AN30888A
Product Standards
Page 15 of 21
(7) Setting output LED current and choosing current sense resistor R
CS
The LED current in this LSI can be set easily by selecting the appropriate R
CS
resistor to be used at CS pin of this
chip.
For Boost Mode and Buck-Boost mode :
R
CS
resistor can be set in the following way :
First is to calculate input current I
IN
at the required operating condition :
I
IN
= (V
OUT
+ V
D
) (I
LED
/ V
IN
) …………………………… Eq[6] (Boost mode, Buck-Boost mode)
V
OUT
= Output voltage
V
D
= Schottky diode forward drop voltage
I
LED
= Required LED current
V
IN
= Input supply voltage
After which the peak input current, I
PK
can be determine by adding I
IN
with half the peak to peak ripple current at
the inductor.
(V
OUT
– V
IN
) T
OFF
I
PK
= I
IN
+ ——————————— …………………… Eq[7] (Boost mode, Buck-Boost mode)
2L
T
OFF
= Fixed off time = 1 µs
L = Inductor value found in part (6) inductor selection
V
OUT
= Output voltage
V
IN
= Input supply voltage
I
IN
= Input current found in Eq[6]
Lastly, R
CS
resistor can be determine by using :
V
FB
R
CS
= ——— ……………………………………………… Eq[8] (Boost mode, Buck-Boost mode)
I
PK
V
FB
= Voltage at CS pin. Refer to data graphs for the V
FB
voltage
at different input voltage condition.
I
PK
= Peak current found in Eq[7]
Using numeric example of operating condition :
V
IN
=6V, V
OUT
=10V, I
LED
= 500 mA, T
OFF
= 1 µs, L = 16 µH, V
D
= 0.4 V, V
FB
=0.1V@V
IN
=6V
From Eq[6] : I
IN
= (10 + 0.4) (0.5 / 6) = 0.8667 A
(10 – 6) 1 µ
From Eq[7] : I
PK
= 0.8667 + ——————— = 0.9971 A
2 16 µ
0.1
From Eq[8] : R
CS
= ———— = 100.8 m
0.9917
OPERATION (continued)
Functions and properties descriptions (continued)
