LTC3214 Datasheet, PDF (7/12 Pages) Linear Technology – 500mA Camera LED Charge Pump

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OPERATIO
R_OL is dependent on a number of factors including the
oscillator frequency, flying capacitor values and switch
resistances. From Figure 1, we can see that the output
current is proportional to:
1.5V_IN – CPO _OR 2V_IN – CPO
R_OL
R_OL
in the 1.5x mode or 2x mode respectively.
LED Current Programming
The LTC3214 includes an accurate, programmable cur-
rent source that is capable of driving LED currents up to
300mA continuously and up to 500mA for pulsed opera-
tion. Pulsed operation may be achieved by toggling the EN
pin. In either continuous or pulsed operation, proper
board layout is required for effective heat sinking.
The current may be programmed using a single external
resistor. Equation 1, used to calculate the resistor value
from the desired current level is repeated below:
R_SET = 3570/I_LED
LTC3214
For applications requiring multiple current levels, several
schemes may be used to change the resistance for the
R_SET resistor. Figure 2 shows one such scheme. The
circuit in Figure 2 uses the I/O output of a microcontroller
to switch a second resistor (R2) in parallel or series with
R1, changing the effective I_SET current.
Mode Switching
The LTC3214 will automatically switch from 1x mode to
1.5x mode, and subsequently from 1.5x mode to 2x mode
whenever the LED forward voltage approaches the maxi-
mum CPO voltage for that mode. The part will wait
approximately 2.5ms before switching to the next mode.
This delay allows the LED to warm up and reduce its
forward voltage which may remove the dropout condition.
The part may be reset to 1X mode by bringing the part into
shutdown by setting the EN pin low. Once the EN pin is
low, it may be immediately brought high to re-enable the
part.
2.2µF
2.2µF
2.9V TO 4.5V
µP
V_IO
2.2µF
ON/OFF
C1⁺ C1^– C2⁺ C2^–
V_IN
CPO
LTC3214
I_LED
EN
I_LED*
4.7µF
V_IO
R2
TORCH/FLASH
I_SET
R1
*I_TORCH = [(1.21V/R1) – ((V_IO – 1.21V)/R2)] • 2950
I_FLASH = [(1.21V/(R1 • R2/(R1 + R2))] • 2950
3214 F02
Figure 2. Recommended Circuit for Attaining Two Current Levels (Torch and Flash Modes)
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