SC446TETRT Semtech, SC446TETRT Datasheet - Page 13

• Manufacturer Part Number: SC446TETRT
• Description: IC LED DRVR WHT BCKLGT TSSOP-16
• Manufacturer: Semtech
• Type: Backlight, White LEDr

Specifications of SC446TETRT

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 3
• Internal Driver: Yes
• Type - Primary: Backlight
• Frequency: 700kHz
• Voltage - Supply: 4.5 V ~ 27 V
• Voltage - Output: 36V
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Current - Output / Channel: 100mA
• Internal Switch(s): Yes
• Efficiency: 90%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Operating Temperature: -
• Other names: SC446TETR

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 (1nF ~ 10nF) 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 short-circuit, that
string will be latched off. Voltage is monitored if it exceeds
2.35V on the internal LED current source (IO pins). (The IO
voltage on an abnormal LED string will rise earlier than
other normal LED strings). If the voltage exceeds 2.35V on
any IO pin, the IO current source will latch off. The latch is
reset if VIN falls below UVLO or it will recycle the EN signal.
Other LED strings operate normally.
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 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 1µA 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)
If the thermal shutdown temperature of 150
a hiccup sequence is initiated where the boost converter
and all IO current sources are turned off. SS is discharged
by a 1µA current source, and a 0.9mA discharge path is
turned on to pull down VOUT. As temperature falls below
© 2010 Semtech Corporation
°C
is reached,
the TSD trip point, SC446 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 SC446 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 SC446 can resolve 10µs (minimum),
PWM dimming pulse-width.
As far as the maximum PWM dimming pulse-width, it
depends on the PWM dimming frequency. Clearly, it is
trivial to get 100% LED brightness by pulling PWM pin
“High” constantly. When the user tries to dim the LED
brightness using PWM signal from 100% down, he or she
needs to observe the following. When the PWM dimming
signal is actively switching from “High” to “Low” and to
“High”, there is a minimal OFF time (T_off_min, 200ns,
guaranteed by design) requirement of the PWM dimming
signal with this IC.
maximum PWM duty ratio before hitting to 100% in the
following way.
For example, if the PWM dimming frequency f_
PWM=200Hz, the D_max=99.996%. If f_PWM=25kHz,
the D_max=99.5%. With most practical dimming
interfaces, the needed dimming steps and resolutions,
it is uncommon to run into the above D_max before
reaching 100%. While most applications will not run into
D_max, the designer should be aware of possible parasitic
D
Such minimal OFF time sets the
max
=
1−
T
off
_
min
f
PWM
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