SC440AEVB datasheet(13/21 Pages) SEMTECH

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SC440A

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Applications Information (continued)

LED Open-Circuit Protection

If any LED is detected as open circuit, that string will be

latched off. Then the COMP pin 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 remaining strings to reach

the shorted LED detection level and those strings are

latched off. The FFLAG pin will also go high. Because of

the open string, VOUT will continue to rise until it reaches

the programmed OVP level.

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

monitored. If any IO voltage is less than 0.2V, that string

will be identified as open and will be latched off. Only

VIN falling below UVLO, recycle EN signal and thermal

shutdown will reset this latch. A hiccup cycle is initiated,

SS is discharged slowly with a 1.6

μA current source and

a 1mA discharge path is 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 sequence is initiated to resume normal operation.

Thermal Shutdown (TSD)

If the thermal shutdown temperature of 150°C is reached,

a hiccup sequence is initiated where the boost converter

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

by a 1.6μA current source, and a 1mA discharge path is

turned on to pull down VOUT. As temperature falls below

TSD trip point, SC440A will retry when SS falls below 0.5V

and VOUT falls to VIN.

PWM Dimming

The PWM input needs to be held high for normal operation.

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 SC440A 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 high

efficiency and color temperature.

Furthermore, advanced lighting effects such as backlight

dim-on can be implemented as the SC440A can resolve

PWM from 10% to 90% duty at its highest frequency.

An additional advantage of PWM dimming comes to

customers who prefer to avoid in-rush currents when

filling the boost output capacitor. Simply apply the PWM

signal to the device at 10% duty for a millisecond or two,

and in-rush current is reduced to less than 50mA. This

dim time will vary based on the number of LEDs and the

size of the output capacitor, but can be easily determined

on the bench and programmed into the μC firmware.

Linear Dimming

The linear dimming control is available for SC440A by

applying an external control voltage on IOSET pin through

an external resistor as shown below. External environment

brightness compensation can also be achieved when the

control voltage is generated by a light sensing circuit.

LED String Connection

Generally, LED strings are connected to the IO1 ~ IO6 pins

through a mechanical connector which cannot support

an electrical connection at times. This connection might

cause noise on the IO1 ~ IO6. If this function is enabled,

the SC440A LED short-circuit protection may false trip

when the noise level is large enough. Certain ceramic

decoupling capacitors (100pF ~ 8.2nF) on pins IO1 ~ IO6

to GND can help prevent the SC440A from entering the

R_EXT

R_IOSET

IOSET

V_EXT

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