LTC3215

7

3215f

the oscillator frequency, flying capacitor values and

switch resistances.

From Figure 1, we can see that the output current is

proportional to:

(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:

(1)

For applications requiring multiple current levels, several

schemes may be used to change the resistance for the

circuit in Figure 2a uses the I/O output of a microcontroller

to switch a second resistor (R2) in parallel or series with

circuit in Figure 2b uses a pulse-width modulator (PWM)

current.

Mode Switching

The LTC3215 will automatically switch from 1x mode to

1.5x mode, and subsequently from 1.5x mode to 2x mode

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

and CPO in 1x mode until the voltage at the CPO pin has

Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit

OPERATIO

–

+

+

–

CPO

3215 F01

OR

2.2µF

2.2µF

4.7µF

2.9V TO 4.4V

2.2µF

ON/OFF

TORCH/FLASH

EN

LTC3215

R2

3215 F02a

CPO

R1

µP

2.2µF

2.2µF

4.7µF

1µF

2.9V TO 4.4V

2.2µF

EN

LTC3215

3215 F02b

CPO

1k

9.31k

1%

9.31k

1%

PWM

(2a)

(2b)

Figure 2