Texas Instruments AN-1907 LM3423, Руководство пользователя

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V
O
x I
LED
V
IN
x I
IN
©
§
¹
·
= 0.85
©
§
¹
·
P
IN
P
O
=
K
f
SW
=
25
C
1
x R
1
©
§
¹
·
= 588 kHz
D =
t
ON
t
ON
+ t
OFF
©
§
¹
·
= t
ON
x f
SW
|
D
V
O
V
IN
+ V
O
©
§
¹
·
|
D
¶
D
©
§
¹
·
V
IN
V
O
©
§
¹
·
t
S
'
T D
S
DT
L
 i
L
i
L(t)
I
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Board Features
The selection of resistors is not arbitrary, for matching and noise performance, the CSH current should be
set to be around 100 µA. This current does not flow in the LEDs and will not affect either off state LED
current or the regulated LED current. CSH current can be above or below this value, but high side
amplifier offset characteristics and jitter performance may be affected slightly.
5.3 Inductor Selection
The inductor should be chosen so that the current ripple ( Δ i
L
) is between 20% and 40% of the average
current (<I
L
>) through the inductor.
Figure 3. Inductor Current Waveform
The worst case ripple is seen when the input voltage is at its lowest magnitude. This is true if we can say
that the output voltage stays relatively constant.
Design Example:
V
O
≊ 28V
V
IN-MIN
= 18V
V
IN-NOM
= 24V
V
IN-MAX
= 36V
I
LED
= 1A
Buck Boost Convertion Ratio:
(5)
Therefore:
(6)
D @ V
IN-MAX
= 0.436
D @ V
IN-MIN
= 0.609
(7)
t
ON
= @ V
IN-MAX
= 0.742 µs
t
ON
= @ V
IN-MIN
= 1.05 µs
Calculate average input current: The average input current is equal to the average inductor current.
(8)
Assume efficiency = 85%
(9)
I
IN
= 915 mA @ V
IN
= 36V
5 SNVA376A – December 2008 – Revised May 2013 AN-1907 LM3423 Buck-Boost Configuration Evaluation Board
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