LT3486 datasheet(9/20 Pages) LINER

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LT3486

3486f

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 VC 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 VC 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

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.

OPERATIO

VOUT1

20V/DIV

VC1

2V/DIV

IL1

1A/DIV

100

µs/DIV

3486 F03a

VIN = 3.6V

CIRCUIT OF

FRONT PAGE

APPLICATION

LED1 DISCONNECTED

AT THIS INSTANT

VOUT1

1V/DIV

AC COUPLED

IL1

1A/DIV

IL2

500mA/DIV

2ms/DIV

3486 F03b

VIN = 3.6V

CIRCUIT OF FRONT PAGE APPLICATION

LED1 DISCONNECTED

VOUT2

10V/DIV

VFB2

200mV/DIV

IIN

200mA/DIV

CTRL2

5V/DIV

0.5ms/DIV

3486 F04

VIN = 3.6V

8 LEDs, 25mA

CIRCUIT OF FRONT PAGE APPLICATION

Figure 3a. Transient Response of Switcher 1 with LED1

Disconnected from the Output

Figure 3b. Switching Waveforms with Output 1 Open Circuit

Figure 4. Start-Up Waveforms

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