LTC3218 Datasheet, PDF (7/12 Pages) Linear Technology – 400mA Single Wire Camera LED Charge Pump

LTC3218
OPERATION
attain the desired current level can be determined by
Equations 1 and 2:
R_SETF
=
3300 • 1.21V
I_LED
(1)
R_SETT
=
850 • 1.21V
I_LED
(2)
Overcurrent shutdown mode will prevent damage to the
part and LED by shutting down the high power sections of
the chip. Choosing an R_SETF or R_SETT value of 5k or greater
will ensure that the part stays out of this mode.
Regulation is achieved by sensing the voltage at the I_LED
pin and modulating the charge pump strength based on
the error signal.
In shutdown mode all circuitry is turned off and the LTC3218
draws a very low current from the V_IN supply. The output is
disconnected from V_IN and is pulled down by a resistance
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
external resistors connected between the I_SETT and I_SETF
pins and GND. The output current modes are shown in
Table 1, where R_SETT is connected between I_SETT and GND,
and R_ESTF is connected between I_SETF and GND.
Since the LTC3218 has three separate LED current ratios
built in, it can be programmed using a single resistor by
connecting I_SETT and I_SETF together, and then connecting
the pins to the resistor.
Table 1. Output Current Modes for All ENT and ENF Settings
ENF
ENT
I_LED
LOW
LOW
SHUTDOWN
LOW
HIGH
1029/R_SETT
HIGH
LOW
2965/R_SETF
HIGH
HIGH
3993/R_SETF
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
V_IN and CPO through a weak pull-up. If the CPO capacitor
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.
3218fa
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