SGM3727 Datasheet, PDF (9/13 Pages) SG Micro Corp – High Efficiency 38V Step-Up WLED Driver

SGM3727
APPLICATION INFORMATION
Setting the Maximum LED Current
The maximum LED current is programmed by
appropriate selection of the resistor R_S connected
between RSET pin and GND. To set the full LED current,
choose the resistor according to the equation:
RS 
V_RSET
ILED

0.314
ILED
where V_RSET is the voltage reference and I_LED is the
desired maximum LED current, such as 30mA. The
current in white LED can be adjusted via the EN/SET
single wire interface to provide a simple means of LED
dimming.
Inductor Selection
The SGM3727 was designed for operation with inductors
in the 2.2μH to 22μH range. Keep in mind that
larger-valued inductors (usually requiring higher V_IN)
generally yield higher efficiency due to lower switching
frequency, while smaller inductors typically yield a
smaller footprint but at the cost of lower efficiency. For
best results, choose an inductor with a low
DC-Resistance (DCR) and be sure to choose an inductor
with a saturation current that exceeds the 1A current limit.
Diode Selection
The SGM3727 requires a Schottky diode as the rectifier.
Select a low forward voltage drop Schottky diode with
forward current (I_F) rating that exceeds the peak current
limit of 1A and a peak repetitive reverse voltage (V_RRM)
rating that exceeds the maximum output voltage, typically
set by the OVP threshold.
High Efficiency,
38V Step-Up WLED Driver
Capacitor Selection
The SGM3727 only requires a tiny 0.47μF output
capacitor for most applications. For better output voltage
filtering, a low ESR output capacitor is recommended.
The selection of the output capacitor value directly
influences the output voltage ripple of the converter
which also influences line regulation. The ripple can be
improved by adding a small feed-forward capacitor from
OUT to FB. 3000pF is a good starting point for most
applications.
For circuits driving 6 or fewer LEDs, a 4.7μF input
capacitor is generally suitable. For circuits driving more
than 6 LEDs, a 10μF input capacitor may be required.
Ceramic capacitors are recommended for most
applications. For best performance, use X5R and X7R
type ceramic capacitors, which possess less degradation
in capacitance over voltage and temperature.
Low Input Voltage Application
In applications that have low input voltage range, such as
those powered from 2~3 AA cells, the SGM3727 may still
be used if there is a suitable system supply (such as 3.3V)
available to power the controller. In such an application,
the inductor may be connected directly to the battery,
while the IC power is supplied by the system supply.
SG Micro Corp
9
www.sg-micro.com