LT3486
OPERATIO
output open circuit are shown in the Typical Performance
Characteristics graphs.
Figure 3a shows the transient response of switcher 1 with
the LEDs disconnected from the output. When the LED1
string is disconnected from the output, the voltage at
the feedback pin (FB1) drops to 0V. As a result, the error
amplifier charges up the V
C
node to the clamp voltage
level of 1.5V (typ). The converter starts switching at peak
current limit and ramps up the output voltage. When the
output voltage reaches the OVP clamp voltage level of
36V (typ), the LT3486 shuts off the converter by pulling
the V
C
node to ground. The converter then regulates the
output voltage at 36V (typ) by switching at a very low
frequency.
In the event one of the converters has an output open-
circuit, its output voltage will be clamped at 36V (typ).
However, the other converter will continue functioning
properly. The photo in Figure 3b shows circuit operation
with converter 1 output open-circuit and converter 2 driving
I
L1
1A/DIV
V
OUT1
20V/DIV
V
C1
2V/DIV
V
IN
= 3.6V
CIRCUIT OF
FRONT PAGE
APPLICATION
100µs/DIV
LED1 DISCONNECTED
AT THIS INSTANT
3486 F03a
Figure 3a. Transient Response of Switcher 1 with LED1
Disconnected from the Output
I
L1
1A/DIV
V
OUT1
1V/DIV
AC COUPLED
I
L2
500mA/DIV
V
IN
= 3.6V
2ms/DIV
CIRCUIT OF FRONT PAGE APPLICATION
LED1 DISCONNECTED
3486 F03b
Figure 3b. Switching Waveforms with Output 1 Open Circuit
U
eight LEDs at 25mA. Converter 1 starts switching at a very
low frequency, reducing its input current.
Soft-Start
The LT3486 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 4
shows the start-up waveforms with converter 2 driving
eight LEDs at 25mA. The filtered input current, as shown
in Figure 4, is well controlled. The soft-start circuit is more
effective when driving a smaller number of LEDs.
Undervoltage Lockout
The LT3486 has an undervoltage lockout circuit which
shuts down both the converters when the input voltage
drops below 2.1V (typ). This prevents the converter to
operate in an erratic mode when powered from low supply
voltages.
Overtemperature Protection
The maximum allowable junction temperature for LT3486 is
125°C. In normal operation, the IC’s junction temperature
should be kept below 125°C at an ambient temperature of
85°C or less. If the junction temperature exceeds 150°C, the
internal thermal shutdown circuitry kicks in and turns-off
both the converters. The converters will remain off until
the die temperature falls below 150°C.
I
IN
200mA/DIV
V
OUT2
10V/DIV
V
FB2
200mV/DIV
CTRL2
5V/DIV
V
IN
= 3.6V
0.5ms/DIV
8 LEDs, 25mA
CIRCUIT OF FRONT PAGE APPLICATION
3486 F04
Figure 4. Start-Up Waveforms
3486f
9
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