SGM3142 Datasheet, PDF (10/14 Pages) SG Micro Corp – Driver with Low Dropout Current Source

SGM3142
6-Channel Charge Pump White LED Driver
with Low Dropout Current Source
APPLICATION INFORMATION
The SGM3142 is a high efficiency charge pump white
LED driver. It provides 6 channels low dropout voltage
current source to regulated 6 white LEDs current. For
high efficiency, the SGM3142 implements a smart mode
transition for charge pump operation. The SGM3142
provides pulse dimming function for LED brightness
control.
Soft-Start
The SGM3142 includes a soft-start circuit to limit the
inrush current at power on and mode switching. Soft-start
circuit limits the input current before output voltage
reaching a desired voltage level.
Input UVLO
The input operating voltage range of the SGM3142 is
2.7V to 5.5V. An input capacitor at the VIN pin could
reduce ripple voltage. It is recommended to use a
ceramic 1µF or larger capacitance as the input capacitor.
This IC provides an under-voltage lockout (UVLO)
function to prevent it from unstable issue when startup.
The UVLO threshold of input rising voltage is set at 2.2V
typically with a hysteresis 100mV.
Thermal Protection
The SGM3142 has a thermal protection circuit that will
shut down the internal LDO and charge pump if the die
temperature rises above the thermal limit.
Mode Decision
The SGM3142 uses a smart mode selection method to
decide the working mode for optimizing the efficiency.
Mode decision circuit senses the output and LED voltage
for up/down selection. The SGM3142 automatically
switches to 1.5× mode whenever the dropout condition is
detected from the current source and returns to 1× mode
whenever the dropout condition releases.
Brightness Control
The SGM3142 implements the pulse dimming method
being used to control the brightness of white LEDs. The
part implements a 4-bit DAC for brightness control. Users
can easily configure the LED current from 1.25mA to
20mA by a serial pulse. The maximum LED current is up
to 20mA that is sufficient for most application in backlight.
The dimming of white LEDs' current can be achieved by
applying a pulse signal to the EN/SET pin. There are
totally 16 steps of current could be set by users. The
detail operation of brightness dimming is showed in the
Figure 2.
Selecting Capacitors
Careful selection of the four external capacitors C_IN, C1,
C2, C_OUT is important because they will affect some
parameters such as input and output ripple, power
efficiency, maximum supply current by charge pump, and
start-up time. To reduce the input and output ripple
effectively, the low ESR ceramic capacitors are
recommended. Generally, to reduce the output ripple,
increasing the output capacitance C_OUT is necessary.
However, this will increase the startup time of output
voltage. For LED driver applications, the input voltage
ripple is more important than output ripple. Input ripple is
controlled by input capacitor C_IN, increasing the value of
input capacitance can further reduce the ripple.
Practically, the input voltage ripple depends on the power
supply’s impedance. If a single input capacitor C_IN cannot
satisfy the requirement of application, it is necessary to
add a low-pass filter. C-R-C filter used to reduce input
ripple. The flying capacitor C1 and C2 determine the
supply current capability of the charge pump. The lower
value will limit the LED’ s current at low input voltage. For
6 × 20mA load over the entire input range of 2.7V to 5.5V,
it is recommended to use a 1µF ceramic capacitor on the
flying capacitor C1 and C2.
SG Micro Corp
10
www.sg-micro.com