Datasheet

9
LT3466-1
34661f
In the event one of the converters has an output open-circuit,
its output voltage will be clamped at 39.5V. However, the
other converter will continue functioning properly. The photo
in Figure 4b shows circuit operation with converter 1 output
open-circuit and converter 2 driving the OLED display. Con-
verter 1 starts switching at a lower inductor current and
begins skipping pulses, thereby reducing its input current.
Converter 2 continues functioning properly.
Soft-Start
The LT3466-1 has a separate internal soft-start circuitry
for each converter. Soft-start helps to limit the inrush
current during start-up. Soft-start is achieved by clamping
the output of the error amplifier during the soft-start
period. This limits the peak inductor current and ramps up
the output voltage in a controlled manner.
The converter enters into soft-start mode whenever the
respective CTRL pin is pulled from low to high. Figure 5
shows the start-up waveforms with converter 1 driving six
LEDs at 20mA. The filtered input current, as shown in
Figure 5, is well controlled. The soft-start circuitry is less
effective when driving a higher number of LEDs.
Undervoltage Lockout
The LT3466-1 has an undervoltage lockout circuit which
shuts down both converters when the input voltage drops
below 2.1V (typ). This prevents the converter from switch-
ing in an erratic mode when powered from low supply
voltages.
OPERATIO
U
Figure 4a. Transient Response of Switcher 1 with LED1
Disconnected from the Output
Figure 4b. Output 1 Open-Circuit Waveforms Figure 5. Start-Up Waveforms
V
OUT1
10V/DIV
200µs/DIV
LED1 DISCONNECTED AT THIS POINT
V
IN
= 3.3V
CIRCUIT OF FIGURE 1
34661 F04a
I
L1
200mA/DIV
V
OUT1
20V/DIV
V
FB1
200mV/DIV
200µs/DIV
V
IN
= 3.6V
6 LEDs, 20mA
CIRCUIT OF FIGURE 1
34661 F05
I
IN
200mA/DIV
CTRL1
5V/DIV
V
SW1
50V/DIV
V
SW2
50V/DIV
1µs/DIV
V
IN
= 3.6V
CIRCUIT OF FIGURE 1
34661 F04b
I
L1
100mA/DIV
I
L2
100mA/DIV