SC441D

4

Applications Information (continued)

tap connected to OVPIN.

Note: If this disconnect function is not desired, bypass

the OVPRTN pin and connect the end of the OVP resistor

divider directly to GND. The OVPIN pin is sensitive to

noise, and a proper decoupling capacitor (nF ~ 0nF) is

required. The combined impedance of the resistor divider

for OVPIN should be greater than 200kΩ.

LED Short-Circuit Protection

If one or more LEDs are detected as shortened, that

corresponding string will be latched off if SCP_EN is

floating. The voltages on all internal LED current sources

(IO pins) are monitored to see if any exceeds the setting

voltage (The IO voltage on abnormal LED string will rise

earlier than other floating LED strings). If any IO pin voltage

exceeds the setting voltage, that IO current source will be

latched off. The latch can be reset if VIN falls below UVLO

or recycle EN signal. Other normal LED strings remain at

their normal operation. The protection will be disabled

if SCP_EN is tied to GND. If all IO pin voltages reach 0.8V

then the internal main switch will be off until any of the IO

voltages is lower than 0.7V.

The setting voltage for SCP can be programmed by R33

(see the schematic on page 8). The relationship can be

described as follows:

VSCP(V)=0.05 X R

Where VSCP is the SCP setting voltage in Volt and R

kohm.

Unused Strings

The SC44D may be operated with less than 4 strings.

In this case, all unused strings should be tied to VIN and

leave the SCP_EN pin floating.

LED Open-Circuit Protection

If any LED string is detected as an open-circuit, that

string will latch off. If any given string is open, the IO

current source will go to deep saturation; the COMP and

FFLAG pins will be driven high and the boost converter

duty cycle will increase causing VOUT to rise. At some

point VOUT will rise high enough to cause all the IO pin

voltages of the intact strings to reach the shorted LED

detection level and latch off those strings. Because of the

LED open string VOUT will continue to rise until it reaches

the programmed OVP level.

When OVP is reached, the voltage on the IO pins are

monitored and if any IO voltage is less than 0.2V that

string will be identified as open and will latch off.

Only when VIN falls below UVLO, or an EN signal is re-

cycled, and if thermal shutdown occurs, can this latch be

reset. A hiccup cycle is then initiated and the SS pin is

discharged slowly with a µA current source and a 0.9mA

discharge path (turned on to pull down VOUT). When SS

falls below 0.5V and VOUT falls below to VIN, the shorted

LED detection latches are reset and a new soft-start se-

quence is initiated to resume normal operation.

Thermal Shutdown (TSD)

a hiccup sequence is initiated where the boost converter

and all IO current sources are turned off. SS is discharged

by a µA current source, and a 0.9mA discharge path is

turned on to pull down VOUT. As temperature falls below

the TSD trip point, SC44D will retry when SS falls below

0.5V and VOUT falls to VIN.

PWM Dimming

ThePWMinputneedstobeheldhighfornormaloperation.

PWM dimming can be done by cycling the PWM input at a

given frequency where a“low”on the PWM input turns off

all IO current sources and a “high” turns on all IO current

sources. The short and open detection latches are blanked

for approximately 2µs as the PWM input transitions from

low-to-high to prevent a false fault detection during PWM

dimming.

The PWM pin can be toggled by external circuitry to allow

PWM dimming. In a typical application, a microcontroller

sets a register – or counter, that varies the pulse-width

on a GPIO pin. The SC44D allows dimming over two

decades in frequency (50Hz–50kHz), in order to allow

compatibility with a wide range of devices, including 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 generated while

keeping the instantaneous LED current at its peak value

for luminescent efficiency and color purity. Furthermore,

advanced lighting effects such as backlight dim-on can be

implemented as the SC44D can resolve 0µs (minimum),

PWM dimming pulse-width.

As far as the maximum PWM dimming pulse-width, it