LTC3215 Datasheet, PDF (6/12 Pages) Linear Technology – 700mA Low Noise High Current LED Charge Pump

LTC3215
U
OPERATIO
The LTC3215 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
LTC3215 will remain in this mode until the LED current
source begins to dropout. When dropout is detected, the
LTC3215 will switch to 1.5x mode after a soft-start period.
Any subsequent dropout detected will cause the part to
enter 2x mode. The part may be reset to 1x mode by
bringing the part into shutdown mode and then reenabling
the part.
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 value of this
current may be selected by choosing the appropriate
programming resistor. The resistor is 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 = 3990/I_LED
(1)
A resistor value of 2k or less (e.g., a short-circuit) will
cause the LTC3215 to enter overcurrent shutdown mode.
This mode will prevent damage to the part by shutting
down the high power sections of the chip.
Regulation is achieved by sensing the voltage at the CPO
pin and modulating the charge pump strength based on
the error signal. The CPO regulation voltages are set
internally, and are dependent on the charge pump mode as
shown in Table 1.
Table 1. Charge Pump Output Regulation Voltages
CHARGE PUMP MODE
V_CPO
1.5x
4.6V
2x
5.1V
In shutdown mode all circuitry is turned off and the
LTC3215 draws a very low current from the V_IN supply.
Furthermore, CPO is weakly connected to V_IN. The LTC3215
enters shutdown mode when the EN pin is brought low.
Since EN is a high impedance CMOS input it should never
be allowed to float. To ensure that its state is defined, it
must always be driven with valid logic levels.
Thermal Protection
The LTC3215 has built-in overtemperature protection.
Thermal shutdown circuitry will shutdown the I_LED output
when the junction temperature exceeds approximately
150°C. It will re-enable the I_LED output once the junction
temperature drops back to approximately 135°C. The
LTC3215 will cycle in and out of thermal shutdown indefi-
nitely without latch up or damage until the heat source is
removed.
Soft-Start
To prevent excessive inrush current during start-up and
mode switching, the LTC3215 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 250µs.
Charge Pump Strength
When the LTC3215 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).
3215f
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