LTC3215 Linear Technology, LTC3215 Datasheet - Page 7

• Manufacturer Part Number: LTC3215
• Description: 700mA Low Noise High Current LED Charge Pump
• Manufacturer: Linear Technology

OPERATIO
R
the 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 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:
Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit
OL
(1.5V
R
TORCH/FLASH
SET
is dependent on a number of factors including
ON/OFF
= 3990/I
IN
– CPO)/R
*I
I
TORCH
FLASH
µP
= [(1.22V/(R1 • R2/(R1 + R2))] • 3270
V
V
= [(1.22V/R1) – ((V
2.9V TO 4.4V
LED
IO
IO
U
1.5V
2V
OR
OL
IN
IN
2.2µF
or (2V
+
–
R2
IO
R
IN
OL
– 1.22V)/R2)] • 3270
V
3215 F01
C1
EN
– CPO)/R
(2a)
IN
2.2µF
+
I
SET
CPO
–
R1
+
C1
LTC3215
–
C2
+
OL
2.2µF
I
CPO
C2
LED
3215 F02a
–
I
LED
(2)
(1)
*
Figure 2
4.7µF
For applications requiring multiple current levels, several
schemes may be used to change the resistance for the
R
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
circuit in Figure 2b uses a pulse-width modulator (PWM)
to vary the current through R
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
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
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
SET
resistor. Figure 2 shows two such schemes. The
2.9V TO 4.4V
2.2µF
PWM
9.31k
1%
C1
V
EN
IN
+
2.2µF
I
SET
SET
1k
9.31k
1%
C1
SET
LTC3215
–
(2b)
current. Alternatively, the
C2
, which changes the LED
+
2.2µF
I
CPO
1µF
C2
LED
3215 F02b
–
LTC3215
LED
IN
4.7µF
pin.
pin.
7
3215f
IN