Global Mixed-mode Technology Inc.
Pin Description
PIN
1
2
3
4
5
6
G5121
NAME
SW
GND
FB
SHDN
FUNCTION
Switch Pin.
The drain of the internal NMOS power switch. Connect this pin to inductor.
Ground Pin.
Feedback Pin.
Connect current setting resistor Rs from this pin to ground. The LED current is set as
0.254V/ Rs.
Active Low Shutdown Pin.
Over Voltage Protection Sense Pin.
Input Supply Pin.
Bypass this pin with a capacitor as close to the device as possible.
OVP
VCC
Function Description
Normal Operation
The G5121 uses a constant frequency control scheme
to provide excellent line and load regulation. Operation
can be best understood by referring to the block dia-
gram. At the start of each oscillator cycle, the SR latch
is set, which turns on the power switch M1. An artificial
ramp is generated to the positive terminal of the PWM
comparator A2. When this voltage exceeds the level at
the negative input of A2, the SR latch is reset turning
off the power switch. The level at the negative input of
A2 is set by the error amplifier A1, and is simply an
amplified version of the difference between the feed-
back voltage and the reference voltage of 0.254V. In
this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the
error amplifier’s output increases, more current is de-
livered to the output, if it decreases, less current is
delivered.
Over Voltage Protection
Over voltage protection function is designed to prevent
the damage of internal NMOS switch in case the in-
creased impedance of the LED load (include the LED
opened). Once the device detects over voltage (typical
19.5V) at the output, the internal NMOS switch is kept
off until the output voltage drops below 15V.
Since the G5121 is designed to operate in discontinu-
ous mode, the inductor current reaches zero during
discharge phase. After the inductor current reaches
zero, the switch pin exhibits ringing due to the LC tank
circuit formed by the inductor in combination with
switch and diode capacitance. This ringing is not
harmful; far less spectral energy is contained in the
ringing than in the switch transitions. The ringing can
be damped by application of a 300Ω resistor across
the inductor, although this will degrade efficiency.
Capacitor Selection
The small size of ceramic capacitors makes them suit-
able for G5121 applications. X5R and X7R types are
recommended because they retain their capacitance
over wider voltage and temperature ranges than other
types such as Y5V or Z5U. A minimum 1µF capacitor
for output is required for most applications. Larger in-
put/output capacitor minimizes input/output ripple.
Diode Selection
Schottky diodes, with their low forward voltage drop
and fast reverse recovery, are the ideal choices for
G5121 applications. The forward voltage drop of a
Schottky diode represents the conduction losses in the
diode, while the diode capacitance (C
T
or C
D
) repre-
sents the switching losses. For diode selection, both
forward voltage drop and diode capacitance need to
be considered. Schottky diodes with higher current
ratings usually have lower forward voltage drop and
larger diode capacitance, which can cause significant
switching losses at the 1MHz switching frequency of
the G5121. A Schottky diode rated at 500mA is suffi-
cient for most G5121 applications.
Application Information
Inductor Selection
A 4.7µF inductor is recommended for G5121 applica-
tions. Small size and high efficiency are the major
concerns for most G5121 applications. Inductor with
low core losses and small DCR (cooper wire resis-
tance) at 1MHz are good choice for G5121 applica-
tions.
Ver: 1.3
Nov 13, 2006
TEL: 886-3-5788833
http://www.gmt.com.tw
8
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
