LT3486 データシートの表示（PDF） - Linear Technology

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LT3486

Dual 1.3A White LED Step-Up Converters with Wide Dimming

Linear Technology 

LT3486

OPERATION

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.

IL1

1A/DIV

VOUT1

20V/DIV

VC1

2V/DIV

VIN = 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

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 con-

verter 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 tempera-

ture 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.

IL1

1A/DIV

VOUT1

1V/DIV

AC COUPLED

IL2

500mA/DIV

VIN = 3.6V

2ms/DIV

CIRCUIT OF FRONT PAGE APPLICATION

LED1 DISCONNECTED

3486 F03b

Figure 3b. Switching Waveforms with Output 1 Open Circuit

IIN

200mA/DIV

VOUT2

10V/DIV

VFB2

200mV/DIV

CTRL2

5V/DIV

VIN = 3.6V

8 LEDs, 25mA

0.5ms/DIV

CIRCUIT OF FRONT PAGE APPLICATION

Figure 4. Start-Up Waveforms

3486 F04

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