LP3954 Datasheet, PDF (23/56 Pages) National Semiconductor (TI) – Advanced Lighting Management Unit

Single High Current Driver
LP3954 has internal constant current driver that is capable for
driving high current mainly targeted for FLASH LED in camera
phone applications.
MAXIMUM CURRENT SETUP FOR FLASH
The user sets the maximum current of FLASH with R_FLASH
resistor based on following equation:
I_MAX = 300 × 1.23V / (R_FLASH + 50Ω),
where
Imax = maximum flash current in Amps (ie. 0.3A)
1.23V = reference voltage
300 = internal current mirror multiplier
R_FLASH = Resistor value in Ohms
50Ω = Internal resistor in the I_FLASH input
For example if 300mA is required for maximum flash current
R_FLASH equals to
R_FLASH = 300 × 1.23V / I_MAX – 50Ω = 369V / 0.3A – 50Ω =
1.18kΩ
CURRENT CONTROL FOR FLASH
To minimize the internal current consumption, the flash func-
tion has an enable bit EN_HCFLASH in the HC_Flash regis-
ter.
EN_
HCFLASH
0
FLASH disabled, no extra current
consumption through R_FLASH
1
FLASH enabled, IFLASH set by
HC_SW[1:0] (see below)
HC[1:0] bits in the HC_Flash register control the FLASH cur-
rent as show in following table.
HC[1:0]
I(FLASH)
00
0.25 × I_MAX(FLASH)
01
0.50 × I_MAX(FLASH)
10
0.75 × I_MAX(FLASH)
11
1.00 × I_MAX(FLASH)
The figure below shows the internal structure for the FLASH
driver.
20132233
FLASH TIMING
Flash output is turned on in lower current View finder mode
when the EN_HCFLASH bit is written high. The actual Flash
at maximum current starts when the EN_FLASH i/o-pin goes
high. The Flash length can be selected from 3 pre-defined
values or EN_FLASH pin pulse length can determine the
length. The pulse length is controlled by the FT_T[1:0] bits as
show in the table below.
FL_T[1:0]
00
01
10
11
Flash duration typ
200ms
400ms
600ms
EN_FLASH on duration
Current during view finder/
focusing
Set by HC[1:0]
Set by HC[1:0]
Set by HC[1:0]
Set by HC[1:0]
Current during FLASH
HC[11] = I_MAX(FLASH)
HC[11] = I_MAX(FLASH)
HC[11] = I_MAX(FLASH)
HC[11] = I_MAX(FLASH)
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