Datasheet

9
LT3466
3466fa
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 driv-
ing eight LEDs at 20mA. Converter 1 starts switching at a
lower peak inductor current and begins skipping pulses,
thereby reducing its input current.
Soft-Start
The LT3466 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
four LEDs at 20mA. The filtered input current, as shown in
Figure 5, is well controlled. The soft-start circuit is less
effective when driving a higher number of LEDs.
Undervoltage Lockout
The LT3466 has an undervoltage lockout circuit which
shuts down both the converters when the input voltage
drops below 2.1V (typ). This prevents the converter from
operating 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 5. Start-Up Waveforms
I
IN
100mA/DIV
V
FB1
200mV/DIV
CRTL1
2V/DIV
V
IN
= 3.6V 100µs/DIV 3466 F05
4 LEDs, 20mA
L = 15µH
C = 0.47µF
V
OUT1
5V/DIV
V
OUT1
10V/DIV
I
L1
200mA/DIV
200µs/DIV 3466 F04a
LED1 DISCONNECTED AT THIS INSTANT
V
IN
= 3.3V
CIRCUIT OF FIGURE 1
I
L1
50mA/DIV
V
SW1
50V/DIV
I
L2
100mA/DIV
V
SW2
50V/DIV
Figure 4b. Switching Waveforms with Output 1 Open-Circuit
V
IN
= 3.3V 1µs/DIV 3466 F04b
CIRCUIT OF FIGURE 1
LED1 DISCONNECTED