LTC3215

6

3215f

Table 1. Charge Pump Output Regulation Voltages

CHARGE PUMP MODE

1.5x

4.6V

2x

5.1V

In shutdown mode all circuitry is turned off and the

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.

when the junction temperature exceeds approximately

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

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

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

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 value of this

current may be selected by choosing the appropriate

programming resistor. The resistor is connected between

the desired current level can be determined by Equation 1.

(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.

OPERATIO