lt3465a Linear Technology Corporation, lt3465a Datasheet - Page 7

• Manufacturer Part Number: lt3465a
• Description: 2.4mhz White Led Step-up Converters With Built-in Schottky In Thinsot
• Manufacturer: Linear Technology Corporation

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.
U
U
W
20mA/DIV
10mV/DIV
20mA/DIV
10mV/DIV
5V/DIV
5V/DIV
V
V
V
V
OUT
OUT
SW
SW
I
I
L
L
Figure 3b. Switching Waveforms (LT3465A)
Figure 3a. Switching Waveforms (LT3465)
V
I
3 LEDs
V
I
3 LEDs
LED
LED
IN
IN
= 4.2V
= 4.2V
= 1.5mA
= 0.2mA
U
0.2µs/DIV
0.1µs/DIV
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 300Ω 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
3465A F03a
3465A F03b
LT3465/LT3465A
3465afa
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