LT3591

6

3591f

Figure 2. Switching Waveforms

OPERATION

The LT3591 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

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 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

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 LT3591 enters into shutdown when

CTRL is pulled lower than 50mV.

Minimum Output Current

The LT3591 can drive a 2-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 remains

regulated down to zero. The photo in Figure 2 details circuit

operation driving two white LEDs at 2mA load. Peak induc-

tor current is less than 40mA 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.

5V/DIV

20mA/DIV

2 LEDs

500ns/DIV

3591 F02