Global Mixed-mode Technology Inc.
Function Description
Normal Operation
The G5105 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 1.28V. In
this manner, the error amplifier sets the correct peak
current level (DCM) or duty (CCM) to keep the output
in regulation.
Over Voltage Protection
Over voltage protect function is designed to prevent
the output accidentally damage the load. Once the
device detects over voltage (nominalx1.05) at the
output, the internal NMOS switch turns off to stop
power input
G5105
ciency are the major concerns for most G5105 applica-
tions. Inductor with low core losses and small DCR
(cooper wire resistance) at 1MHz are good choice for
G5105 applications.
Capacitor Selection
The small size of ceramic capacitors makes them suit-
able for G5105 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 10µF or 22µF capacitor
for input and output are recommended for most appli-
cations. Smaller input/output capacitor enlarges the
input/output ripple.
Diode Selection
Schottky diodes, with their low forward voltage drop
and fast reverse recovery, are the ideal choices for
G5105 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 di-
ode capacitance, which can cause significant switching
losses at the 1MHz switching frequency of the G5105.
A Schottky diode rated at 1A is sufficient for most
G5105 applications.
Application Information
Inductor Selection
A 4.7µH or 10µH inductor is recommended for small
ripple applications. Small form factor and high effi-
Ver: 0.6 Preliminary
May 10, 2006
TEL: 886-3-5788833
http://www.gmt.com.tw
9
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
