I
LED
=
4000
î
V
BG
R
ISET
10
15
20
25
30
35
40
45
200
300
400
500
600
700
800
900
1000
M
a
x
im
u
m
co
n
ve
rte
r
o
u
tp
u
t
vo
lta
g
e
(
V
)
R12 (k
)
C011
V
MAX BOOST
= (
V
BG
R13
+ 0.0387 ) x R12 + V
BG
Boost Converter
7.3
Voltage Control
Maximum boost converter voltage is defined by the feedback voltage divider R12 and R13.
Recommended value for R13 is 130 k
Ω
. R12 value can be calculated with the following equation:
where
•
V
BG
= 1.2 V
•
R12, R13 values are in k
Ω
(2)
Figure 7. Converter Maximum Output Voltage vs R12
Maximum voltage should be chosen based on the maximum voltage required for LED strings.
Recommended maximum voltage is about 30% higher than maximum LED string voltage. Initial boost
voltage is about 88% of maximum voltage. Boost output voltage is adjusted automatically based on LED
current sink headroom voltage.
Default setting for boost maximum voltage is 37 V (R12 is 750 k
Ω
).
8
LED Current Sinks
8.1
Output Configuration
The LP8862-Q1 detects LED output configuration during start-up. Output connected to ground is disabled
and excluded from adaptive boost control and fault detections. Non-used LED output must be grounded,
with jumper J11 or J12.
8.2
Current Setting
Maximum current for the LED outputs is controlled with an external resistor (R4) connected to ISET pin.
Maximum current can be calculated as follows:
where
•
V
BG
= 1.2 V
•
R
ISET
– current setting resistor (R4), k
Ω
•
I
LED
– output current per output, mA
(3)
10
LP8862-Q1EVM Evaluation Module
SNVU478A – July 2015 – Revised October 2015
Copyright © 2015, Texas Instruments Incorporated