an30888b Panasonic Corporation of North America, an30888b Datasheet - Page 25

• Manufacturer Part Number: an30888b
• Description: High Brightness Led Driver Ic
• Manufacturer: Panasonic Corporation of North America

(7) Setting output LED current and choosing current sense resistor R
For Buck Mode :
R
First is to calculate the peak current I
current plus half of actual current ripple through the inductor.
Lastly, R
Using numeric example of operating condition :
Please note that for component deviation such as inductor, diodes, etc, these deviation can cause the designed I
than the calculated value.
Users may need to fine tune the value of R
Please take note of total impedance including parasitic impedance of PCB trace at CS pin to ground when designing the required R
value. This is especially important if the designed I
significant to the total impedance seen at CS pin
AN30888B
CS
Technical Data (continued)
resistor can be set in the following way :
Functions and properties descriptions (continued)
CS
I
resistor can be determine by using :
R
V
From Eq[9] :
From Eq[10] : R
PK
CS
IN
= I
= 12 V, V
= ——— ………………………………………………
LED
V
V
T
I
L = Inductor value found in part (6) inductor selection
V
I
V
LED
PK
I
OFF
OUT
IN
PK
FB
FB
+ ——————————— …………………… Eq[9]
is peak current found in Eq[9] .
= Input supply voltage
= LED output current
is voltage at CS pin. Refer to data graphs for the V
=T
= Output voltage
OUT
I
OFF
(V
PK
CS
= 2 V, I
IN
= 0.5 + —————— = 0.575 A
= Fixed off time design at 1 μs
= ———— = 348 mΩ
– V
2L
0.575
OUT
PK
0.2
LED
using Eq[9]. During buck mode, peak current sense correspond to the average output LED
(12 – 2) × 1μ
) × T
= 500 mA, T
CS
2 × 66μ
OFF
from the calculated values in order to obtain accurate I
LED
OFF
is high as R
= 1 μs, L = 66 μH, V
SDG00005AEB
CS
value will be small and in turn making parasitic impedance
Eq[10]
FB
CS
voltage at different input voltage condition.
(continued)
FB
= 0.2 V
(Buck mode)
(Buck mode)
LED
measurement.
PK
to be higher or lower
CS
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