LTC3214 Datasheet, PDF (6/12 Pages) Linear Technology – 500mA Camera LED Charge Pump

LTC3214
U
OPERATIO
The LTC3214 uses a fractional switched capacitor charge
pump to power a high current LED with a programmed
regulated current. The part starts up into the 1x mode. In
this mode, V_IN is directly connected to CPO. This mode
provides maximum efficiency and minimum noise. The
LTC3214 will remain in this mode until the forward voltage
(V_F) approaches the maximum CPO voltage possible in
this mode. When this dropout condition occurs, the
LTC3214 will switch to 1.5x mode after a soft-start period.
Any subsequent dropout detected will cause the part to
enter 2x mode.
A two phase nonoverlapping clock activates the charge
pump switches. In the 2x mode, the flying capacitors are
charged on alternate clock phases from V_IN. While one
capacitor is being charged from V_IN, the other is stacked
on top of V_IN and connected to the output. Alternatively, in
the 1.5x mode the flying capacitors are charged in series
during the first clock phase, and stacked in parallel on top
of V_IN on the second clock phase. This sequence of
charging and discharging the flying capacitors continues
at a free running frequency of 900kHz (typ).
The current delivered to the LED load is controlled by the
internal programmable current source. The current is
programmed by a resistor connected between the I_SET pin
and GND. The resistor value needed to attain the desired
current level can be determined by Equation 1.
R_SET = 3570/I_LED
(1)
Overcurrent shutdown mode will prevent damage to the
part by shutting down the high power sections of the chip.
Choosing an R_SET 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
LTC3214 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 43kΩ. The LTC3214 enters
shutdown mode when the EN pin is brought low.
Thermal Protection
The LTC3214 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 LTC3214 will cycle in
and out of thermal shutdown indefinitely without latch up
or damage until the heat source is removed.
Short-Circuit Protection
When EN is 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 LTC3214 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 150µs.
Charge Pump Strength
When the LTC3214 operates in either the 1.5x mode or 2x
mode, the charge pump can be modeled as a Thevenin
equivalent circuit to determine the amount of current
available from the effective input voltage and effective
open-loop output resistance, R_OL(Figure 1).
1.5V_IN
OR
2V_IN
–+
R_OL
+
CPO
–
3214 F01
Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit
3214fa
6