LT3498
OPERATION—LED DRIVER
The LED portion of the LT3498 uses a constant-frequency,
current mode control scheme to provide excellent line
and load regulation. Operation can be best understood
by referring to the Block Diagram.
At power-up, the capacitor at the CAP1 pin is charged up
to V
IN
(input supply voltage) through the inductor and the
internal Schottky diode. If CTRL1 is pulled higher than
125mV, 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
the difference between the V
CAP1
and V
LED1
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 CTRL1 pin is used to
adjust the LED current. The LED Driver is shutdown when
CTRL1 is pulled lower than 75mV.
Minimum Output Current
The LED Driver of the LT3498 can drive a 4-LED string at
2mA LED current, without pulse-skipping, using the same
external components 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 1 details circuit operation driving four 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 SW1
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.
I
L
50mA/DIV
V
SW
10V/DIV
V
IN
= 4.2V
I
LED
= 2mA
4 LEDs
200ns/DIV
3498 F01
Figure 1. Switching Waveforms with
Four White LEDs at 2mA Load
3498f
8
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