LTC3214

6

3214fa

OPERATIO

Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit

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CPO

3214 F01

OR

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

provides maximum efficiency and minimum noise. The

LTC3214 will remain in this mode until the forward voltage

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

the 1.5x mode the flying capacitors are charged in series

during the first clock phase, and stacked in parallel on top

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

and GND. The resistor value needed to attain the desired

current level can be determined by Equation 1.

(1)

Overcurrent shutdown mode will prevent damage to the

part by shutting down the high power sections of the chip.

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

a resistance of approximately 43k

Ω. 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

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 150

µs.

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