SC5014A View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC5014A
Semtech Corporation
'SC5014A' PDF : 27 Pages View PDF
SC5014A
Applications Information
General Description
The SC5014A contains a high frequency, current-mode,
internally compensated boost controller with 2 constant
current sinks for driving LED strings. The LED current for
both strings is programmed by an external resistor. 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 per each string, with current driven up to 240mA.
The unique control loop of the SC5014A allows fast tran-
sient response in dealing with line and load disturbances.
The SC5014A, operating with an external power MOSFET,
regulates the boost converter output voltage based on
the instantaneous requirement of the 2 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 2mA typical
when both 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 the UVLO
pin voltage is greater than 1.23V, the SC5014A goes into a
start-up 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 applica-
tion circuit on page 20, Figure 2) as shown by the follow-
ing equations:
VIN_TurnOn [V] = 1.23 X (R1 + R2) / R1
VIN_Hysteresis [V] = 10-5 X R2 [Ω]
In the next phase, the SC5014A checks each IO pin to
determine if the respective LED string is enabled. Each IO
pin is pulled up with a 100µA current source. If any IO pin
is connected to ground, it will be detected as an unused
string, and will be turned off. This unused string checking
procedure typically takes 1ms. After this, the SC5014A
enters into a soft-start sequence.
The soft-start function helps to prevent excess inrush
current through the input rail during start-up. In the
SC5014A, 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 the SC5014A is controlled by an internal
timing circuit, which is used during start-up and is based
on the boost converter switching frequency. For example,
with switching frequency at 1MHz, it is 8ms typical and
becomes 4ms typical when the switching frequency is
2MHz.
If the PWM voltage goes low while the SC5014A is in soft-
start operation, the SC5014A switches to standby mode,
where the external power MOSFET and the LED current
sources will be turned off immediately. The internal soft-
start timer is turned off and the soft-start value is saved.
When the PWM voltage goes high again, the soft-start
resumes from the previously saved value.
Each LED current source 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 0.9V
typical.
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 where the EN pin voltage is high, but VCC
voltage falls below the respective UVLO threshold, the
SC5014A 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 typical 150°C is
reached, the boost converter and all IO current sources
are turned off. The FLT pin is forced low in this condition.
When the temperature falls below the TSD trip point by
10°C, the SC5014A will restart following the start-up
sequence as described before. The FLT pin is latched and
will stay low, it is reset by cycling the EN, VCC or UVLO.
12
Share Link: 