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LT3466

Dual Full Function White LED Step-Up Converter with Built-In Schottky Diodes

Linear Technology 

LT3466

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OPERATIO

Main Control Loop

The LT3466 uses a constant frequency, current mode

control scheme to provide excellent line and load regula-

tion. It incorporates two identical, but fully independent

PWM converters. Operation can be best understood by

referring to the Block Diagram in Figure 2. The oscillator,

start-up bias and the bandgap reference are shared be-

tween the two converters. The control circuitry, power

switch, Schottky diode etc., are all identical for both the

converters.

At power-up, the output voltages VOUT1 and VOUT2 are

charged up to VIN (input supply voltage) via their respec-

tive inductor and the internal Schottky diode. If either

CTRL1 and CTRL2 or both are pulled high, the bandgap

reference, start-up bias and the oscillator are turned on.

Working of the main control loop can be understood by

following the operation of converter 1. At the start of each

oscillator cycle, the power switch Q1 is turned on. A

voltage proportional to the switch current is added to a

stabilizing ramp and the resulting sum is fed into the

positive terminal of the PWM comparator A2. When this

voltage exceeds the level at the negative input of A2, the

PWM logic turns off the power switch. The level at the

negative input of A2 is set by the error amplifier A1, and is

simply an amplified version of the difference between the

feedback voltage and the 200mV reference voltage. In this

manner, the error amplifier A1 regulates the feedback

voltage to 200mV reference voltage. The output of the

error amplifier A1 sets the correct peak current level in

inductor L1 to keep the output in regulation. The CTRL1

pin voltage is used to adjust the reference voltage.

If only one of the converters is turned on, the other con-

verter will stay off and its output will remain charged up to

VIN (input supply voltage). The LT3466 enters into shut-

down, when both CTRL1 and CTRL2 are pulled lower than

50mV. The CTRL1 and CTRL2 pins perform independent

dimming and shutdown control for the two converters.

Minimum Output Current

The LT3466 can drive an 8-LED string at 2.5mA LED

current without pulse skipping. As current is further

reduced, the device may begin skipping pulses. This will

result in some low frequency ripple, although the LED

current remains regulated on an average basis down to

zero. The photo in Figure 3 shows circuit operation with 16

white LEDs (eight per converter) at 2.5mA current driven

from 3.6V supply. Peak inductor current is less than 50mA

and the regulator operates in discontinuous mode imply-

ing that the inductor current reached zero during the

discharge phase. After the inductor current reaches zero,

the switch pin exhibits ringing due to the LC tank circuit

formed by the inductor in combination with switch and

diode capacitance. This ringing is not harmful; far less

spectral energy is contained in the ringing than in the

switch transitions. The ringing can be damped by applica-

tion of a 300Ω resistor across the inductors, although this

will degrade efficiency.

VOUT1

10mV/DIV

VSW1

20V/DIV

IL1

50mA/DIV

VIN = 3.6V

0.5µs/DIV

ILED1 = 2.5mA

CIRCUIT OF FIGURE 1

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Figure 3. Switching Waveforms

Open-Circuit Protection

The LT3466 has internal open-circuit protection for both

the converters. When the LEDs are disconnected from the

circuit or fail open, the converter output voltage is clamped

at 39.5V (typ). Figure 4a shows the transient response of

Figure 1’s step-up converter with LED1 disconnected.

With LED1 disconnected, the converter starts switching at

the peak inductor current limit. The converter output starts

ramping up and finally gets clamped at 39.5V (typ). The

converter will then switch at low inductor current to

regulate the converter output at the clamp voltage. Output

voltage and input current during output open circuit are

shown in the Typical Performance Characteristics graphs.

In the event one of the converters has an output open-

circuit, its output voltage will be clamped at 39.5V.

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