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LT3465/LT3465A

3465afa

APPLICATIO S I FOR ATIO

Operation

The LT3465 uses a constant frequency, current mode

control scheme to provide excellent line and load regula-

tion. Operation can be best understood by referring to the

block diagram in Figure 2. At the start of each oscillator

cycle, the SR latch is set, which turns on the power switch

Q1. 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

SR latch is reset turning 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 reference voltage of

200mV. In this manner, the error amplifier sets the

correct peak current level to keep the output in regulation.

If the error amplifier’s output increases, more current is

delivered to the output; if it decreases, less current is

delivered. The CTRL pin voltage is used to adjust the

reference voltage. The block diagram for the LT3465A (not

shown) is identical except that the oscillator frequency

is 2.4MHz.

Minimum Output Current

The LT3465 can drive a 3-LED string at 1.5mA LED

current without pulse skipping. As current is further

reduced, the device will 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 3a details circuit operation

driving three white LEDs at a 1.5mA load. Peak inductor

current is less than 40mA and the regulator operates in

discontinuous mode, meaning the inductor current

reaches zero during the discharge phase. After the induc-

tor current reaches zero, the SW pin exhibits ringing due

to the LC tank circuit formed by the inductor in combina-

tion 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 application of a 300

Ω resistor across the

inductor, although this will degrade efficiency. Because

of the higher switching frequency, the LT3465A can drive

a 3-LED string at 0.2mA LED current without pulse

VSW

5V/DIV

20mA/DIV

VOUT

10mV/DIV

VIN = 4.2V

0.2

µs/DIV

3465A F03a

ILED = 1.5mA

3 LEDs

Figure 3b. Switching Waveforms (LT3465A)

VSW

5V/DIV

20mA/DIV

VOUT

10mV/DIV

VIN = 4.2V

0.1

µs/DIV

3465A F03b

ILED = 0.2mA

3 LEDs

Figure 3a. Switching Waveforms (LT3465)

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