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LT3491ESC8

White LED Driver with Integrated Schottky in SC70 and 2mm × 2mm DFN

Linear Technology 

LT3491

U

OPERATIO

The LT3491 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 1.

At power up, the capacitor at the CAP pin is charged up to

VIN (input supply voltage) through the inductor and the

internal Schottky diode. If CTRL is pulled higher than

100mV, the bandgap reference, the start-up bias and the

oscillator are turned on. At the start of each oscillator

cycle, the power switch Q1 is turned on. A voltage propor-

tional 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 VCAP and VLED voltage and

the bandgap reference. In this manner the error amplifier,

A1, sets the correct peak current level in inductor L1 to

keep the output in regulation. The CTRL pin is used to

adjust the LED current. The LT3491 enters into shutdown

when CTRL is pulled lower than 50mV.

Minimum Output Current

The LT3491 can drive a 3-LED string at 2mA LED current

without pulse skipping using the same external compo-

nents shown in the application circuit on the front page of

this data sheet. As current is further reduced, the device

will begin skipping pulses. This will result in some low

frequency ripple, although the average LED current re-

mains regulated down to zero. The photo in Figure 2

details circuit operation driving three white LEDs at 2mA

load. Peak inductor current is less than 60mA and the

regulator operates in discontinuous mode, meaning the

inductor current reaches zero during the discharge phase.

After the inductor current reaches zero, the SW pin

exhibits ringing due to the LC tank circuit formed by the

inductor in combination with the switch and the diode

capacitance. This ringing is not harmful; far less spectral

energy is contained in the ringing than in the switch

transitions.

IL

50mA/DIV

VSW

10V/DIV

VIN = 4.2V

ILED = 2mA

3 LEDs

200ns/DIV

3491 F02

Figure 2. Switching Waveforms

3491fa

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