SC5014AEVB datasheet(14/27 Pages) SEMTECH

14 / 27 page

SC5014A

LED Short-Circuit Protection (SCP)

The SC504A features a programmable LED short-circuit

protection (SCP). This allows the part to be customized

based on the LED forward voltage (V

F) mismatches between

the LED strings. If one or more LEDs are detected as short-

circuited, the corresponding string will be latched off. The

voltages on both IO pins are monitored to check if any IO

pin exceeds the SCP trip point. The IO voltage for LED

string(s) with faulty short-circuit LED(s) will be higher than

other normal IO pin voltage. This LED short-circuit protec-

tion trip level (see detailed application circuit on page 20,

Figure 2) is expressed by the following equation:

SCP_Trip [V] = 20 X (.23 X R4) / (R4 + R5)

If any IO pin voltage exceeds the trip voltage, the IO current

sink will be latched off and the FLT will go low.This latch can

be reset by cycling UVLO, VCC or EN. Other LED string is

unaffected and continue in normal operation. This protec-

tion will be disabled if SCP is tied to VCC.

There is a typical 0μs SCP detection time in PWM dimming

applications. If the PWM dimming on-time is less than the

SCP detection time, the SCP cannot be enabled.

In many applications, LED strings are connected to the IO

pins through a mechanical connector, which cannot support

an electrical connection at specific times. This connection

might cause noise on the IO pins. If this noise is large enough,

it may trigger a false SCP mode. Under such condition, a

ceramic decoupling capacitor (00pF ~ 8.2nF) between IO

pin to ground can help prevent the SC504A from entering

the protection mode by false trigger. Or, simply disable this

feature by connecting SCP pin to VCC pin.

LED Open-Circuit Protection

If any LED string becomes open, the respective IO pin

voltage will be pulled to ground. Consequently, the inter-

nal COMP node (output of error amplifier) is driven high,

which causes the boost output voltage to increase. The

output voltage will be eventually clamped to a voltage set

by the OVP resistor divider. Under this condition, the faulty

string is latched off and the FLT pin is pulled low. The boost

voltage gets regulated to the voltage required to set all

non-faulty IO pins above 0.9V (typ). The other string

remain in normal operation. The FLT and the faulty LED

Applications Information (continued)

current sink latch-off can be reset by cycling UVLO, VCC or

EN.

LED Analog Dimming Control

The LED current in SC504A can be dimmed via the 5-bit

analog dimming register (register address: 0x02). The LED

current can be adjusted in 32 steps from 0mA to maximum

value, which is determined by the R

ISET resistor.

The SC504A has a unique DAC architecture which allows

it to have excellent LED current accuracy and string-to-

string matching over the entire DAC range.

The analog dimming method can be used in conjunction

with PWM dimming to increase the dimming resolution.

The fast loop response of the SC504A allows the LED

current to transition to a new value within 60µs or so.

Please refer to the graphs in the typical characteristics

section.

LED PWM Dimming Control

The PWM input needs to be held high for normal opera-

tion. PWM dimming can be achieved by cycling the PWM

input at a given frequency where a “low” on the PWM

input turns off both IO current sinks and a“high”turns on

all IO current sinks. The PWM pin can be toggled by exter-

nal circuitry to allow PWM dimming. In a typical applica-

tion, a microcontroller sets a register or counter that

varies the pulse width on a GPIO pin. The SC504A allows

dimming over a wide frequency range (00Hz-30kHz) in

order to allow compatibility with a wide range of devices.

This includes the newest dimming strategies that avoid

the audio band by using high frequency PWM dimming.

In this manner, a wide range of illumination can be gener-

ated while keeping the instantaneous LED current at its

peak value for high efficiency and color temperature. The

SC504A provides a 000: dimming range at kHz PWM

frequency. The LED current sinks turn on/off very rapidly

(<25ns, typical). This allows a wide dimming ratio. An

additional advantage of PWM dimming is that it avoids in-

rush currents when filling the boost output capacitor.

Simply apply the PWM signal to the device at 0% duty for

a millisecond or two, and in-rush current is reduced. This

dimming time will vary based on the number of LEDs and

the size of the output capacitor. This can be easily deter-

mined during testing and programmed into the microcon-

troller firmware.

SC5014AEVB датащи(PDF) 14 Page - Semtech Corporation

SC5014AEVB датащи(HTML) 14 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

датащи скачать

ссылки URL