SGM3138 Datasheet, PDF (8/11 Pages) SG Micro Corp – Low Dropout Current Source

SGM3138
6-Channel Charge Pump White LED
Driver with Low Dropout Current Source
APPLICATION INFORMATION
The SGM3138 uses a fractional switched capacitor
charge pump to power up to 6 white LEDs with a
programmable current for uniform intensity. The part
integrates current sources and automatic mode selection
charge pump. It maintains the high efficiency by utilizing
a 1×/1.5× fractional charge pump and current sources.
The small equivalent 1× mode open loop resistance and
ultra-low dropout voltage of current source extend the
operating time of 1× mode and optimize the efficiency in
white LED applications.
Input UVLO
The input operating voltage range of the SGM3138 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 start-up.
The UVLO threshold of input rising voltage is set at 2.15V
typically with a hysteresis 50mV.
Soft-Start
The SGM3138 employs a soft start feature to limit the
inrush current. The soft-start circuit prevents the
excessive inrush current and input voltage droop. The
soft-start clamps the input current over a typical period of
190s.
Mode Decision
The SGM3138 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 SGM3138 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.
Chip Enable and Shutdown
When EN/SET is in “High” status, SGM3138 will enter
into active status, When EN/SET is in “Low” status for
3ms, and SGM3138 will enter into shutdown status.
LED Dimming Control Interface
An internal resistor sets the maximum LED current to be
20mA. EN/SET one wire interface is used to adjust the
current of LED1 to LED6, this is 16 steps pulse dimming.
The programming pulse is shown in Figure 2.
Brightness Control
The SGM3138 implements a pulse dimming method to
control the brightness of white LEDs. Users can easily
configure the LED current from 1.25mA to 20mA by a
serial pulse. 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.
LED Connection
The SGM3138 supports up to 6 white LEDs. The 6 LEDs
are connected from VOUT to pin 1, 2, 3, 4, 5 and 6
respectively. Figure 1 shows the connection for 6-WLEDs
application. LED1 to LED6 are controlled by EN/SET.
The SGM3138 internal current source reference circuit
bases feedback from current sensed on the LED1 and
LED5 outputs. For best operation, the only requirement
for this type of application is the output LED1 and LED5
should always be connected to the load circuit. The other
LED pins (LED2, 3, 4,&6) can be left floating if those
white LEDs are not used.
Over-Voltage Protection
The SGM3138 equips over-voltage protection function.
When LED is open, the voltage between input and output
will be clamped at a certain voltage level.
Selecting Capacitors
To get the better performance of SGM3138, the selection
of peripherally appropriate capacitor and value is very
important. These capacitors determine some parameters
such as input/output ripple voltage, power efficiency, and
maximum supply current by charge pump. To reduce the
input and output ripple effectively, the low ESR ceramic
capacitors are recommended. 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 impedance.
The flying capacitor C1 and C2 determine the supply
current capability of the charge pump and to influence the
overall efficiency of system. The lower value will improve
efficiency, but it 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
8
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