SC5014A View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC5014A
Semtech Corporation
'SC5014A' PDF : 27 Pages View PDF
SC5014A
Applications Information (continued)
LED Short-Circuit Protection (SCP)
The SC5014A features a programmable LED short-circuit
protection (SCP). This allows the part to be customized
based on the LED forward voltage (VF) 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:
current sink latch-off can be reset by cycling UVLO, VCC or
EN.
LED Analog Dimming Control
The LED current in SC5014A 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 RISET resistor.
The SC5014A has a unique DAC architecture which allows
it to have excellent LED current accuracy and string-to-
string matching over the entire DAC range.
VSCP_Trip [V] = 20 X (1.23 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 10μs 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 (100pF ~ 8.2nF) between IO
pin to ground can help prevent the SC5014A 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
The analog dimming method can be used in conjunction
with PWM dimming to increase the dimming resolution.
The fast loop response of the SC5014A allows the LED
current to transition to a new value within 160µs 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 SC5014A allows
dimming over a wide frequency range (100Hz-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
SC5014A provides a 1000:1 dimming range at 1kHz 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 10% 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.
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