LT3497
8
3497f
OPERATION
Main Control Loop
The LT3497 uses a constant frequency, current mode con-
trol 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 band gap reference are shared between the two
converters. The control circuitry, power switch, Schottky
diode etc., are identical for both the converters.
At power up, the capacitors at CAP1 and CAP2 pins are
charged up to V
IN
(input supply voltage) via their respective
inductor and the internal Schottky diode. If either CTRL1
and CTRL2 or both are pulled higher than 100mV, the
bandgap reference, the start-up bias and the oscillator
are turned on.
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 amplifi er, A1, and is simply an amplifi ed version of
the difference between the V
CAP1
and V
LED1
voltage and
the bandgap reference. In this manner the error amplifi er,
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.
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 LT3497 enters into shutdown
when both CTRL1 and CTRL2 pins are pulled lower than
50mV. The CTRL1 and CTRL2 pins perform independent
dimming and shutdown control for the two converters.
Minimum Output Current
The LT3497 can drive a 4-LED string at 2mA 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
average LED current remains regulated down to zero. The
photo in Figure 2 details circuit operation driving 4 white
LEDs at 2mA. Peak inductor current is less than 50mA and
the regulator operates in discontinuous mode, meaning
the inductor current reaches zero during the discharge
phase. After the inductor current reaches zero, the SW
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.
Figure 2. Switching Waveforms
I
L
50mA/DIV
V
SW
10V/DIV
V
IN
= 4.2V
I
LED
= 2mA
4 LEDs
200ns/DIV
3497 F02
