BESCHREIBUNG

The LTC3453 is a synchronous buck-boost DC/DC converter optimized for driving up to 4 white LEDs at a combined current of up to 500mA from a single Li-Ion battery input. The regulator operates in either synchronous buck, synchronous boost, or buck-boost mode, depending on input voltage and LED maximum forward voltage. Optimum efficiency is achieved using a proprietary architecture that determines which LED requires the largest forward voltage drop at its programmed current, and regulates the common output rail for lowest dropout. Efficiency of 90% can be achieved over the entire usable range of a Li-Ion battery (2.7V to 4.2V). LED current is programmable to one of four levels (including shutdown) with dual current setting resistors and dual enable pins. In shutdown, the supply current is only 6µA. A high constant operating frequency of 1MHz allows the use of a small external inductor. The LTC3453 is offered in a low profile (0.75mm) thermally enhanced 16-lead (4mm x 4mm) QFN package.

 

FEATURES

High Efficiency: 90% Typical Over Entire Li-Ion Battery Range

Wide VIN Range: 2.7V to 5.5V

Up to 500mA Continuous Output Current

Internal Soft-Start

Open/Shorted LED Protection

LED Current Matching Typically <2%

Constant Frequency 1MHz Operation

Low Shutdown Current: 6µA

Overtemperature Protection

Small Thermally Enhanced 16-Lead (4mm x 4mm) QFN Package

 

ANWENDUNGEN

Cell Phones

Digital Cameras

PDAs

Portable Devices

 

OPERATION

Buck-Boost DC-DC Converter

The LTC3453 employs an LTC proprietary buck-boost DC/DC converter to generate the output voltage required to drive the LEDs. This architecture permits high-efficiency, low noise operation at input voltages above, below or equal to the output voltage by properly phasing four internal power switches. The error amp output voltage on the VC pin determines the duty cycle of the switches. Since the VC pin is a filtered signal, it provides rejection of frequencies well below the factory trimmed switching frequency of 1MHz. The low RDS(ON), low gate charge synchronous switches provide high frequency pulse width modulation control at high efficiency. Schottky diodes across synchronous rectifier switch B and synchronous rectifier switch D are not required, but if used do provide a lower voltage drop during the break-before-make time (typically 20ns), which improves peak efficiency by typically 1% to 2% at higher loads.

 

Undervoltage Lockout

To prevent operation of the power switches at high RDS(ON), an undervoltage lockout is incorporated on the LTC3453. When the input supply voltage drops below approximately 1.9V, the four power switches and all control circuitry are turned off except for the undervoltage block, which draws only several microamperes.

 

Overtemperature Protection

If the junction temperature of the LTC3453 exceeds 130°C for any reason, all four switches are shut off immediately. The overtemperature protection circuit has a typical hysteresis of 11°C.

 

Soft-Start

The LTC3453 includes an internally fixed soft-start which is active when powering up or coming out of shutdown. The soft-start works by clamping the voltage on the VC node and gradually releasing it such that it requires 0.65ms to linearly slew from 0.9V to 2.1V. This has the effect of limiting the rate of duty cycle change as VC transitions from the buck region through the buck-boost region into the boost region. Once the soft-start times out, it can only be reset by entering shutdown, or by an undervoltage or overtemperature condition.

 

LED Current Sources

Each LED pin is driven by a current source specifically designed for low dropout. The LTC3453 employs a proprietary architecture that determines which of the four LEDs requires the largest forward voltage drop at its programmed current, and then generates a feedback voltage based on this one for closing the buck-boost regulation loop. This results in the lowest output voltage required for regulating all of the LEDs and thus the highest LED power efficiency. The voltage present at the LED pin of the “controlling LED” will be typically 130mV at 75mA of current.