Texas Instruments LM3448 Evaluation Board LM3448-EDSNEV/NOPB LM3448-EDSNEV/NOPB 데이터 시트
제품 코드
LM3448-EDSNEV/NOPB
L E D
L E D
B U C K
2
S W
V
1
V
1-
V
L
f
i
§
·
§
·
u
¨
¸
¨
¸
¨
¸
¨
¸
K
©
¹
©
¹
u '
L E D
B U C K
O F F
S W
V
1
1-
V
t
f
§
·
u
¨
¸
¨
¸
K
©
¹
L E D
O F F
V
L 2
t
i
#
u
'
L E D
O F F
V
i
t
L 2
' #
u
M A X
M IN
L ( O F F - T IM E )
L E D
I
I
V
V
L
t
§
·
u ¨
¸
'
©
¹
L(OFF-TIME)
LED
i
V
V
L
t
'
§
·
u ¨
¸
'
©
¹
L(OFF TIME)
LED
V
V
L(ON TIME)
BUCK
LED
V
V
V
L(ON TIME)
BUCK
LED
DS
L2
V
V
(V
V
I
R3)
u
v
t
di
L
d
SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013
The equation for an ideal inductor is:
(16)
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,
(17)
Since the voltage across the SW FET (V
DS
) is relatively small as is the voltage across sense resistor R3, we can
simplify this as approximately,
(18)
During the off-time, the voltage seen by the inductor is approximately,
(19)
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,
(20)
(21)
Re-arranging this gives,
(22)
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 inductance (L2).
These equations can be rearranged to calculate the desired value for inductor L2.
(23)
where,
(24)
and finally,
(25)
Refer to “Design Example” section of the datasheet to better understand the design process.
SETTING THE LED CURRENT
shows the inductor current waveform (I
L2
) when operating in continuous conduction mode (CCM). The
LM3448 constant off-time control loop regulates the peak inductor current (I
L2-PK
). Since the average inductor
current equals the average LED current (I
AVE
), LED current is controlled by regulating the peak inductor current.
26
Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: