LP39542 Datasheet, PDF (24/58 Pages) National Semiconductor (TI) – Advanced Lighting Management Unit

Single High Current Driver
LP39542 has internal constant current driver that is capable
of driving high current LED, 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 400mA is required for the maximum flash cur-
rent, R_FLASH equals to
R_FLASH = 300 × 1.23V / I_MAX – 50Ω = 369V / 0.4A – 50Ω =
873Ω e.g. 910Ω resistor can be used
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
MODE
FLASH disabled, no extra current
0
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.
30008533
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 FLASH_EN digital input
pin goes high. The Flash length can be selected from 3 pre-
defined values or the FLASH_EN pin pulse length can deter-
mine how long the flash pulse is. After flash pulse the flash is
shut down completely. To enable flash again, EN_HCFLASH
bit must be set to 0 and then 1.The pulse length is controlled
by the FT_T[1:0] bits in register 10H as show in the table be-
low.
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)
After the flash pulse the EN_HCFLASH bit has to be written
low, the LP39542 does not clear this bit automatically. If 11b
is selected in the FL_T[1:0] register, then it is possible to use
safety bit EN_SAFETY in register 10H. When EN_SAFETY
is 1, then the flash is shut down automatically, if the
FLASH_EN pulse duration is longer than 1.2 seconds (typ.).
This prevents any damage to the application circuitry, if the
FLASH_EN pin is stuck high because of user or program er-
ror.
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