LTC3215EDD 查看數據表（PDF） - Linear Technology

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LTC3215EDD

700mA Low Noise High Current LED Charge Pump

Linear Technology 

LTC3215

OPERATION

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

VCPO

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 VIN supply.

Furthermore, CPO is weakly connected to VIN. 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 ILED output

when the junction temperature exceeds approximately

150°C. It will re-enable the ILED output once the junc-

tion temperature drops back to approximately 135°C.

The LTC3215 will cycle in and out of thermal shutdown

indefinitely 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, ROL(Figure 1).

1.5VIN

OR

2VIN

–+

ROL

+

CPO

–

3215 F01

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

ROL is dependent on a number of factors including the

oscillator frequency, flying capacitor values and switch

resistances.

From Figure 1, we can see that the output current is

proportional to:

(1.5VIN – CPO)/ROL or (2VIN – CPO)/ROL

(2)

in the 1.5x mode or 2x mode respectively.

LED Current Programming

The LTC3215 includes an accurate, programmable cur-

rent source that is capable of driving LED currents up to

350mA continuously and up to 700mA for pulsed opera-

tion. Pulsed operation may be achieved by toggling the

EN pin. In either continuous or pulsed operation, proper

board layout is required for effective heat sinking.

The current may be programmed using a single external

resistor. Equation 1, used to calculate the resistor value

from the desired current level is repeated below:

RSET = 3990/ILED

(1)

For applications requiring multiple current levels, several

schemes may be used to change the resistance for the RSET

resistor. Figure 2 shows two such schemes. The circuit

in Figure 2a uses the I/O output of a microcontroller to

switch a second resistor (R2) in parallel or series with R1,

changing the effective ISET current. Alternatively, the circuit

in Figure 2b uses a pulse-width modulator (PWM) to vary

the current through RSET, which changes the LED current.

3215fc

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