LT3477 datasheet(19/26 Pages) AD

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LT3950

Rev. 0

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Excepting LED Open, all reported faults will result in inter-

ruption of switching. The Short LED and Overcurrent

faults trigger a cooldown period, after which the system

will enter soft-start as if it had just been turned on via the

EN/UVLO pin. Upon successful completion of the soft-

start process with no faults, the FAULT signal will be de-

asserted. During soft-start, the PWM Dimming logic low

state is ignored for both internal and external PWM until

soft start is complete or current in the LED reaches 10%

of full scale value.

Table 6.

CONDITION (TYP.)

TYPE OF FAULT

FB Pin >1.3V

Overvoltage

1.17V < FB pin < 1.3V and VISP-ISN < 25mV

Open LED

FB pin < 300mV

Short LED

VISP-ISN > ~700mV

Overcurrent

Soft Start

To prevent a surge of current at start-up, LT3950 uses

an internal soft-start. This feature affects both current

limit and switching frequency. At start-up, the system will

switch at around 40% fSW with near zero current limit. As

the clock runs, both constraints relax, leading to normal

operation after ~1800 • TSW(NOM). Note that this process

does not take 1800 cycles to complete, rather the above

number takes into account the reduction in clock fre-

quency enforced by the soft-start feature. In total, 1,024

cycles are spent in soft-start, with fSW adjustments every

16 cycles.

The cooldown period triggered by certain faults dis-

cussed in the LED Faults Section lasts for at least

32,768 • TSW(NOM). At the end of the cooldown period,

the system attempts to start again, and enters soft-start

as if it had just been powered on for the first time.

Regardless of whether it is the first-time soft-start or a

soft-start resulting from a fault condition, the LT3950 will

wait until the first PWM dimming pulse arrives before

beginning to power on the system. This PWM pulse can

be from an external source or from the internal dimming

PWM generator. At the arrival of the first PWM pulse,

soft-start begins and operation continues until the system

has reached steady state. Until the system either finishes

APPLICATIONS INFORMATION

soft-start or observes at least 10% of full-scale current

flowing through the sense resistor, the PWM low logic

state is ignored and the part runs continuously. If any

fault condition caused the part to re-enter soft-start, the

FAULT pin will remain asserted until the part successfully

completes the soft-start process.

Figure 8. LED Short Hiccup Mode

IL (500mA/DIV)

ILED (500mA/DIV)

TIME (ms)

–2.6 –0.6 1.4 3.4 5.3 7.3 9.3 11.3 13.3 15.3 17.3

3950 F08

Using the EN/UVLO Pin

The EN/UVLO pin offers two modes of operation. First, it

can be driven high (above 1.4V) or low (below 1V) to act

as an enable pin, turning the part on and off. In addition to

this, the pin can also be used to create an accurate exter-

nal under voltage lockout (UVLO). To use this feature,

connect the EN/UVLO pin to a resistive voltage divider

from the VIN pin to ground. Select resistors to program

the desired minimum VIN value for operation.

VIN FALLING

(

) =1.25V • 1+

RUV1

RUV2

⎛

⎝⎜

⎞

⎠⎟

VIN RISING

(

) =1.28V • 1+

RUV1

RUV2

⎛

⎝⎜

⎞

⎠⎟

+2µA •RUV1

Figure 9.

LT3950

3950 F09

RUV1

RUV2

EN/UVLO

VIN

Programming VIN Undervoltage Setpoint

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