LTC3218

7

3218fb

attain the desired current level can be determined by

Equations 1 and 2:

R

V

I

SETF

LED

=

3300 1 21

•.

R

V

I

SETT

LED

=

850 1 21

•.

Overcurrent shutdown mode will prevent damage to the

part and LED by shutting down the high power sections of

will ensure that the part stays out of this mode.

pin and modulating the charge pump strength based on

the error signal.

In shutdown mode all circuitry is turned off and the LTC3218

of approximately 90kΩ. The LTC3218 enters shutdown

mode when the ENF and ENT pins are brought low.

LED Current Programming

The LTC3218 includes an accurate, programmable cur-

rent source that is capable of driving LED currents up to

150mA continuously and up to 400mA for pulsed operation.

Pulsed operation may be achieved by toggling the ENT or

ENF pins. In either continuous or pulsed operation, proper

board layout is required for effective heat sinking.

The output current of the LTC3218 is programmed using

pins and GND. The output current modes are shown in

Since the LTC3218 has three separate LED current ratios

built in, it can be programmed using a single resistor by

the pins to the resistor.

OPERATION

Thermal Protection

The LTC3218 has built-in overtemperature protection.

Thermal shutdown circuitry will shut down the part when

the junction temperature exceeds approximately 165°C. It

will re-enable the part once the junction temperature drops

back to approximately 150°C. The LTC3218 will cycle in

and out of thermal shutdown indefinitely without latchup

or damage until the heat source is removed.

ENF Timeout

The ENF input is used to select the high current setting for

use as a camera flash. To prevent damage to the LED, the

ENF pin has a 2-second timeout. If the LTC3218 is enabled

for greater than approximately 2 seconds using the ENF

pin, the part will enter a low-power mode, preventing cur-

rent from being delivered to the LED. Normal operation

can be restored by bringing the part into shutdown and

re-enabling it.

Short-Circuit Protection

When ENF or ENT are brought high, the part will connect

fails to charge up to over 1V (i.e., CPO is shorted), the chip

will not be enabled. Similarly, during operation if CPO is

pulled down below 1V, the part will be disabled.

Soft-Start

To prevent excessive inrush current during start-up and

mode switching, the LTC3218 employs built-in soft-start

circuitry. Soft-start is achieved by increasing the amount

of current available to the output charge storage capacitor

linearly over a period of approximately 80μs.

Table 1. Output Current Modes for All ENT and ENF Settings

ENF

ENT

LOW

LOW

SHUTDOWN

LOW

HIGH

HIGH

LOW

HIGH

HIGH

(1)

(2)