LT3486
OPERATION
Main Control Loop
The LT3486 uses a constant frequency, current mode
control scheme to provide excellent line and load regulation.
It incorporates two identical, but fully independent PWM
converters. Operation can be best understood by referring
to the block diagram in Figure 1. The oscillator, start-up
bias and the bandgap reference are shared between the two
converters. The control circuitry, power switch, dimming
control etc., are all identical for both converters.
At power-up, the output capacitors of both converters are
charged up to V
IN
(input supply voltage) via their respective
inductor and the Schottky diode. If the
SHDN
pin is taken
above 1.6V, the bandgap reference, start-up bias and the
oscillator are turned on. Grounding the
SHDN
pin shuts
down the part.
The CTRL1 and CTRL2 pins perform independent dimming
and shutdown control for the two converters. Taking
the CTRL pins high, enables the respective converters.
Connecting these pins to ground, shuts down each
converter by pulling their respective V
C
pin low.
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 manner, the error amplifier A1 regulates the feedback
voltage to 200mV reference voltage. The output of the
error amplifier A1 sets the correct peak current level in
inductor L1 to keep the output in regulation. The CTRL1
pin voltage is used to adjust the reference voltage.
The PWM1, 2 control pins are used to extend the dimming
range for the individual converter. The LED current in each
string can be controlled down to µA levels by feeding
a PWM signal to these pins. Refer to the Applications
Information section for more detail.
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).
Minimum Output Current
The LT3486 can drive an 8-LED string at 4mA 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 2 shows circuit operation with 8 white LEDs
at 4mA current driven from 3.6V supply. Peak inductor
current is less than 200mA and the regulator operates in
discontinuous 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 300Ω resistor across the inductors,
although this will degrade efficiency.
V
OUT2
10mV/DIV
V
SW2
20V/DIV
I
L2
200mA/DIV
V
IN
= 3.6V
0.5µs/DIV
I
LED2
= 4mA (8 LEDs)
CIRCUIT OF FRONT PAGE APPLICATION
3486 F02
Figure 2. Switching Waveforms
Open-Circuit Protection
The LT3486 has internal open-circuit protection for both
the converters. Connect the overvoltage protection pins
(OVP1, OVP2) to the output of the respective converter.
When the LEDs are disconnected from the circuit or fail
open, the on-chip voltage detectors monitor the voltages at
the OVP1 and OVP2 pins and limits these voltages to 36V
(typ) by turning off the respective switcher. The converter
will then switch at a very low frequency to minimize the
input current. Output voltage and input current during
3486fe
8
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