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LT3465AES6 Fiches technique(PDF) 5 Page - Linear Technology

No de pièce LT3465AES6
Description  1.2MHz/2.4MHz White LED Step-Up Converters with Built-In Schottky in ThinSOT
Download  12 Pages
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
Logo LINER - Linear Technology

LT3465AES6 Fiches technique(HTML) 5 Page - Linear Technology

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LT3465/LT3465A
5
3465af
BLOCK DIAGRA
+
+
RQ
S
0.2
SW
DRIVER
COMPARATOR
2
CTRL
4
6
VOUT
VIN
3
FB
200mV
5
+
Σ
RAMP
GENERATOR
OVERVOLTAGE
PROTECT
RC
10k
40k
CC
1.2MHz*
OSCILLATOR
*2.4MHz FOR LT3465A
GND
3465A F02
Q1
A2
A1
VREF
1.25V
1
Figure 2. LT3465 Block Diagram
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.
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
VSW
5V/DIV
IL
20mA/DIV
VOUT
10mV/DIV
VIN = 4.2V
0.2
µs/DIV
3465A F03a
ILED = 1.5mA
3 LEDs
Figure 3b. Switching Waveforms (LT3465A)
VSW
5V/DIV
IL
20mA/DIV
VOUT
10mV/DIV
VIN = 4.2V
0.1
µs/DIV
3465A F03b
ILED = 0.2mA
3 LEDs
Figure 3a. Switching Waveforms (LT3465)


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