6
LT3491
3491fa
OPERATIO
U
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
V
IN
(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 V
CAP
and V
LED
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.
Figure 2. Switching Waveforms
V
SW
10V/DIV
I
L
50mA/DIV
V
IN
= 4.2V
I
LED
= 2mA
3 LEDs
200ns/DIV
3491 F02
