LTC3215 Datasheet, PDF (7/12 Pages) Linear Technology – 700mA Low Noise High Current LED Charge Pump

U
OPERATIO
1.5V_IN
OR
2V_IN
–+
R_OL
+
CPO
–
3215 F01
Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit
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)/R_OL or (2V_IN – CPO)/R_OL
(2)
in the 1.5x mode or 2x mode respectively.
LED Current Programming
The LTC3215 includes an accurate, programmable cur-
rent source that is capable of driving LED currents up to
350mA continuously and up to 700mA 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 = 3990/I_LED
(1)
2.2µF
2.2µF
LTC3215
For applications requiring multiple current levels, several
schemes may be used to change the resistance for the
R_SET resistor. Figure 2 shows two such schemes. The
circuit in Figure 2a 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. Alternatively, the
circuit in Figure 2b uses a pulse-width modulator (PWM)
to vary the current through R_SET, which changes the LED
current.
Mode Switching
The LTC3215 will automatically switch from 1x mode to
1.5x mode, and subsequently from 1.5x mode to 2x mode
whenever a dropout condition is detected at the I_LED pin.
The part will wait approximately 2ms 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.
In order to reset the part back into 1x mode, the LTC3215
must be brought into shutdown (EN = LOW). Immediately
after the part has been brought to shutdown, it may be
enabled into the 1x mode via the EN pin. An internal
comparator will not allow the main switches to connect V_IN
and CPO in 1x mode until the voltage at the CPO pin has
decayed to less than or equal to the voltage at the V_IN pin.
2.2µF
2.2µF
2.9V TO 4.4V
µP
V_IO
2.2µF
ON/OFF
V_IO
R2
TORCH/FLASH
C1⁺ C1^– C2⁺ C2^–
V_IN
CPO
LTC3215
I_LED
EN
I_SET
R1
I_LED*
4.7µF
*I_TORCH = [(1.22V/R1) – ((V_IO – 1.22V)/R2)] • 3270
I_FLASH = [(1.22V/(R1 • R2/(R1 + R2))] • 3270
(2a)
3215 F02a
Figure 2
2.9V TO 4.4V
2.2µF
C1⁺ C1^– C2⁺ C2^–
V_IN
CPO
LTC3215
I_LED
EN
PWM
I_SET
9.31k _1k
1%
9.31k
1%
3215 F02b
1µF
(2b)
4.7µF
3215f
7