LTC3214EDD Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTC3214EDD

500mA Camera LED Charge Pump

Linear Technology 

LTC3214

OPERATION

ROL 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:

1.5VIN – CPO OR 2VIN – CPO

ROL

ROL

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:

RSET = 3570/ILED

For applications requiring multiple current levels, several

schemes may be used to change the resistance for the

RSET resistor. Figure 2 shows one such scheme. The cir-

cuit 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 ISET 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

maximum 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 for-

ward 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

VIO

2.2μF

ON/OFF

C1+ C1– C2+ C2–

VIN

CPO

LTC3214

ILED

EN

ILED*

VIO

R2

TORCH/FLASH

ISET

R1

*ITORCH = [(1.21V/R1) – ((VIO – 1.21V)/R2)] • 2950

IFLASH = [(1.21V/(R1 • R2/(R1 + R2))] • 2950

3214 F02

4.7μF

Figure 2. Recommended Circuit for Attaining Two Current Levels (Torch and Flash Modes)

3214fb

7

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