#
t
OFF
x
V
LED
'
i
'
i
#
t
OFF
x
V
LED
L2
V
L(OFF-TIME)
= V
LED
= L x
'
i
'
t
V
L(OFF-TIME)
= V
LED
= L x
(I
(MAX)
- I
(MIN)
)
'
t
di
Q
= L dt
-
C12
R3
Q2
-
D10
V
LED
V
BUCK
V
L2
L2
SNVS570L – JANUARY 2009 – REVISED MAY 2013
Figure 27. LM3445 External Components of the Buck Converter
The equation for an ideal inductor is:
(14)
Given a fixed inductor value, L, this equation states that the change in the inductor current over time is
proportional to the voltage applied across the inductor.
proportional to the voltage applied across the inductor.
During the on-time, the voltage applied across the inductor is,
V
L(ON-TIME)
= V
BUCK
- (V
LED
+ V
DS(Q2)
+ I
L2
x R3)
(15)
Since the voltage across the MOSFET switch (Q2) is relatively small, as is the voltage across sense resistor R3,
we can simplify this to approximately,
we can simplify this to approximately,
V
L(ON-TIME)
= V
BUCK
- V
LED
(16)
During the off-time, the voltage seen by the inductor is approximately:
V
L(OFF-TIME)
= V
LED
(17)
The value of V
L(OFF-TIME)
will be relatively constant, because the LED stack voltage will remain constant. If we
rewrite the equation for an inductor inserting what we know about the circuit during the off-time, we get:
(18)
Re-arranging this gives:
(19)
From this we can see that the ripple current (
Δ
i) is proportional to off-time (t
OFF
) multiplied by a voltage which is
dominated by V
LED
divided by a constant (L2).
These equations can be rearranged to calculate the desired value for inductor L2.
(20)
Where:
22
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