LTC3214 Linear Technology, LTC3214 Datasheet - Page 7

• Manufacturer Part Number: LTC3214
• Description: 500mA Camera LED Charge Pump
• Manufacturer: Linear Technology

OPERATIO
R
oscillator frequency, flying capacitor values and switch
resistances. From Figure 1, we can see that the output
current is proportional to:
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:
OL
R
1 5
SET
.
is dependent on a number of factors including the
V
IN
R
= 3570/I
OL
–
CPO
LED
OR
U
Figure 2. Recommended Circuit for Attaining Two Current Levels (Torch and Flash Modes)
2
V
IN
R
–
OL
CPO
TORCH/FLASH
ON/OFF
*I
I
TORCH
FLASH
µP
= [(1.21V/(R1 • R2/(R1 + R2))] • 2950
V
V
= [(1.21V/R1) – ((V
2.9V TO 4.5V
IO
IO
2.2µF
R2
IO
– 1.21V)/R2)] • 2950
C1
V
EN
IN
+
2.2µF
I
SET
For applications requiring multiple current levels, several
schemes may be used to change the resistance for the
R
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
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.
R1
C1
LTC3214
SET
–
C2
resistor. Figure 2 shows one such scheme. The
+
2.2µF
I
CPO
C2
LED
–
3214 F02
I
LED
*
4.7µF
SET
current.
LTC3214
7
3214f