SC5010 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC5010
Semtech Corporation
'SC5010' PDF : 36 Pages View PDF
SC5010
Applications Information
General Description
The SC5010 contains a high frequency, current-mode,
internally compensated boost controller and eight con-
stant current sinks for driving LED strings. The LED current
for all strings is programmed by an external resistor and
the boost converter operates to maintain minimal required
output voltage for regulating the LED current to the pro-
grammed value. A typical backlight application uses 3 to
14 LEDs for each string, with current driven up to 30mA.
The unique control loop of the SC5010 allows fast tran-
sient response in dealing with line and load disturbances.
The SC5010 operating with an external power MOSFET
regulates the boost converter output voltage based on
instantaneous requirement of the eight string current
sources. This provides power to the entire lighting subsys-
tem with increased efficiency and reduced component
count. It supports PWM dimming frequencies from 100Hz
to 30kHz and the supply current is reduced to typical 2mA
when all LED strings are off.
Start-Up
When the EN pin is pulled up high (>2.1V), the device is
enabled and the UVLO and VCC pin voltages are checked.
The VCC voltage has fixed under-voltage rising and falling
trip points. If the VCC pin is higher than 4.2V and UVLO pin
voltage is greater than 1.23V, the SC5010 goes into a
startup sequence. The UVLO pin voltage can be used to
program the input power source voltage VIN turn on
threshold and its hysteresis (refer to the Detailed
Application Circuit on page 21) as shown by the following
equations:
VIN_TurnOn [V] = 1.23 X (R1 + R2) / R1
VIN_Hysteresis [V] = 10-5 X R2 [Ω]
In the next phase, the SC5010 checks each IO pin to deter-
mine if the respective LED string is enabled. Each IO pin is
pulled up with a 100μA current source. If any IO pin is con-
nected to GND, it will be detected as an unused string,
and will be turned off. This unused string checking proce-
dure takes 1ms (typical). After this the SC5010 enters into
a soft-start sequence.
SC5010, the soft-start is implemented by slowly ramping
up the reference voltage fed to the error amplifier. This
closed loop start-up method allows the output voltage
to ramp up without any overshoot. The duration of the
soft-start in SC5010 is controlled by an internal timing
circuit which is used during start-up and it’s based on the
boost converter switching frequency. For example, with
switching frequency at 1MHz, it is typical 8ms and it
becomes typical 4ms when the switching frequency is
2MHz.
If PWMI voltage goes low while the SC5010 is in soft start
operation, the SC5010 switches to standby mode. Under
such mode, the external power MOSFET and the LED
current sources will be turned off immediately. The inter-
nal soft-start timer is turned off and the soft-start value is
saved. When the PWMI voltage goes high again, the soft-
start resumes from the previously saved value.
Each LED current source (IO1 to IO8) tries to regulate the
LED current to its set point. The control loop will regulate
the output voltage such that all the IO pin voltages are at
least typical 0.6V.
Shutdown
When the EN pin is pulled down below 0.8V, the device
enters into shutdown mode. In this mode, all the internal
circuitry is turned off and the supply current is less than
1μA(max).
In the scenario when the EN pin voltage is high, but either
V or VCC voltage falls below their respective UVLO
IN
threshold, the SC5010 goes into a suspend mode. In this
mode, all the internal circuitry except the reference and
the oscillator are turned off.
Thermal Shutdown (TSD)
If the thermal shutdown temperature of 150°C is reached,
the boost converter and all IO current sources are turned
off. FLT pin is forced low in this situation. As temperature
falls below the TSD trip point by 10°C, the SC5010 will
restart following the startup sequence as described
before. The FLT pin is latched and will stay low, it is reset
by cycling the EN, VCC, or UVLO.
The soft-start function helps to prevent excess inrush
current through the input rail during startup. In the
15
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