SC440A_10 Datasheet, PDF (13/21 Pages) Semtech Corporation – High Efficiency Integrated Driver for 6-Strings of 30mA LEDs

SC440A
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 1.6μA 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.
V_EXT
R _E XT
R _IOS E T
IOS E T
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
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