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LT3466-1
OPERATIO
Main Control Loop
The LT3466-1 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. It incorporates two similar, but fully independent PWM
converters. Operation can be best understood by referring
to the Block Diagram in Figure 2. The oscillator, start-up
bias and the bandgap reference are shared between the
two converters. The control circuitry, power switch, Schot-
tky diode etc., are similar for both converters.
At power-up, the output voltages V OUT1 and V OUT2 are
charged up to V IN (input supply voltage) via their respec-
tive inductor and the internal Schottky diode. If either
CTRL1 and CTRL2 or both are pulled high, the bandgap
reference, start-up bias and the oscillator are turned on.
Working of the main control loop can be understood by
following the operation of converter 1. 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
feedback voltage and the 200mV reference voltage. In this
Minimum Output Current
The LT3466-1 can drive a 6-LED string at 3mA LED current
without pulse skipping. As current is further reduced, the
device may 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 3 shows circuit operation with 6 white LEDs at 3mA
current driven from 3.6V supply. Peak inductor current is
less than 50mA and the regulator operates in discontinu-
ous mode implying that the inductor current reached zero
during the discharge phase. After the inductor current
reaches zero, the switch pin exhibits ringing due to the LC
tank circuit formed by the inductor in combination 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 300 ? resistor across the inductors, al-
though this will degrade efficiency.
V OUT1
20mV/DIV
(AC-COUPLED)
V SW1
20V/DIV
I L1
50mA/DIV
manner, the error amplifier A1 regulates the voltage at the
FB1 pin to 200mV. The output of the error amplifier A1 sets
V IN = 3.6V 0.5 μ s/DIV
I LED1 = 3mA
CIRCUIT OF FIGURE 1
34661 F03
the correct peak current level in inductor L1 to keep the
output in regulation. The CTRL1 pin voltage is used to
adjust the feedback voltage.
The working of converter 2 is similar to converter 1 with
the exception that the feedback 2 reference voltage is
800mV. The error amplifier A1 in converter 2 regulates the
voltage at the FB2 pin to 800mV. If only one of the
converters is turned on, the other converter will stay off
and its output will remain charged up to V IN (input supply
voltage). The LT3466-1 enters into shutdown, when both
CTRL1 and CTRL2 are pulled lower than 70mV. The CTRL1
and CTRL2 pins perform independent dimming and shut-
down control for the two converters.
Figure 3. Switching Waveforms
Overvoltage Protection
The LT3466-1 has internal overvoltage protection for both
converters. In the event the white LEDs are disconnected
from the circuit or fail open, the converter 1 output voltage
is clamped at 39.5V (typ). Figure 4(a) shows the transient
response of the circuit in Figure 1 with LED1 disconnected.
With the white LEDs disconnected, the converter 1 starts
switching at the peak current limit. The output of converter
1 starts ramping up and finally gets clamped at 39.5V (typ).
The converter 1 will then switch at low inductor current to
regulate the output voltage. Output voltage and input
current during output open circuit are shown in the Typical
Performance Characteristics graphs.
34661f
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